BANCWEST CORP/HI
S-4, 1999-04-14
STATE COMMERCIAL BANKS
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<PAGE>   1
 
     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON APRIL 14, 1999
 
                                                 REGISTRATION NO. 333-
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------
                                    FORM S-4
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                            ------------------------
 
                              BANCWEST CORPORATION
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)
 
<TABLE>
<S>                                  <C>                                  <C>
              DELAWARE                               6711                              99-0156159
  (STATE OR OTHER JURISDICTION OF        (PRIMARY STANDARD INDUSTRIAL               (I.R.S. EMPLOYER
   INCORPORATION OR ORGANIZATION)        CLASSIFICATION CODE NUMBER)              IDENTIFICATION NO.)
</TABLE>
 
                               999 BISHOP STREET
                             HONOLULU, HAWAII 96813
                                 (808) 525-7000
                 (ADDRESS AND TELEPHONE NUMBER OF REGISTRANT'S
                          PRINCIPAL EXECUTIVE OFFICES)
                            ------------------------
                              WALTER A. DODS, JR.
                      CHAIRMAN AND CHIEF EXECUTIVE OFFICER
                              BANCWEST CORPORATION
                               999 BISHOP STREET
                             HONOLULU, HAWAII 96813
                                 (808) 525-7000
                    (NAME, ADDRESS, INCLUDING ZIP CODE, AND
                     TELEPHONE NUMBER OF AGENT FOR SERVICE)
                            ------------------------
                                WITH COPIES TO:
 
<TABLE>
<S>                                <C>                                <C>
       RODNEY R. PECK, ESQ.                LEE MEYERSON, ESQ.                 JAMES M. ROCKETT, ESQ.
  PILLSBURY MADISON & SUTRO LLP        SIMPSON THACHER & BARTLETT     MCCUTCHEN, DOYLE, BROWN & ENERSEN, LLP
      235 MONTGOMERY STREET               425 LEXINGTON AVENUE         THREE EMBARCADERO CENTER, SUITE 2700
 SAN FRANCISCO, CALIFORNIA 94104        NEW YORK, NEW YORK 10017         SAN FRANCISCO, CALIFORNIA 94111
          (415) 983-1000                     (212) 455-2000                       (415) 393-2000
</TABLE>
 
                            ------------------------
     APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO PUBLIC: As soon as
practicable after this Registration Statement becomes effective.
 
     If the securities being registered on this Form are being offered in
connection with the formation of a holding company and there is compliance with
General Instruction G, check the following box.  [ ]
 
                        CALCULATION OF REGISTRATION FEE
 
<TABLE>
<CAPTION>
- ---------------------------------------------------------------------------------------------------------------------------------
- ---------------------------------------------------------------------------------------------------------------------------------
                                                                PROPOSED MAXIMUM       PROPOSED MAXIMUM
       TITLE OF EACH CLASS OF              AMOUNT TO BE          OFFERING PRICE       AGGREGATE OFFERING         AMOUNT OF
   SECURITIES TO BE REGISTERED(1)         REGISTERED(2)             PER UNIT               PRICE(2)         REGISTRATION FEE(3)
- ---------------------------------------------------------------------------------------------------------------------------------
<S>                                   <C>                    <C>                    <C>                    <C>
Common Stock
  (par value $1.00 per share).......     4,624,004 shares        Not applicable         Not applicable            $39,730
- ---------------------------------------------------------------------------------------------------------------------------------
- ---------------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) This registration statement relates to securities of the Registrant issuable
    to holders of common stock of SierraWest Bancorp.
 
(2) The number of shares to be registered is based upon a product of (a) the
    number of shares of common stock, no par value, of SierraWest ("SierraWest
    Common Stock") outstanding, assuming exercise of all SierraWest stock
    options (whether or not currently exercisable) multiplied by (b) 0.82, the
    exchange ratio in the merger.
 
(3) Pursuant to Rule 457(f), the registration fee was computed on the basis of
    $30.907, the market value of the common stock of SierraWest Bancorp to be
    exchanged for the Registrant's common stock in the merger, computed in
    accordance with Rule 457(c) on the basis of the average of the high and low
    price as quoted on the Nasdaq National Market on April 8, 1999. In
    accordance with Rule 457(b), the total registration fee of $39,730 has been
    reduced by $35,456, which was paid on March 25, 1999, at the time of the
    filing under the Securities Exchange Act of 1934, as amended, of preliminary
    copies of SierraWest's proxy materials included herein. Therefore the
    registration fee payable upon filing of this Registration Statement is
    $4,274.
                            ------------------------
     THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SHALL SPECIFICALLY STATE THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OF 1933, AS AMENDED OR UNTIL THE REGISTRATION STATEMENT SHALL
BECOME EFFECTIVE ON SUCH DATE AS THE SECURITIES AND EXCHANGE COMMISSION, ACTING
PURSUANT TO SAID SECTION 8(a), MAY DETERMINE.
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>   2
 
THE INFORMATION IN THIS PROSPECTUS IS NOT COMPLETE AND MAY BE CHANGED. WE MAY
NOT SELL THESE SECURITIES UNTIL THE REGISTRATION STATEMENT FILED WITH THE
SECURITIES AND EXCHANGE COMMISSION IS EFFECTIVE. THIS PROSPECTUS IS NOT AN OFFER
TO SELL THESE SECURITIES AND IT IS NOT SOLICITING AN OFFER TO BUY THESE
SECURITIES IN ANY STATE WHERE THE OFFER OR SALE IS NOT PERMITTED.
 
              PRELIMINARY PROXY STATEMENT -- SUBJECT TO COMPLETION
                           [SierraWest Bancorp Logo]
                                                                  April   , 1999
 
Dear Shareholder,
 
     You are cordially invited to attend the Annual Meeting of Shareholders of
SierraWest Bancorp which we will hold at the North Tahoe Conference Center, 8318
North Lake Boulevard, Kings Beach, California on Thursday, May 27, 1999 at 4:00
p.m.
 
     As you have probably read, we have signed an agreement to merge with Bank
of the West, a subsidiary of BancWest Corporation. In the proposed merger, you
will receive 0.82 of a share of BancWest common stock for each share of
SierraWest common stock that you own. You will not incur federal income tax as a
result of the merger, except with respect to cash you receive instead of
fractional shares.
 
     BancWest common stock is traded on the New York Stock Exchange under the
symbol "BWE" and on April 13, 1999, the BancWest common stock closed at $40.63
per share. SierraWest common stock is traded on the Nasdaq National Market under
the symbol "SWBS" and on April 13, 1999, the SierraWest common stock closed at
$31.19 per share. Based on that closing stock price of the BancWest common
stock, you would receive for each share of SierraWest common stock BancWest
common stock with a value equal to $33.31.
 
     The most important issue on the agenda for our meeting will be a
shareholder vote to approve the merger. We believe the merger is in your best
interests as shareholders and we hope you will support it. Information about the
proposed merger is included in the enclosed proxy statement-prospectus. We will
also be voting on the election of 13 people to SierraWest's Board of Directors
to serve until the proposed merger is completed or until the next annual
meeting.
 
     Please give these proxy materials your careful attention. Your Board of
Directors has unanimously approved the merger and recommends that you vote to
approve it as well.
 
                                            Sincerely,
 
                                            /s/ William T. Fike
 
                                            William T. Fike
                                            President and Chief Executive
                                            Officer
                                            SierraWest Bancorp
 
     Neither the Securities and Exchange Commission nor any state securities
regulators have approved this transaction or the shares of BancWest common stock
to be issued under this proxy statement-prospectus or determined if this proxy
statement-prospectus is accurate or adequate. Any representation to the contrary
is a criminal offense.
 
     The shares of BancWest common stock offered by this proxy
statement-prospectus are not savings accounts, deposits or other obligations of
any bank or nonbank subsidiary of BancWest Corporation and are not insured by
the Federal Deposit Insurance Corporation or any other governmental agency.
 
         THE DATE OF THIS PROXY STATEMENT-PROSPECTUS IS APRIL   , 1999,
AND IS FIRST BEING MAILED TO SIERRAWEST SHAREHOLDERS ON OR ABOUT APRIL 21, 1999.
<PAGE>   3
 
              PRELIMINARY PROXY STATEMENT -- SUBJECT TO COMPLETION
 
                               SIERRAWEST BANCORP
                        10181 TRUCKEE-TAHOE AIRPORT ROAD
                           TRUCKEE, CALIFORNIA 96161
 
                    NOTICE OF ANNUAL MEETING OF SHAREHOLDERS
                           TO BE HELD ON MAY 27, 1999
                           -------------------------
 
To the Shareholders of SierraWest Bancorp:
 
     Notice is hereby given that we will hold an Annual Meeting of Shareholders
of SierraWest Bancorp, a California corporation, at the North Tahoe Conference
Center, 8318 North Lake Boulevard, Kings Beach, California on Thursday, May 27,
1999 at 4:00 p.m., for the following purposes:
 
        1.  Approving the Agreement and Plan of Merger dated as of February 25,
            1999, as amended, among BancWest Corporation, Bank of the West and
            SierraWest Bancorp. The merger agreement provides for the merger of
            SierraWest and its subsidiary, SierraWest Bank, into Bank of the
            West and the conversion of each outstanding share of SierraWest
            common stock into the right to receive 0.82 of a share of BancWest
            common stock.
 
        2.  Electing 13 people to SierraWest's Board of Directors.
 
        3.  Acting upon any other matters that may properly come before the
            annual meeting or any adjournment or postponement thereof.
 
     You are entitled to notice of and to vote at the annual meeting and any
postponements or adjournments if you were listed in SierraWest's records as a
holder of common stock at the close of business on April 19, 1999. The merger
agreement and other related matters are described in more detail in the
accompanying proxy statement-prospectus and the attached Appendices, which are
incorporated by reference into and should be considered a part of this Notice.
 
     If the merger is completed and you comply with the requirements of Chapter
13 of the California General Corporation Law, you may have dissenters' rights
giving you the right to receive from BancWest a cash payment of the fair market
value of your shares determined in accordance with Chapter 13. BancWest does not
have to complete the merger if you properly exercise your dissenters' rights.
See "THE MERGER -- Dissenters' Rights of Appraisal" in the attached proxy
statement-prospectus for a discussion of the availability of dissenters' rights
and a description of the procedures which you must follow to enforce those
rights under Chapter 13. A copy of Chapter 13 is included as Appendix D to the
attached proxy statement-prospectus.
 
                                            By Order of the Board of Directors,
                                            /s/ A. Morgan Jones
                                            A. Morgan Jones, Secretary
                                            Truckee, California
                                            April   , 1999
 
                                   IMPORTANT
 
     YOUR VOTE IS IMPORTANT. WHETHER OR NOT YOU PLAN TO ATTEND THIS MEETING,
PLEASE SIGN AND RETURN THE ENCLOSED PROXY AS PROMPTLY AS POSSIBLE IN THE
ENCLOSED POSTAGE-PAID ENVELOPE.
<PAGE>   4
 
                               TABLE OF CONTENTS
 
<TABLE>
<S>                                    <C>
QUESTIONS AND ANSWERS ABOUT THE
  MERGER.............................    1
SUMMARY..............................    2
  Comparative Market Price Data......    2
  Selected Historical Consolidated
     Financial Data of BancWest
     Corporation.....................    7
  Selected Historical Consolidated
     Financial Data of SierraWest
     Bancorp.........................    9
  Comparative Per Common Share
     Data............................   11
INFORMATION REGARDING FORWARD LOOKING
  STATEMENTS.........................   12
ANNUAL MEETING OF SIERRAWEST
  SHAREHOLDERS.......................   13
  Date, Time and Place...............   13
  The Annual Meeting.................   13
  Record Date; Quorum................   13
  Vote Required......................   13
  Voting of Proxies..................   14
  Revocability of Proxies............   14
  Solicitation of Proxies............   15
PROPOSAL ONE.........................   16
THE MERGER...........................   16
  General............................   16
  Background of the Merger...........   16
  Reasons for the Merger;
     Recommendation of the Board of
     Directors.......................   19
  Opinion of Financial Advisor.......   20
  Regulatory Approvals Required......   26
  Stock Exchange Listing.............   27
  Interest of Certain Officers and
     Directors in the Merger.........   27
  Effect on SierraWest's Employee
     Benefits Plans..................   29
  Accounting Treatment...............   29
  Certain Federal Income Tax
     Consequences....................   30
  Dissenters' Rights of Appraisal....   31
  SierraWest's Rights Agreement......   33
  Resales of BancWest Common Stock...   33
THE MERGER AGREEMENT.................   35
  Structure of the Merger; Effective
     Time............................   35
  Conversion of SierraWest Common
     Stock...........................   35
  Options............................   36
  Exchange Agent; Exchange
     Procedure.......................   36
  Representations and Warranties.....   37
  Conduct of Business Pending the
     Merger..........................   38
  Additional Agreements..............   40
  Conditions to the Completion of the
     Merger..........................   42
  Termination........................   43
  Fees and Expenses..................   45
  Amendment..........................   45
  Extension; Waiver..................   46
STOCK OPTION AGREEMENT BETWEEN
  SIERRAWEST AND BANCWEST............   47
  Exercise of Stock Option...........   47
  Termination of Stock Option........   48
  Adjustment of Number of Shares
     Subject to Option...............   48
  Repurchase of Option Shares........   49
  Registration Rights................   50
  Effect of Stock Option Agreement...   50
OPERATIONS FOLLOWING THE MERGER......   51
BANCWEST CORPORATION.................   52
  First Hawaiian, Inc./BancWest
     Corporation Merger..............   52
  First Hawaiian Bank................   52
  Bank of the West...................   55
  First Hawaiian Capital I...........   57
  Employees..........................   57
  Monetary Policy and Economic
     Conditions......................   58
</TABLE>
 
                                        i
<PAGE>   5
<TABLE>
<S>                                    <C>
  Competition........................   58
  Supervision and Regulation.........   58
SIERRAWEST BANCORP...................   63
MARKET PRICE AND DIVIDEND
  INFORMATION........................   64
  Dividend Policy....................   64
DESCRIPTION OF BANCWEST CAPITAL
  STOCK..............................   65
  Common Stock.......................   65
CERTAIN DIFFERENCES IN RIGHTS OF
  HOLDERS OF SIERRAWEST COMMON STOCK
  AND BANCWEST COMMON STOCK..........   69
  Dividends..........................   69
  Indemnification of Directors and
     Executive Officers..............   69
  Cumulative Voting..................   70
  Classified Board of Directors......   71
  Dissenters' Rights in Mergers and
     Other Reorganizations...........   71
  Anti-Takeover Statutes.............   72
  Shareholder Vote for Mergers and
     Asset Sales.....................   72
  Inspection of Stockholder Lists....   74
  Nomination of Directors............   74
  Amendment of Certificate of
     Incorporation and Bylaws........   74
PROPOSAL TWO.........................   76
ELECTION OF DIRECTORS................   76
  The Board of Directors and
     Committees......................   79
  Compensation of Directors..........   79
EXECUTIVE COMPENSATION...............   81
  Summary Compensation Table.........   81
  Aggregated Option/SAR Exercises in
     Last Fiscal Year and FY-End
     Option/SAR Value................   83
  Salary Continuation Plan...........   83
  Employment Agreements..............   84
  Personnel/Compensation Committee
     Report on Executive
     Compensation....................   85
  Personnel/Compensation Committee
     Interlocks and Insider
     Participation...................   86
SHAREHOLDINGS OF CERTAIN BENEFICIAL
  OWNERS AND MANAGEMENT..............   87
SECTION 16(a) BENEFICIAL OWNERSHIP
  AND COMPLIANCE.....................   89
CERTAIN RELATIONSHIPS AND RELATED
  TRANSACTIONS.......................   89
LEGAL MATTERS........................   89
EXPERTS..............................   89
SUBMISSIONS OF SHAREHOLDERS'
  PROPOSALS..........................   90
INFORMATION CONCERNING BANCWEST
  MANAGEMENT.........................   90
WHERE YOU CAN FIND MORE
  INFORMATION........................   91
APPENDICES
  Appendix A -- Agreement and Plan of
     Merger..........................  A-1
  Appendix B -- Stock Option
     Agreement.......................  B-1
  Appendix C -- Fairness Opinion by
     NationsBanc Montgomery
     Securities LLC..................  C-1
  Appendix D -- Chapter 13 of the
     California General Corporation
     Law Regarding Dissenters'
     Rights..........................  D-1
</TABLE>
 
                                       ii
<PAGE>   6
 
                     QUESTIONS AND ANSWERS ABOUT THE MERGER
 
     Q:  WHY IS THIS MERGER PROPOSED?
 
     A:  SierraWest is proposing this merger because its board of directors has
concluded that this merger is in the best interest of its shareholders and that
the combined companies can offer SierraWest's customers a broader array of
services and products than SierraWest could offer on its own.
 
     Q:  WHAT WILL I RECEIVE IN THIS MERGER?
 
     A:  Under the merger agreement, you will have the right to receive 0.82 of
a share of BancWest common stock for each share of SierraWest common stock that
you own.
 
     Q:  WHAT WILL HAPPEN TO SIERRAWEST BANK IN THIS MERGER?
 
     A:  Immediately after the merger, SierraWest Bank will merge into Bank of
the West. The resulting bank will continue under the name "Bank of the West" as
a wholly owned subsidiary of BancWest.
 
     Q:  HOW DO I VOTE?
 
     A:  Simply indicate on your proxy card how you want to vote and then sign
and mail your proxy card in the enclosed return envelope as soon as possible so
that your shares may be represented at the SierraWest annual meeting.
 
     Q:  IF MY SHARES ARE HELD IN "STREET NAME" BY MY BROKER, WILL MY BROKER
VOTE MY SHARES FOR ME?
 
     A:  Your broker will not vote your shares for you unless you provide
instructions to your broker on how to vote. It is important therefore that you
follow the directions provided by your broker regarding how to instruct your
broker to vote your shares. If you fail to instruct your broker how to vote your
shares, the effect will be the same as a vote against the merger agreement.
 
     Q:  CAN I CHANGE MY VOTE AFTER I HAVE MAILED MY SIGNED PROXY CARD?
 
     A:  Yes. You may change your vote at any time before your proxy is voted at
the annual meeting. If your shares are held in your name you may do this in one
of three ways. First, you may send a written notice stating that you would like
to revoke your proxy. Second, you may complete and submit a new proxy card. If
you choose either of these two methods, you must submit your notice of
revocation or your new proxy card to SierraWest at the address at the top of the
SierraWest notice of annual meeting. Third, you may attend the meeting and vote
in person if you tell the Secretary that you want to cancel your proxy and vote
in person. Simply attending the SierraWest annual meeting, however, will not
revoke your proxy. If you have instructed a broker to vote your shares, you must
follow directions received from your broker to change your vote or to vote at
the SierraWest annual meeting.
 
     Q:  SHOULD I SEND IN MY CERTIFICATES NOW?
 
     A:  No. After the merger is completed, we will send you written
instructions for exchanging your stock certificates.
 
     Q:  WHEN DO YOU EXPECT THIS MERGER TO BE COMPLETED?
 
     A:  We are working toward completing this merger as quickly as possible. We
currently expect to complete this merger in mid-1999.
 
     Q:  WHY HAVE YOU SENT ME THIS DOCUMENT?
 
     A:  This proxy statement-prospectus contains important information
regarding this proposed merger, as well as information about BancWest and
SierraWest. It also contains important information about what the SierraWest
board of directors and management considered in evaluating this proposed merger.
We urge you to read this proxy statement-prospectus carefully, including its
appendices. You may also want to review the documents listed under "WHERE YOU
CAN FIND MORE INFORMATION" on page 91.
 
                                        1
<PAGE>   7
 
                                    SUMMARY
 
     This summary, together with the "Questions and Answers" on the preceding
pages, highlights important selected information from this proxy
statement-prospectus. To understand the merger fully and for a more complete
description of the legal terms of the merger, you should read carefully this
entire document and the other information available to you. We have included
page references parenthetically to direct you to a more complete description of
the topics presented in this summary.
 
SIERRAWEST SHAREHOLDERS WILL RECEIVE 0.82 OF A SHARE OF BANCWEST COMMON STOCK IN
THE MERGER (page 35)
 
     When the merger is completed, you will receive 0.82 of a share of BancWest
common stock for each share of SierraWest common stock that you hold. Cash will
be paid instead of fractional shares. For example, if you hold 115 shares of
SierraWest common stock, you will have the right to receive 94.3 shares of
BancWest common stock in the merger. Since cash will be paid instead of
fractional shares, you would only receive 94 shares of BancWest common stock and
a check in an amount equal to 0.3 of a share multiplied by the closing price of
the BancWest common stock on the trading day before the closing date.
 
     SierraWest has the option to terminate the merger agreement with the
approval of a majority of its entire board during a two business day period
following the approval of the merger by the Federal Deposit Insurance
Corporation (the "FDIC") if:
 
     - the average closing price of BancWest common stock for the 20 trading
       days before the receipt of the FDIC approval is less than 85% of
       $43.8375, or $37.2619, the average closing price of BancWest common stock
       for the 5 trading days before the public announcement of the merger, AND
 
     - the ratio of the average closing price of BancWest common stock during
       the 20 trading day period to $43.8375 is less than 85% of the ratio of
       Standard & Poor's Mid-Cap Regional Bank Index during the 20 trading day
       period to $223.854, the average of the bank stock index for the 5 trading
       days before the public announcement of the merger.
 
     SierraWest cannot terminate the merger agreement, if within 5 business days
of being notified by SierraWest of SierraWest's intention to terminate, BancWest
adjusts the exchange ratio so that the value of the BancWest shares you receive
in the merger will not have declined more than 15% in relation to any decline in
the bank stock index. In this event, you will receive more than 0.82 of a share
of BancWest stock for each share of SierraWest common stock that you hold.
 
COMPARATIVE MARKET PRICE DATA
 
     BancWest common stock is listed on the New York Stock Exchange under the
symbol "BWE." Before November 2, 1998, BancWest's common stock was quoted under
the symbol "FHWN" on the Nasdaq National Market. SierraWest common stock is
traded on the Nasdaq National Market under the symbol "SWBS." The following
table sets forth historical per share market values for BancWest common stock
and SierraWest common stock based on the last sales prices and the equivalent
pro forma market values for SierraWest common stock on:
 
     - February 25, 1999, the last trading day before public announcement of the
       merger, and
                                        2
<PAGE>   8
 
     - April 13, 1999, the most recent date before the mailing of this proxy
       statement-prospectus.
 
The equivalent pro forma sales price of SierraWest common stock is determined by
multiplying the exchange ratio (0.82) by the BancWest stock price.
 
<TABLE>
<CAPTION>
                                    HISTORICAL MARKET PRICE
                                    ------------------------    SIERRAWEST EQUIVALENT
                                    BANCWEST     SIERRAWEST     PRO FORMA MARKET VALUE
                                    ---------    -----------    ----------------------
<S>                                 <C>          <C>            <C>
February 25, 1999.................   $43.00        $28.88               $35.26
April 13, 1999....................   $40.63        $31.19               $33.31
</TABLE>
 
     BancWest cannot assure you that actual stock prices for its common stock
will be equal to or greater than the prices shown in the table at the time of
the merger or at any time after the completion of the merger. In the merger,
SierraWest will be merged into Bank of the West and there will be no further
public market for SierraWest common stock after the merger. BancWest common
stock will continue to be traded on the New York Stock Exchange after the
merger.
 
THE MERGER WILL BE A TAX-FREE TRANSACTION IN WHICH SIERRAWEST SHAREHOLDERS WILL
NOT RECOGNIZE GAIN OR LOSS (page 30)
 
     The merger is intended to be a tax-free reorganization so that no gain or
loss will be recognized by either BancWest or SierraWest or their respective
shareholders for federal income tax purposes, except with respect to cash that
SierraWest shareholders will receive instead of fractional shares.
 
SIERRAWEST BOARD RECOMMENDS SHAREHOLDER APPROVAL (page 20)
 
     SierraWest's board of directors believes that the merger is in the best
interests of SierraWest and its shareholders and has unanimously approved the
merger agreement. SierraWest's board recommends that you vote "FOR" approval of
the merger agreement.
 
FINANCIAL ADVISOR GIVES OPINION THAT CONSIDERATION IS FAIR TO SIERRAWEST
SHAREHOLDERS (page 20)
 
     In deciding to approve the merger, the SierraWest board of directors
considered the opinion of its financial advisor, NationsBanc Montgomery
Securities LLC, dated as of February 25, 1999, as to the fairness of the merger
consideration to SierraWest shareholders from a financial point of view. This
opinion is attached as Appendix C to this proxy statement-prospectus. We
encourage you to read this opinion carefully. Under an agreement with
SierraWest, NationsBanc Montgomery received a fee of $600,000 when we executed
the merger agreement and will receive an additional fee of about $1.9 million
when the merger closes.
 
ANNUAL SHAREHOLDERS' MEETING TO BE HELD ON MAY 27, 1999 (page 13)
 
     We will hold the annual meeting of shareholders at 4:00 p.m. on Thursday,
May 27, 1999, at the North Tahoe Conference Center, 8318 North Lake Boulevard,
Kings Beach, California. At the annual meeting, you will be asked (1) to approve
the merger agreement and (2) to elect 13 people to the board of directors to
serve until the merger is completed or until the next annual meeting.
 
RECORD DATE SET AT APRIL 19, 1999; VOTE REQUIRED FOR APPROVAL OF MERGER (page
13) AND ELECTION OF DIRECTORS (page 13)
 
     You can vote at the annual meeting if you owned SierraWest common stock at
the close of business on April 19, 1999. A majority of the outstanding shares of
SierraWest
                                        3
<PAGE>   9
 
common stock must vote to approve the merger agreement in order for the merger
to occur. In the election of directors, the 13 nominees receiving the greatest
number of votes will be elected.
 
DISSENTERS' RIGHTS OF APPRAISAL (page 31 and Appendix D)
 
     No holder of SierraWest common stock will be entitled to dissenters' rights
unless holders of at least 5% of the outstanding shares of SierraWest common
stock have perfected their dissenters' rights in accordance with Chapter 13 of
the California General Corporation Law. BancWest will not be obligated to
complete the merger if shareholders of SierraWest are entitled to exercise
dissenters' rights. Therefore, if you perfect your dissenters' rights, the
merger may not be completed.
 
INFORMATION REGARDING BANCWEST AND SIERRAWEST (pages 52 and 63)
 
       BANCWEST CORPORATION
       999 Bishop Street
       Honolulu, Hawaii 96813
       (808) 525-7000
       http://www.bancwestcorp.com
 
     BancWest is a $15 billion regional financial services company with
operations in five western states. Its headquarters are in Honolulu, with
administrative headquarters in San Francisco. Its major subsidiaries, First
Hawaiian Bank and Bank of the West, operate as separate institutions. First
Hawaiian Bank has 59 branches in Hawaii, Guam, and Saipan. Bank of the West is
California's fifth largest commercial bank with 146 branches serving nearly
400,000 households and businesses in California, Oregon, Washington, and Idaho.
At December 31, 1998, BancWest had consolidated total assets of $15.0 billion,
total deposits of $11.3 billion and total stockholders' equity of $1.7 billion.
 
     On November 1, 1998, First Hawaiian, Inc. merged with BancWest Corporation,
which was a subsidiary of Banque Nationale de Paris and the parent corporation
of Bank of the West, and changed its name to BancWest Corporation. In the
merger, Banque Nationale de Paris received shares of Class A Common Stock of
First Hawaiian, Inc. representing 45% of the outstanding voting stock of First
Hawaiian, Inc. following that merger. Banque Nationale de Paris owns 45% of the
outstanding voting stock of BancWest as of the date of this proxy
statement-prospectus. The ownership percentage of Banque Nationale de Paris will
decline as a result of the issuance of BancWest shares to SierraWest
shareholders in the merger. However, under the terms of a standstill agreement
with BancWest, Banque Nationale de Paris will have the right to purchase
additional shares of BancWest common stock to maintain its 45% ownership
interest.
 
       SIERRAWEST BANCORP
       10181 Truckee-Tahoe Airport Road
       Truckee, California 96161
       (530) 582-3000
 
     SierraWest is a bank holding company for SierraWest Bank, headquartered in
Truckee, California. SierraWest Bank has 20 branches in Solano and Contra Costa
Counties, Sacramento, the Sierra foothills and Lake Tahoe regions of California
and northern Nevada. SierraWest is a significant originator of loans under the
U.S. government's Small Business Administration loan program, with loan
production offices or agency relationships in California, Nevada, Arizona,
Washington, Colorado, Tennessee, Oregon, Georgia, Florida, Texas and Alabama. At
December 31, 1998, SierraWest had
                                        4
<PAGE>   10
 
consolidated total assets of $879 million, total deposits of $783 million and
total shareholders' equity of $78 million.
 
BANCWEST TO USE "POOLING OF INTERESTS" ACCOUNTING TREATMENT (page 29)
 
     BancWest expects to account for the merger as a "pooling of interests."
Under the pooling of interests accounting method, BancWest will carry forward on
its books the assets and liabilities of SierraWest at their historical recorded
values.
 
BENEFITS TO CERTAIN OFFICERS AND DIRECTORS IN THE MERGER (page 27)
 
     In considering the recommendation of the board of directors of SierraWest
to approve the merger agreement, you should be aware that certain officers and
directors of SierraWest have certain interests in, and will receive benefits as
a consequence of, the merger that are different from the benefits to SierraWest
shareholders generally. These interests include:
 
     - SierraWest directors who exercise nonqualified stock options after the
       merger will receive a payment equal to the estimated tax benefit of the
       option exercise to BancWest,
 
     - ten of SierraWest's directors will receive a cash payment at the time of
       the merger in accordance with SierraWest's Amended and Restated Payment
       Continuation Agreements,
 
     - SierraWest's president and chief executive officer and its chief
       financial officer have each entered into salary continuation agreements
       with SierraWest under which each will receive annual payments for 20
       years after the merger,
 
     - some officers have entered into severance agreements with SierraWest
       under which each will receive a severance payment after the merger, and
 
     - following the merger, BancWest will indemnify the directors and officers
       of SierraWest and SierraWest Bank.
 
CONDITIONS THAT MUST BE SATISFIED FOR THE MERGER TO OCCUR (page 42)
 
     We will not complete the merger unless a number of conditions are met.
These include:
 
     - approval of the merger agreement by SierraWest shareholders,
 
     - receipt of all required regulatory approvals,
 
     - absence of any court order prohibiting the merger,
 
     - listing on the New York Stock Exchange of the BancWest shares which will
       be issued to SierraWest shareholders,
 
     - absence of any orders suspending the effectiveness of the registration
       statement filed by BancWest to register the shares to be issued to
       SierraWest shareholders, and
 
     - receipt of opinions of independent public accountants to both BancWest
       and SierraWest that the merger will qualify for "pooling of interests"
       accounting treatment.
 
     In addition, BancWest does not have to complete the merger if 5% or more of
SierraWest shareholders take all steps required to exercise dissenters' rights.
 
REGULATORY APPROVALS WE MUST OBTAIN FOR THE MERGER (page 26)
 
     The merger requires the prior approval of the FDIC and the banking
authority of the State of California.
                                        5
<PAGE>   11
 
TERMINATION OF THE MERGER AGREEMENT (page 43)
 
     We can mutually agree to terminate the merger agreement at any time before
the merger is completed, even if SierraWest's shareholders have approved the
merger agreement.
 
     Either BancWest or SierraWest can terminate the merger agreement if:
 
     - any federal or state banking regulatory agency issues an order denying
       approval of the merger or any governmental entity issues a final
       permanent order prohibiting the merger,
 
     - the merger is not completed by September 30, 1999, or
 
     - SierraWest shareholders do not approve the merger at the annual meeting
       of shareholders.
 
     BancWest can terminate the merger agreement if:
 
     - any governmental entity issues an order in connection with the merger
       which imposes burdensome conditions on BancWest or the surviving
       corporation, or
 
     - SierraWest's board of directors withdraws its recommendation of the
       merger.
 
     SierraWest can terminate the merger agreement once the merger has been
approved by the FDIC if during a measurement period, the BancWest stock price
falls below 85% of $43.8375, or $37.2619, AND the BancWest stock price declines
more than 15% in relation to a bank stock index. SierraWest, however, cannot
exercise this termination right if BancWest chooses to adjust the exchange ratio
so that the value of the BancWest shares you receive in the merger will not have
declined more than 15% in relation to any decline in the bank stock index. This
is more fully described under "SierraWest shareholders will receive 0.82 of a
share of BancWest common stock in the merger" on page 2.
 
STOCK OPTION AGREEMENT BETWEEN BANCWEST AND SIERRAWEST (page 47)
 
     When we signed the merger agreement we also signed a stock option
agreement. Under the stock option agreement, SierraWest gave BancWest an option
to purchase up to 1,059,490 shares of SierraWest common stock representing
approximately 19.9% of the outstanding shares of SierraWest common stock under
certain circumstances. BancWest has the right to purchase the shares for $28.875
per share.
 
     SierraWest agreed to grant the option to BancWest in order to induce
BancWest to enter into the merger agreement. The option could have the effect of
discouraging other companies from trying to acquire SierraWest.
 
     The stock option agreement is attached to this document as Appendix B.
                                        6
<PAGE>   12
 
                SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA
                            OF BANCWEST CORPORATION
 
     This financial information for BancWest for the fiscal years 1994 to 1998
is only a summary. You should read it with the audited consolidated financial
statements and the accompanying notes of BancWest, which BancWest has filed with
the Securities and Exchange Commission. For more information, see "WHERE YOU CAN
FIND MORE INFORMATION."
 
<TABLE>
<CAPTION>
                                           AT OR FOR THE YEAR ENDED DECEMBER 31,
                                  --------------------------------------------------------
                                    1998      1997(2)     1996(2)     1995(2)     1994(2)
                                  --------    --------    --------    --------    --------
                                                       (IN THOUSANDS)
<S>                               <C>         <C>         <C>         <C>         <C>
INCOME STATEMENTS:
Total interest income...........  $684,439    $592,483    $574,140    $559,957    $475,760
Total interest expense..........   290,202     258,011     252,795     265,297     179,688
                                  --------    --------    --------    --------    --------
Net interest income.............   394,237     334,472     321,345     294,660     296,072
Provision for credit losses.....    28,555      17,211      23,627      38,107      22,922
Total noninterest income........   119,581      98,513      87,455      82,106      75,512
Total noninterest expense.......   353,807     292,210     269,339     216,521     237,161
                                  --------    --------    --------    --------    --------
Income before income taxes......   131,456     123,564     115,834     122,138     111,501
Provision for income taxes......    54,850      39,303      35,538      45,133      38,990
                                  --------    --------    --------    --------    --------
Net income......................  $ 76,606(2) $ 84,261    $ 80,296    $ 77,005    $ 72,511
                                  ========    ========    ========    ========    ========
COMMON STOCK DATA:
Per share:
  Basic:
     Earnings...................  $   2.16    $   2.66    $   2.56    $   2.43    $   2.25
     Cash earnings(3),(4).......      3.08        2.86        2.74        2.56        2.37
  Diluted:
     Earnings...................      2.15        2.64        2.55        2.43        2.25
     Cash earnings(3),(4).......      3.06        2.83        2.73        2.56        2.37
  Cash dividends................      1.24        1.24        1.20        1.18        1.18
  Book value (at December 31)...     29.07       23.34       22.22       20.86       19.61
  Market price (at December
     31)........................     48.00       39.75       35.00       30.00       23.75
Average shares outstanding
  (in thousands)................    35,534      31,726      31,399      31,735      32,259
BALANCE SHEETS:                                        (IN MILLIONS)
Average balances:
  Total assets..................  $  9,199    $  7,918    $  7,755    $  7,528    $  7,200
  Total earning assets..........     8,289       7,128       7,071       6,876       6,558
  Loans and leases..............     7,105       5,980       5,510       5,461       5,172
  Deposits......................     6,967       5,903       5,618       5,178       5,082
  Stockholders' equity..........       865         726         676         640         618
At December 31:
  Total assets..................  $ 15,050    $  8,093    $  8,002    $  7,565    $  7,535
  Loans and leases..............    11,340       6,239       5,807       5,260       5,534
  Deposits......................    11,260       6,089       5,937       5,358       5,152
  Long-term debt and capital
     securities.................       730         319         206         239         219
  Stockholders' equity..........     1,668         732         706         650         628
</TABLE>
 
                                        7
<PAGE>   13
 
<TABLE>
<CAPTION>
                                           AT OR FOR THE YEAR ENDED DECEMBER 31,
                                  --------------------------------------------------------
                                    1998      1997(2)     1996(2)     1995(2)     1994(2)
                                  --------    --------    --------    --------    --------
                                                       (IN THOUSANDS)
<S>                               <C>         <C>         <C>         <C>         <C>
SELECTED RATIOS:
Return on average:
  Total assets..................      0.83%       1.06%       1.04%       1.02%       1.01%
  Total stockholders' equity....      8.86       11.61       11.88       12.03       11.73
Dividend payout ratio...........     57.41       46.62       46.68       48.56       52.44
Average stockholders' equity to
  average total assets..........      9.40        9.17        8.72        8.50        8.58
Net interest margin.............      4.76        4.70        4.57        4.36        4.63
At December 31:
  Risk-based capital ratios:
     Tier 1.....................      8.17        9.51        8.42        9.03        9.31
     Total......................     10.06       11.81       11.85       11.88       12.06
  Tier 1 leverage ratio.........      9.16        9.14        7.32        7.72        7.51
  Allowance for credit losses to
     total loans and leases.....      1.32        1.32        1.47        1.50        1.11
  Nonperforming assets to total
     loans and leases and other
     real estate owned..........      1.08        1.38        1.68        1.75        1.14
  Allowance for credit losses to
     nonperforming loans
     and leases.................      1.67x       1.49x       1.18x        .95x       1.04x
</TABLE>
 
- -------------------------
 
(1) On November 1, 1998, the former BancWest Corporation ("Old BancWest") merged
    with and into First Hawaiian, Inc., which changed its name to "BancWest
    Corporation." BancWest used the purchase method of accounting for the
    merger. As a result, the financial information presented in this table at
    and for the year ended December 31, 1998, includes the two months of
    combined operations of First Hawaiian, Inc. and Old BancWest from November
    1, 1998. The increase in substantially all categories of BancWest's
    consolidated financial data between amounts reported at December 31, 1998
    and those reported in previous years resulted from the merger of First
    Hawaiian, Inc. and Old BancWest. In accordance with purchase accounting,
    financial information at and for the years ended before 1998 is historical
    information of First Hawaiian, Inc. and is not restated to reflect the
    merger with Old BancWest.
 
(2) Net income before restructuring, merger related and other nonrecurring costs
    of $21.9 million in connection with the merger of Old BancWest Corporation
    with and into First Hawaiian, Inc. on November 1, 1998 was $98.5 million.
 
(3) Cash earnings per share (which is unaudited) is defined as earnings per
    share in accordance with generally accepted accounting principles before the
    after-tax amortization of goodwill and core deposit intangible.
 
(4) Excluding after-tax restructuring, merger-related and other nonrecurring
    costs of $21.9 million in 1998.
                                        8
<PAGE>   14
 
                SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA
                             OF SIERRAWEST BANCORP
 
     This financial information for SierraWest for the fiscal years 1994 to 1998
is only a summary. You should read it with the audited consolidated financial
statements and the accompanying notes of SierraWest, which SierraWest has filed
with the Securities and Exchange Commission. For more information, see "WHERE
YOU CAN FIND MORE INFORMATION."
 
<TABLE>
<CAPTION>
                                                 AT OR FOR THE YEAR ENDED DECEMBER 31,
                                          ----------------------------------------------------
                                            1998     1997(2)    1996(2)    1995(2)    1994(2)
                                          --------   --------   --------   --------   --------
                                                             (IN THOUSANDS)
<S>                                       <C>        <C>        <C>        <C>        <C>
STATEMENTS OF OPERATIONS DATA
  Total interest income.................  $ 65,102   $ 58,565   $ 46,371   $ 38,141   $ 30,610
  Total interest expense................    25,620     23,221     17,960     13,972      9,693
                                          --------   --------   --------   --------   --------
  Net interest income...................    39,482     35,344     28,411     24,169     20,917
  Provision for possible loan and lease
     losses.............................     2,370      2,799      1,421      1,594      1,141
                                          --------   --------   --------   --------   --------
  Net interest income after provision
     for possible loan and lease
     losses.............................    37,112     32,545     26,990     22,575     19,776
  Total non-interest income.............    14,601     13,686      9,370     10,147     10,839
  Total non-interest expense............    38,268     31,610     28,478     27,574     23,964
  Provision for income taxes............     5,767      5,673      2,995      1,827      2,427
                                          --------   --------   --------   --------   --------
  Net income............................  $  7,678   $  8,948   $  4,887   $  3,321   $  4,224
                                          ========   ========   ========   ========   ========
STATEMENTS OF
  FINANCIAL CONDITION DATA
  Total assets..........................  $879,169   $786,746   $639,718   $497,601   $416,707
  Loans and leases, net.................   616,274    545,822    430,745    345,358    278,541
  Allowance for possible loan and lease
     losses.............................     8,709      7,891      5,647      5,003      4,654
  Total deposits........................   782,552    701,001    569,994    435,388    359,603
  Convertible debentures................         0      2,468     12,210     14,025     14,025
  Notes payable.........................     2,650      2,650      2,650          0          0
  Shareholders' equity..................    78,270     69,383     47,285     42,095     38,889
PER SHARE DATA(1)
  Book value............................  $  14.76   $  13.82   $  12.67   $  11.95   $  10.98
  Net income:
     Basic..............................      1.49       1.96       1.35       0.94       1.20
     Diluted............................      1.41       1.73       1.10       0.81       1.00
  Cash dividends declared...............      0.40       0.40       0.38       0.31       0.14
  Shares used to compute net income per
     share:
     Basic..............................     5,151      4,566      3,622      3,525      3,518
     Diluted............................     5,474      5,280      5,014      4,951      4,859
</TABLE>
 
                                        9
<PAGE>   15
 
<TABLE>
<CAPTION>
                                                 AT OR FOR THE YEAR ENDED DECEMBER 31,
                                          ----------------------------------------------------
                                            1998     1997(2)    1996(2)    1995(2)    1994(2)
                                          --------   --------   --------   --------   --------
                                                             (IN THOUSANDS)
<S>                                       <C>        <C>        <C>        <C>        <C>
  Dividend payout ratio:
     Basic..............................      26.7%      20.2%      28.0%      33.2%      11.3%
     Diluted............................      28.2       23.0       34.3       38.5       13.5
SELECTED RATIOS
  Return on average assets..............       0.9%       1.2%       0.9%       0.7%       1.0%
  Return on average shareholders'
     equity.............................      10.5       14.9       11.0        8.2       11.3
  Net interest margin...................       5.3        5.5        5.8        6.3        6.0
  Average shareholders' equity to
     average assets.....................       8.8        8.4        8.1        9.2        9.1
ASSET QUALITY RATIOS
  Allowance for possible loan and lease
     losses to total loans and leases...       1.4%       1.4%       1.3%       1.4%       1.6%
  Allowance for possible loan and lease
     losses to nonaccrual loans.........     100.5      113.6      103.9       76.7      124.4
  Net charge-offs to average loans
     outstanding........................       0.3        0.3        0.2        0.4        0.4
  Nonaccrual and restructured performing
     loans to total loans and leases....       1.7        1.6        1.5        2.0        1.4
  Nonperforming assets to total
     assets.............................       1.1%       1.1%       0.9%       1.5%       1.3%
</TABLE>
 
- -------------------------
 
(1) All per share data has been adjusted to reflect stock dividend and stock
    splits and has been restated under the guidelines of SFAS 128. Book value
    per share is calculated as total shareholders' equity divided by the number
    of shares outstanding at the end of the period.
 
(2) Restated on a historical basis to reflect the acquisition of California
    Community Bancshares Corporation on April 15, 1998, under the pooling of
    interests method of accounting.
                                       10
<PAGE>   16
 
                       COMPARATIVE PER COMMON SHARE DATA
 
     We have summarized below the historical per share information for BancWest
and SierraWest and additional information as if the companies had been combined
for the periods shown ("pro forma") calculated based on an exchange ratio of
0.82 of a share of BancWest common stock per share of SierraWest common stock.
 
     You should read this information with our historical financial statements
and related notes contained in the annual reports and other information that we
have filed with the Securities and Exchange Commission. See "WHERE YOU CAN FIND
MORE INFORMATION" on page 91.
 
     SierraWest equivalent pro forma share amounts are calculated by multiplying
the pro forma book value per share, net income per share and BancWest's
historical per share dividends by the exchange ratio (0.82) so that the per
share amounts equate to the respective values for one share of SierraWest common
stock. You should not rely on the pro forma information as being indicative of
the historical results that we would have had or the future results that will
occur after the merger.
 
<TABLE>
<CAPTION>
                                                               YEAR ENDED DECEMBER 31,
                                                              --------------------------
                                                               1998      1997      1996
                                                              ------    ------    ------
<S>                                                           <C>       <C>       <C>
BANCWEST
Net income:
  Basic:
     Historical.............................................  $ 2.16(1) $ 2.66    $ 2.56
     Pro forma..............................................    2.12(1)   2.63      2.48
  Diluted:
     Historical.............................................    2.15(1)   2.64      2.55
     Pro forma..............................................    2.10(1)   2.58      2.41
Dividends:
  Historical................................................    1.24      1.24      1.20
Book value:
  Historical................................................   29.07
  Pro forma.................................................   28.29
SIERRAWEST
Net income:
  Basic:
     Historical.............................................  $ 1.49    $ 1.96    $ 1.35
     Equivalent pro forma(2)................................    1.74      2.16      2.03
  Diluted:
     Historical.............................................    1.41      1.73      1.10
     Equivalent pro forma(2)................................    1.72      2.11      1.98
  Dividends:
     Historical.............................................    0.40      0.40      0.38
     Equivalent pro forma(3)................................    1.02      1.02      0.98
  Book value:
     Historical.............................................   14.76
     Equivalent pro forma(2)................................   23.20
</TABLE>
 
- -------------------------
 
(1) Includes after tax restructuring, merger related and other nonrecurring
    costs of $21.9 million in connection with the merger of Old BancWest with
    and into First Hawaiian, Inc. (now known as BancWest Corporation) on
    November 1, 1998.
 
(2) Pro forma amounts for BancWest multiplied by 0.82 (the exchange ratio).
 
(3) Historical amounts for BancWest multiplied by 0.82 (the exchange ratio).
                                       11
<PAGE>   17
 
                INFORMATION REGARDING FORWARD LOOKING STATEMENTS
 
     This proxy statement-prospectus contains forward looking statements
regarding each of SierraWest and BancWest and the combined company following the
merger, including statements relating to:
 
     - the financial condition, results of operations and business of BancWest
       following completion of the merger,
 
     - cost savings, enhanced revenues and accretion to reported earnings that
       are expected to be realized from the merger, and
 
     - the restructuring charges expected to be incurred in connection with the
       merger.
 
These forward-looking statements involve certain risks and uncertainties.
Factors that may cause actual results to differ materially from those
contemplated by such forward looking statements include, among others, the
following possibilities:
 
     - expected cost savings from the merger cannot be fully realized or
       realized within the expected time frame,
 
     - revenues following the merger are lower than expected or deposit
       withdrawals, operating costs or customer loss and business disruption
       following the merger may be greater than expected,
 
     - competitive pressures among depository and other financial services
       companies increase significantly,
 
     - costs or difficulties related to the integration of the businesses of
       BancWest and SierraWest are greater than expected,
 
     - changes in the interest rate environment reduce interest margins, cause
       an increase in the prepayment rate on mortgages and other loans or reduce
       the demand for new loans,
 
     - general economic or business conditions, either internationally or
       nationally or in the states in which the combined company will be doing
       business, are less favorable than expected, resulting in, among other
       things, a deterioration in credit quality or a reduced demand for credit,
 
     - legislation or regulatory requirements or changes adversely affect the
       businesses in which the combined company would be engaged,
 
     - technology-related changes, including "Year 2000" data systems compliance
       issues, may be harder to make or more expensive than expected, and
 
     - changes in the securities markets.
 
                                       12
<PAGE>   18
 
                   ANNUAL MEETING OF SIERRAWEST SHAREHOLDERS
 
DATE, TIME AND PLACE
 
     The annual meeting of the shareholders of SierraWest will be held at the
North Tahoe Conference Center, 8318 North Lake Boulevard, Kings Beach,
California on Thursday, May 27, 1999, at 4:00 p.m., local time.
 
THE ANNUAL MEETING
 
     At the annual meeting, holders of shares of common stock, no par value, of
SierraWest will consider and vote upon the approval of the Agreement and Plan of
Merger, dated as of February 25, 1999, as amended, among BancWest Corporation,
its wholly-owned subsidiary, Bank of the West, and SierraWest Bancorp. The
merger agreement provides for, among other things,
 
     - the merger of SierraWest with and into Bank of the West, and
 
     - the conversion of all the outstanding shares of SierraWest's common stock
       into shares of common stock, par value $1.00 per share, of BancWest.
 
For more information see "THE MERGER" and "THE MERGER AGREEMENT."
 
     THE BOARD OF DIRECTORS OF SIERRAWEST HAS, BY UNANIMOUS VOTE, APPROVED THE
MERGER AGREEMENT AND RECOMMENDS A VOTE FOR APPROVAL OF THE MERGER AGREEMENT.
 
     In addition to the proposal to approve the merger agreement, at the annual
meeting you will be asked to:
 
     - elect 13 directors to serve until completion of the merger or until the
       next annual meeting of shareholders, and
 
     - transact any other business properly brought at the annual meeting.
 
     THE BOARD OF DIRECTORS OF SIERRAWEST ALSO RECOMMENDS A VOTE FOR EACH OF THE
PROPOSED NOMINEES FOR ELECTION TO THE BOARD OF DIRECTORS.
 
RECORD DATE; QUORUM
 
     Only holders of record of SierraWest common stock at the close of business
on April 19, 1999 are entitled to notice of and will be entitled to vote at the
annual meeting. As of the record date, there were           shares of SierraWest
common stock issued and outstanding and entitled to vote at the annual meeting.
The shares were held by approximately           holders of record.
 
     The required quorum for the transaction of business at the annual meeting
is a majority of the outstanding shares of SierraWest's common stock issued and
outstanding on the record date, which shares may be present in person or
represented by proxy.
 
VOTE REQUIRED
 
     The approval of the merger agreement will require the affirmative vote of
the holders of record of a majority of the outstanding shares of SierraWest
common stock on the record date.
 
     In the election of directors, the 13 nominees receiving the most votes will
be elected.
 
                                       13
<PAGE>   19
 
     Holders of record of SierraWest common stock on the record date are each
entitled to one vote per share on the merger agreement. Shareholders are
entitled to cumulate votes for the election of directors.
 
     As of the record date, directors and executive officers of SierraWest
beneficially owned or were entitled to vote                shares of common
stock, representing approximately      % of the outstanding shares of common
stock on the record date. Each director has indicated his or her present
intention to vote the common stock so owned by him or her (1) for the approval
of the merger agreement and (2) for each of the nominees for election to the
board of directors. In addition, as of the record date, SierraWest Bank, as
fiduciary, custodian or agent, held a total of                shares of common
stock, representing approximately      % of the outstanding shares of common
stock on the record date.
 
VOTING OF PROXIES
 
     The proxy accompanying this proxy statement-prospectus is solicited on
behalf of the board of directors of SierraWest for use at the annual meeting.
Please complete, date and sign the accompanying proxy and promptly return it in
the accompanying envelope or otherwise mail it to SierraWest. All proxies that
are properly executed and returned, and that are not revoked, will be voted at
the annual meeting in accordance with the instructions indicated on the proxies.
IF YOU SIGN AND SEND IN YOUR PROXY AND DO NOT INDICATE HOW YOU WANT TO VOTE,
YOUR PROXY WILL BE COUNTED AS A VOTE FOR EACH OF THE PROPOSALS.
 
     For voting purposes at the annual meeting, only shares affirmatively voted
in favor of approval of the merger agreement will be counted as favorable votes
for approval. IF YOU FAIL TO EITHER RETURN YOUR PROXY CARD OR VOTE IN PERSON OR
CHOOSE TO ABSTAIN FROM VOTING, THE EFFECT WILL BE THE SAME AS A VOTE AGAINST THE
MERGER.
 
     If your shares are held in "street name" by your broker and you fail to
instruct your broker regarding how to vote your shares, the effect will be the
same as a vote against the merger.
 
     The management of SierraWest is not aware of any other matters to be voted
on at the annual meeting. If other matters should properly come before the
annual meeting, the proxy holders will vote on such matters in accordance with
their judgment.
 
     The persons named as proxies by a shareholder may propose and vote for one
or more adjournments of the annual meeting to permit further solicitations of
proxies in favor of any proposal. However, no proxy which is voted against the
merger will be voted in favor of any such adjournment.
 
REVOCABILITY OF PROXIES
 
     You may revoke your proxy at any time before it is voted by:
 
     - filing with the Secretary of SierraWest a duly executed revocation of
       proxy,
 
     - submitting a duly executed proxy bearing a later date, or
 
     - attending the annual meeting and voting in person.
 
Simply attending the annual meeting, however, will not revoke your proxy.
 
                                       14
<PAGE>   20
 
SOLICITATION OF PROXIES
 
     All expenses of our solicitation of proxies, including the cost of mailing
this proxy statement-prospectus to you, will be paid by us. In addition to
solicitation by use of the mails, proxies may be solicited from shareholders by
our directors, officers and employees in person or by telephone, telegram or
other means of communication. Such directors, officers and employees will not
receive additional compensation, but we may reimburse them for their reasonable
out-of-pocket expenses in connection with such solicitation. We have retained
Morrow & Co., Inc., a proxy solicitation firm, for assistance in connection with
the solicitation of proxies for the annual meeting at a cost of approximately
$10,000 plus reimbursement of reasonable out-of-pocket expenses. Arrangements
will also be made with brokerage houses, custodians, nominees and fiduciaries
for the forwarding of proxy solicitation materials to beneficial owners of
shares held of record by such brokerage houses, custodians, nominees and
fiduciaries, and we will reimburse such brokerage houses, custodians, nominees
and fiduciaries for their reasonable expenses incurred in connection with such
solicitation.
 
                                       15
<PAGE>   21
 
                                  PROPOSAL ONE
 
                                   THE MERGER
 
GENERAL
 
     The board of directors of SierraWest has approved the merger agreement,
which provides for the merger of SierraWest with and into Bank of the West. Upon
completion of the merger, the separate corporate existence of SierraWest will
end. This section of the proxy statement-prospectus describes certain aspects of
the merger, including the background of the merger and SierraWest's reasons for
the merger.
 
     In the merger, each outstanding share of SierraWest common stock will be
converted into the right to receive 0.82 of a share of BancWest common stock.
Based on the number of shares of SierraWest common stock outstanding as of April
9, 1999, BancWest will issue approximately 4,373,335 shares of BancWest common
stock in the merger, representing approximately 13.9% of the number of shares of
BancWest common stock and 7.6% of the number of shares of BancWest common stock
and Class A Common Stock that will be outstanding after the merger.
 
     The merger agreement also provides that before the merger, BancWest may
specify that, for any reasonable business, tax or regulatory purpose, the
parties to the merger agreement and SierraWest Bank enter into transactions that
are different from the merger described in this section. No such change,
however, may materially and adversely affect the timing of the completion of the
merger or adversely affect the economic benefits, the form of consideration or
the tax effect of the merger to you.
 
BACKGROUND OF THE MERGER
 
     The board of directors of SierraWest has consistently applied a policy of
identifying and reviewing any serious expression of interest in a business
combination. To that end, in late 1996 the board created a Merger and
Acquisition Committee and a Strategic Planning Committee that regularly met to
evaluate the strategic and shareholder value implications for SierraWest when
possible business combinations were considered. Those committees directed
management of SierraWest to communicate with parties who expressed an interest
in SierraWest. Such meetings took place from time to time and were reported to
the board committees and full board, as appropriate.
 
     In July 1997, SierraWest commenced negotiations with California Community
Bancshares Corporation for the acquisition and merger of California Community
Bancshares and its subsidiary bank, Continental Pacific Bank. As those
negotiations were proceeding, SierraWest was contacted by the president of Bank
A, a large regional bank, to express interest in a possible acquisition of
SierraWest. Management representatives of SierraWest held informal discussions
with representatives of Bank A and in mid-October 1997, executed a
confidentiality agreement pursuant to which SierraWest provided non-public
information to Bank A.
 
     In early November 1997, Bank A expressed its interest in pursuing a
possible transaction with SierraWest. At a special meeting, the SierraWest board
considered the implications of the expression of interest in light of the fact
that the transaction with California Community Bancshares was in its final stage
of documentation and about to be announced. SierraWest's advisers addressed the
legal and financial considerations. SierraWest then discussed these issues with
Bank A. Bank A responded that it would not accommodate the combination of
SierraWest and California Community Bancshares as part of its proposed
transaction. The SierraWest board in another special meeting voted
 
                                       16
<PAGE>   22
 
unanimously to proceed with the California Community Bancshares transaction and
announced the transaction three days later. Subsequently in May 1998, Bank A had
further conversations with SierraWest's management but those conversations
terminated when Bank A entered into a larger merger which precluded the
acquisition of SierraWest.
 
     In February 1998, the chief executive officer of Bank B, a regional banking
company, contacted the management of SierraWest to seek a meeting. A meeting
took place but further discussions were deferred until the completion of the
California Community Bancshares transaction in April 1998. In June 1998, after
additional discussions, Bank B signed a confidentiality agreement under which
SierraWest provided non-public information to Bank B. After receiving the
confidential information, Bank B did not communicate further with SierraWest for
an extended period of time.
 
     In August 1998, the SierraWest board met with NationsBanc Montgomery
Securities and SierraWest's legal advisers to review the strategic alternatives
available to SierraWest and to consider commencing an active process of
evaluating these strategic alternatives. The board subsequently voted to proceed
with a process by authorizing the preparation of confidential materials for use
in approaching possible acquirers. However, immediately after that decision, a
significant stock market decline, which disproportionately affected bank, and
particularly community bank, stocks caused the board to slow down the process
until the marketplace stabilized.
 
     On October 19, 1998, SierraWest formally retained NationsBanc Montgomery as
its investment bank to advise it on strategic alternatives and to establish a
process by which a possible business combination could be best pursued. In this
role, NationsBanc Montgomery provided SierraWest with financial summaries and
public information regarding Bank B and other banks that had previously
indicated an interest in merger discussions as well as a broad range of
institutions that might be potential partners. The board decided that if a sale
of SierraWest were to take place, it would be in the best interest of the
shareholders, employees and customers of SierraWest if the potential acquirers
approached were limited to a select group of highly qualified banks selected
based upon ability to pay, potential interest in SierraWest's market, size,
stock price and liquidity, and future prospects. Consequently, 11 potential
buyers, including Bank A and Bank B, were approached. Seven of the 11 potential
buyers contacted returned confidentiality agreements, signaling interest in
reviewing the confidential memorandum and potentially proceeding further with
possible merger discussions.
 
     In early November 1998, five of the parties indicated their interest in
proceeding with a possible transaction by expressing ranges of pricing and
preliminary deal structures. On November 19, 1998, the board reviewed the
expressions of interest and decided that one proposal was below an acceptable
level to be considered. On November 30 and December 1, 1998, the board met
individually with representatives of each of the four remaining potential buyers
to determine if proceeding with such parties was in the best interests of the
shareholders of SierraWest. As a result of those meetings, the board eliminated
one prospective party because the board did not believe that SierraWest fit well
with the party's strategic focus and growth pattern and chose to proceed with
three parties, including BancWest, through the due diligence process.
 
     One of the remaining parties informed NationsBanc Montgomery that it would
not proceed with due diligence unless it was permitted to do so on an exclusive
basis. The Merger and Acquisition Committee of the SierraWest board met on
December 17, 1998 to consider this request. The committee determined that it was
appropriate and in the best interests of the shareholders to proceed with the
process which it had initiated and
 
                                       17
<PAGE>   23
 
informed that specific party that without the completion of its due diligence,
that party's expression of interest could not be considered a binding proposal
and that SierraWest would proceed with its process. The party declined to
participate further in the process.
 
     During December 1998, BancWest and the other remaining party conducted due
diligence on SierraWest. BancWest and the other bidder submitted final offers on
January 6, 1999. On January 8, 1999, the Merger and Acquisition Committee met
with its legal and financial advisors to discuss the two bids. The committee
also directed SierraWest representatives to meet with BancWest representatives
to review various issues and to determine if the pricing structure could be
enhanced. The committee also decided to seek the advice of a respected economist
to review the status of the Hawaii economy and the possible effect of the
continuing Asian financial crisis on the performance of BancWest.
 
     On January 19, 1999, the Merger and Acquisition Committee met again to
review the information obtained in response to the committee's directions. The
committee indicated its preference to proceed exclusively with BancWest and to
make such a recommendation to the board.
 
     The BancWest board held a meeting on January 21, 1999, at which BancWest's
management described the results of its due diligence and the terms of the
expression of interest that had been communicated to SierraWest.
 
     On January 26, 1999, the Merger and Acquisition Committee again met to
review the report of the economist and voted unanimously to recommend the
BancWest proposal to the full board. On January 27, 1999, the board voted to
pursue negotiation of a definitive agreement on an exclusive basis with
BancWest. An exclusivity agreement was thereafter executed with BancWest.
 
     Between January 27, 1999, and February 25, 1999, the managements of
SierraWest and BancWest, together with their respective outside counsels,
negotiated the terms of the merger agreement and the stock option agreement.
 
     The BancWest board held another meeting on February 18, 1999, at which
BancWest's directors discussed the proposed merger in detail with BancWest's
management, including the consideration to be paid by BancWest and the related
transactions. At the meeting, the BancWest board approved the merger agreement
and the related agreements contemplated thereby and authorized and directed
BancWest management to take all action reasonably necessary to effect the
merger.
 
     On February 25, 1999, at a meeting of the SierraWest board, NationsBanc
Montgomery delivered its oral opinion that the consideration to be received was
fair to the SierraWest shareholders, as of that date. NationsBanc Montgomery's
oral opinion was subsequently confirmed in writing as of such date. At that
meeting, the SierraWest board approved the merger agreement and the transactions
contemplated by the merger agreement as in the best interests of SierraWest and
its shareholders. The SierraWest board also authorized the SierraWest management
to finalize the terms of the merger agreement and the stock option agreement.
 
     Following the February 25, 1999 SierraWest board meeting and after the
close of the market, the parties signed the merger agreement and the stock
option agreement and issued a joint press release publicly announcing the
merger.
 
     On March 24, 1999, the parties entered into an amendment to the merger
agreement to effect certain technical amendments.
 
                                       18
<PAGE>   24
 
REASONS FOR THE MERGER; RECOMMENDATION OF THE BOARD OF DIRECTORS
 
     In evaluating the terms of the merger, the board of directors of SierraWest
considered a number of different factors, including the following material
factors:
 
     HISTORICAL AND RECENT MARKET PRICES OF SIERRAWEST SHARES COMPARED TO
BANCWEST SHARES.  The board of directors reviewed the historical and recent
trading prices for SierraWest stock. The board of directors considered as
favorable the fact that, upon completion of the merger, SierraWest shareholders
would receive BancWest shares with a market value as of February 25, 1999 (the
date SierraWest's board approved the merger) of $35.26 per share, representing a
premium of 22% over the closing sales price for SierraWest shares of $28.88, on
such date. The board of directors also considered that after the trading price
of SierraWest's stock price fell in August 1998, it did not recover to the same
extent as many of the bank stocks in its peer group did in the last quarter of
1998 and the beginning of 1999. The board of directors also compared BancWest's
stock price to its book value ratio to those of other potential acquirers. The
board of directors found to be favorable the fact that BancWest's stock price to
book value ratio was relatively low, suggesting that, in this respect, the
BancWest shares that you will receive in the merger may represent a greater
value than the shares that you would receive in an identically priced stock
merger with a different acquirer whose shares trade at higher multiples of book
value.
 
     SIERRAWEST'S BUSINESS, CONDITIONS AND PROSPECTS.  The board of directors
considered information with respect to the financial condition, results of
operations and business risks of SierraWest on both a historical and prospective
basis and current industry, economic and market conditions. Among other things,
the board considered the negative impact that continuing lower interest rates
could have on SierraWest's Small Business Administration loan servicing assets.
Lower interest rates lead borrowers to refinance their loans. SierraWest's loan
servicing assets must be amortized as a charge against income over the expected
future life of the underlying loans. The increase in prepayments increases the
amortization charge which adversely affects SierraWest's results of operations.
 
     BANCWEST'S BUSINESS, CONDITIONS AND PROSPECTS.  The board of directors
considered information with respect to the financial condition, financial
performance, business operations, capital levels, assets quality, loan portfolio
breakdown and prospects of BancWest on both a historical and prospective basis.
The board of directors also considered information regarding current industry,
economic and market conditions in the financial services industry. SierraWest's
financial advisors, NationsBanc Montgomery, made presentations to and provided
the board of directors with information regarding BancWest's financial condition
and prospects after conducting business and financial due diligence. Officers of
SierraWest also made presentations to the board of directors regarding
BancWest's financial condition, business and prospects, all but one of four
presentations were made prior to conducting due diligence. In evaluating
BancWest's prospects, the board of directors considered, among other things,
BancWest's financial performance, the geographic areas in which BancWest
conducts business compared to those in which SierraWest conducts business, the
state of the economies in Northern California and Hawaii, the primary areas in
which BancWest conducts its business, and the effect those economies may have on
BancWest's performance. The board also considered the report of an economist
hired by the board to review the status of the Hawaiian economy and the possible
effect of the continuing Asian financial crisis on the performance of BancWest.
In addition, the board of directors considered and found favorable the fact that
BancWest's stock has greater market capitalization and liquidity compared to
SierraWest.
 
                                       19
<PAGE>   25
 
     OPINION OF NATIONSBANC MONTGOMERY.  At its February 25, 1999 meeting, the
board of directors considered NationsBanc Montgomery's oral opinion delivered to
the board of directors at that meeting (which NationsBanc Montgomery
subsequently confirmed in a written opinion of that same date) that the
consideration to be received by SierraWest shareholders in the merger was, as of
that date, fair to SierraWest's shareholders, from a financial point of view.
 
     TERMS OF THE MERGER.  The board of directors considered the number and
value of shares of BancWest stock to be issued in exchange for each outstanding
share of SierraWest stock and found such consideration was the most favorable of
the final offers provided to SierraWest. In addition, the board considered the
terms, conditions, covenants and representations contained in the merger
agreement and the stock option agreement. The board also considered and found
favorable the fixed exchange ratio and absence of any collar, which would allow
you to benefit from any increase in the price of BancWest common stock, in view
of its relatively low stock price to book value ratio.
 
     THE TAX-FREE NATURE OF THE MERGER.  The board of directors considered and
found favorable the fact that the merger is structured to be tax free for
federal income tax purposes. You will recognize no gain for federal income tax
purposes in connection with the exchange of SierraWest stock for BancWest stock
(except with respect to cash received for fractional shares of BancWest stock).
 
     IMPACT ON DEPOSITORS, CUSTOMERS AND EMPLOYEES.  The board of directors
considered the impact of the merger upon SierraWest's depositors, customers,
employees, the overall compatibility of SierraWest's office and branch structure
compared to those of BancWest's.
 
     The board of directors did not assign any relative or specific value to any
of the factors.
 
     THE BOARD OF DIRECTORS OF SIERRAWEST HAS, BY UNANIMOUS VOTE, APPROVED THE
MERGER AGREEMENT AND RECOMMENDS A VOTE FOR APPROVAL OF THE MERGER AGREEMENT.
 
OPINION OF FINANCIAL ADVISOR
 
     GENERAL.  SierraWest engaged NationsBanc Montgomery on October 19, 1998, to
identify opportunities for maximizing shareholder value, including, among other
things, a sale of SierraWest. As part of its engagement, NationsBanc Montgomery
agreed to render to SierraWest's board an opinion with respect to the fairness
from a financial point of view of the consideration to be received by you in a
potential sale of SierraWest. NationsBanc Montgomery is a nationally recognized
investment banking firm and, as part of its investment banking activities, is
regularly engaged in the valuation of businesses and their securities in
connection with merger transactions and other types of acquisitions, negotiated
underwritings, private placements and valuations for corporate and other
purposes. SierraWest selected NationsBanc Montgomery to render the opinion on
the basis of its experience and expertise and its reputation in the banking and
investment communities.
 
     At a meeting of SierraWest's board on February 25, 1999, NationsBanc
Montgomery delivered its oral opinion that the consideration to be received by
the holders of SierraWest's common stock in the merger was fair to such
shareholders from a financial point of view, as of the date of such opinion.
NationsBanc Montgomery's oral opinion was subsequently confirmed in writing as
of that date.
 
     THE FULL TEXT OF NATIONSBANC MONTGOMERY'S WRITTEN OPINION TO SIERRAWEST'S
BOARD, DATED FEBRUARY 25, 1999, WHICH SETS FORTH THE ASSUMPTIONS MADE, MATTERS
CONSIDERED, AND LIMITATIONS OF THE REVIEW BY NATIONSBANC MONTGOMERY, IS ATTACHED
AS APPENDIX C TO THIS
                                       20
<PAGE>   26
 
PROXY STATEMENT-PROSPECTUS. YOU SHOULD READ THE NATIONSBANC MONTGOMERY OPINION
CAREFULLY AND IN ITS ENTIRETY. THE SUMMARY OF NATIONSBANC MONTGOMERY'S OPINION
INCLUDED IN THIS DOCUMENT IS QUALIFIED IN ITS ENTIRETY BY REFERENCE TO THE FULL
TEXT OF NATIONSBANC MONTGOMERY'S OPINION. IN FURNISHING SUCH OPINION,
NATIONSBANC MONTGOMERY DOES NOT ADMIT THAT IT IS AN EXPERT WITH RESPECT TO THE
REGISTRATION STATEMENT OF WHICH THIS PROXY STATEMENT-PROSPECTUS IS A PART WITHIN
THE MEANING OF THE TERM "EXPERT" AS USED IN THE SECURITIES ACT OF 1933 AND THE
RULES AND REGULATIONS OF THE SECURITIES AND EXCHANGE COMMISSION, NOR DOES IT
ADMIT THAT ITS OPINION CONSTITUTES A REPORT OR VALUATION WITHIN THE MEANING OF
SECTION 11 OF THE SECURITIES ACT. NATIONSBANC MONTGOMERY'S OPINION IS DIRECTED
TO SIERRAWEST'S BOARD, COVERS ONLY THE FAIRNESS FROM A FINANCIAL POINT OF VIEW
OF THE CONSIDERATION TO BE RECEIVED BY YOU AS OF THE DATE OF THE OPINION AND
DOES NOT CONSTITUTE A RECOMMENDATION TO YOU AS TO HOW YOU SHOULD VOTE AT
SIERRAWEST'S ANNUAL MEETING.
 
     In connection with its February 25, 1999 opinion, NationsBanc Montgomery,
among other things:
 
     - reviewed certain publicly available financial and other data with respect
       to SierraWest and BancWest, including the consolidated financial
       statements for recent years and interim periods to December 31, 1998, and
       certain other relevant financial and operating data relating to
       SierraWest and BancWest made available to NationsBanc Montgomery from
       published sources and from the internal records of SierraWest and
       BancWest,
 
     - reviewed the February 21, 1999 draft of the merger agreement,
 
     - reviewed certain publicly available information concerning the trading
       of, and market for, SierraWest common stock and BancWest common stock,
 
     - compared certain financial data of SierraWest and BancWest with those of
       certain other companies in the banking industry which NationsBanc
       Montgomery deemed to be relevant,
 
     - considered the financial terms, to the extent publicly available, of
       selected recent business combinations of companies in the banking
       industry which NationsBanc Montgomery deemed to be comparable, in whole
       or in part, to the merger,
 
     - conducted discussions with representatives of the senior management of
       SierraWest and BancWest concerning their respective businesses and
       prospects,
 
     - reviewed certain information, including financial forecasts and related
       assumptions, furnished to NationsBanc Montgomery by SierraWest and
       BancWest, respectively, and
 
     - performed such other analyses and examinations as NationsBanc Montgomery
       deemed appropriate.
 
     In connection with NationsBanc Montgomery's review, NationsBanc Montgomery
did not assume any obligation independently to verify the information listed
above and relied on its accuracy and completeness in all material respects. With
respect to the financial forecasts for SierraWest and BancWest provided to
NationsBanc Montgomery by their respective managements, upon their advice and
with SierraWest's consent, NationsBanc Montgomery assumed for purposes of its
opinion that the forecasts were reasonably prepared on bases reflecting the best
available estimates and judgments of their respective managements at the time of
preparation as to the future financial performance of SierraWest and BancWest
and that they provided a reasonable basis upon which
                                       21
<PAGE>   27
 
NationsBanc Montgomery could form its opinion. NationsBanc Montgomery assumed
that there were no material changes in SierraWest's or BancWest's assets,
financial condition, results of operations, business or prospects since the
respective dates of their last financial statements made available to
NationsBanc Montgomery. NationsBanc Montgomery relied on advice of counsel to
SierraWest as to all legal matters with respect to SierraWest, the merger and
the merger agreement. NationsBanc Montgomery assumed that the merger will be
completed in a manner that complies in all respects with the applicable
provisions of the Securities Act of 1933, the Securities Exchange Act of 1934
and all other applicable federal and state statutes, rules and regulations. In
addition, NationsBanc Montgomery did not assume responsibility for reviewing any
individual credit files, or making an independent evaluation, appraisal or
physical inspection of any of the assets or liabilities (contingent or
otherwise) of SierraWest or BancWest, nor was NationsBanc Montgomery furnished
with any such appraisals. NationsBanc Montgomery is not an expert in the
evaluation of loan portfolios for purposes of assessing the adequacy of the
allowances for losses with respect thereto and assumed, with SierraWest's
consent, that such allowances for each of SierraWest and BancWest were in the
aggregate adequate to cover such losses. SierraWest informed NationsBanc
Montgomery, and NationsBanc Montgomery assumed, that the merger will be recorded
as a pooling of interests under generally accepted accounting principles.
Finally, NationsBanc Montgomery's opinion was based on economic, monetary and
market and other conditions as in effect on, and the information made available
to NationsBanc Montgomery as of, the date of the opinion.
 
     Set forth below is a brief summary of the information presented by
NationsBanc Montgomery to SierraWest's board on February 25, 1999, in connection
with its opinion.
 
     DISCOUNTED DIVIDEND ANALYSIS.  In performing the discounted dividend
analysis, NationsBanc Montgomery used SierraWest's management estimates of
earnings per share and dividend payments over a four-year period. The estimated
earnings per share in the year 2002 was multiplied by an estimated price to
earnings multiple ranging from 13.0x to 15.0x. This product was then added to
the cumulative estimated dividends for the prior four years and the sum of these
two numbers was discounted to the present using discount rates ranging from
12.5% to 17.5%. This analysis indicated that the present value of SierraWest's
future stock price plus dividends ranged from $23.89 to $32.52 per share. The
$36.59 offer from BancWest, based on a closing price for BancWest common stock
of $44.625 on February 23, 1999 exceeded the range of values implied by this
analysis.
 
     COMPARABLE PUBLIC COMPANY ANALYSIS.  NationsBanc Montgomery analyzed the
trading multiples of a comparison group of 17 publicly traded California banks
(a group of banks published in the NationsBanc Montgomery Western Bank Monitor).
The multiples that were analyzed were price to book value, price to tangible
book value, price to latest twelve months ("LTM") earnings, price to deposits,
and the ratio of the premium (i.e., purchase price in excess of tangible book
value) to core deposits. The median values of these multiples were then
multiplied by SierraWest's current values. This analysis indicated a reference
range for SierraWest's current common stock price from $27.03 to $32.20 per
share. Based on the most common measures of public market valuation, price to
book value, price to tangible book value and price to LTM earnings, the analysis
indicated a reference range for SierraWest's current common stock price from
$27.03 to $27.85 per share with a median value of $27.58, as compared to
BancWest's offer of $36.59 per share.
 
     ANALYSIS OF SELECTED MERGER TRANSACTIONS.  NationsBanc Montgomery reviewed
the consideration paid in selected categories of bank transactions for which the
relevant
 
                                       22
<PAGE>   28
 
information was publicly available. Specifically, NationsBanc Montgomery
reviewed selected bank transactions from January 1, 1997 to February 25, 1999,
involving (1) mergers in California with transaction values greater than $50
million and less than $500 million, (2) national mergers with transaction values
greater than $50 million and less than $500 million, and (3) all national
mergers. The multiples paid to SierraWest in this transaction are in line with
multiples paid in transactions in each of the above categories. For each
transaction, NationsBanc Montgomery analyzed data illustrating, among other
things, purchase price to book value, purchase price to "adjusted" book value
(assuming a premium is paid on a 6% "normalized" equity to assets level and
dollar for dollar for equity above 6%), purchase price to tangible book value,
purchase price to LTM earnings, purchase price to deposits, the ratio of the
premium (i.e., purchase price in excess of tangible book value) to core
deposits, and the premium paid to the seller's closing stock price thirty days
prior to announcement. For purposes of this analysis, NationsBanc Montgomery
specifically focused on bank transactions that have been announced since January
1, 1997.
 
     A summary of the median multiples in the analysis is as follows:
 
<TABLE>
<CAPTION>
                                                    PRICE TO
                              PRICE TO   PRICE TO    TANG.     PRICE TO              PREMIUM       PREMIUM
TRANSACTION                     BOOK     ADJ. BK.     BOOK       LTM      PRICE TO   TO CORE      TO PRICE
CATEGORIES                     VALUE      VALUE      VALUE       EPS      DEPOSITS   DEPOSITS   30 DAYS PRIOR
- -----------                   --------   --------   --------   --------   --------   --------   -------------
<S>                           <C>        <C>        <C>        <C>        <C>        <C>        <C>
1997-1999 Year-to-Date
  Mergers in California with
  Transaction Values Greater
  than $50 Million and Less
  than $500 Million.........    2.4x       4.0x       2.9x      21.2x       32.3%      23.0%        26.9%
1997-1999 Year-to-Date
  National Mergers with
  Transaction Values Greater
  than $50 Million and Less
  than $500 Million.........    2.9x       3.7x       3.0x      22.5x       32.3%      23.4%        31.0%
1997-1999 Year-to-Date All
  National Mergers..........    2.4x       3.1x       2.4x      20.0x       25.9%      17.4%        32.5%
</TABLE>
 
A summary of the results of NationsBanc Montgomery's analysis of the multiples
to be paid in the merger with BancWest is as follows:
 
<TABLE>
<CAPTION>
                        PRICE TO
  PRICE TO   PRICE TO    TANG.     PRICE TO               PREMIUM        PREMIUM
    BOOK     ADJ. BK.     BOOK       LTM      PRICE TO    TO CORE       TO PRICE
   VALUE      VALUE      VALUE       EPS      DEPOSITS    DEPOSITS    30 DAYS PRIOR
  --------   --------   --------   --------   --------   ----------   -------------
  <S>        <C>        <C>        <C>        <C>        <C>          <C>
   2.5x        3.2x       2.5x      19.2x       25.7%       19.5%         42.8%
</TABLE>
 
     CONTRIBUTION ANALYSIS.  NationsBanc Montgomery analyzed the contribution of
each of SierraWest and BancWest to, among other things, assets, loans, deposits,
and tangible equity of the pro forma combined company for the period ending
December 31, 1998, and projected net income and cash net income for the calendar
year ending December 31, 1999. This analysis showed, among other things, that
based on pro forma combined balance sheets for SierraWest and BancWest at
December 31, 1998, SierraWest would have contributed approximately 6% of the
assets, 5% of the loans, 6% of the deposits, and 7% of the tangible equity. The
pro forma projected income statement for the period ending December 31, 1999,
showed that SierraWest would contribute approximately 7% and 5% of the net
income and cash net income, respectively, in 1999. Based on an exchange ratio of
 
                                       23
<PAGE>   29
 
0.82 of a share of BancWest common stock for each share of SierraWest's common
stock, holders of SierraWest's common stock would own approximately 7.3% of the
combined company based on common shares outstanding at December 31, 1998, a
20.7% premium to its median contribution.
 
     PRO FORMA EARNINGS DILUTION ANALYSIS.  Using earnings estimates and
projected growth rates for SierraWest and BancWest provided by their respective
managements, NationsBanc Montgomery compared estimated reported earnings per
share ("Reported EPS") and estimated cash earnings per share ("Cash EPS") of
BancWest's common stock on a stand-alone basis to the estimated Reported EPS and
estimated Cash EPS of the common stock for the pro forma combined company for
the calendar year ending December 31, 1999. NationsBanc Montgomery noted that
the merger would result in dilution of 0.7% and 2.0% to BancWest's Reported EPS
and Cash EPS, respectively, for the year ending December 31, 1999. These
estimates were used for purposes of this analysis only and are not necessarily
indicative of expected results or plans of BancWest, SierraWest, or the combined
institution. Additionally, this analysis did not incorporate any anticipated
cost savings or revenue enhancements.
 
     PRO FORMA BOOK VALUE/TANGIBLE BOOK VALUE DILUTION ANALYSIS.  Using the book
value and the tangible book value for the period ending December 31, 1998, and
the exchange ratio of 0.82 of a share of BancWest common stock for each share of
SierraWest's common stock, NationsBanc Montgomery compared the book value and
the tangible book value of BancWest on a stand-alone basis to the pro forma book
value and tangible book value for the combined company. This analysis showed,
among other things, that the merger would be dilutive to BancWest's book value
by 2.9% and accretive to BancWest's tangible book value by 0.1%.
 
     PICKUP ANALYSIS.  This analysis attempts to illustrate the increase in per
share values to you as if the merger had been completed on January 1, 1999. As
shown below, the analysis shows an increase to you in dividends, book value and
earnings per share. In performing the pickup analysis, NationsBanc Montgomery
applied the exchange ratio of 0.82 of a share of BancWest common stock for each
share of SierraWest's common stock and applied it to the pro forma per share
values for BancWest. The results of this analysis were:
 
<TABLE>
<CAPTION>
                                                                         ESTIMATED
                                            CURRENT      PRO FORMA     INCREASE FROM
                                           SIERRAWEST    SIERRAWEST       CURRENT
                                             VALUE        VALUE(1)         VALUE
                                           ----------    ----------    -------------
<S>                                        <C>           <C>           <C>
Annualized Dividend Per Share............    $ 0.40        $ 1.02          154.2%
Estimated 1999 Reported EPS..............    $ 2.20        $ 2.40            9.2%
Estimated 1999 Cash EPS..................    $ 2.10        $ 2.81           33.8%
Book Value Per Share.....................    $14.76        $23.14           56.7%
Tangible Book Value Per Share............    $14.40        $13.72           (4.7)%
</TABLE>
 
- -------------------------
 
(1) Adjusted for exchange of 0.82 of a share of BancWest common Stock for each
    share of SierraWest common stock.
 
     RELATIVE VALUE ANALYSIS.  In performing the relative value analysis,
NationsBanc Montgomery compared the multiples paid to SierraWest relative to
BancWest's current trading multiples and compared such multiples to multiples
paid to other sellers relative to
 
                                       24
<PAGE>   30
 
their buyers' trading multiples at the time of announcement of other
transactions. For the purposes of this analysis, NationsBanc Montgomery analyzed
other California bank transactions announced since 1997 with transaction values
greater than $10 million. The multiples that were compared included: purchase
price to book value, purchase price to "adjusted" book value (assuming a premium
is paid on a 6% "normalized" equity to assets level and dollar for dollar for
equity above 6%), purchase price to tangible book value, purchase price to LTM
earnings, purchase price to deposits, and the ratio of the premium (i.e.,
purchase price in excess of tangible book value) to core deposits. The multiples
paid to SierraWest relative to BancWest's current trading multiples ranged from
95% to 161%, with a median value of 116%. This compared favorably to multiples
paid to other sellers relative to their buyers in other transactions which
ranged from 78% to 103%, with a median value of 89%.
 
     EARNINGS GROWTH ANALYSIS.  Using a 14.2x terminal multiple (the median
price to 1998 earnings multiple of banks in the NationsBanc Montgomery's Western
Bank Monitor) and a 15% discount rate, NationsBanc Montgomery determined that
SierraWest would have to achieve an average annual earnings growth rate of 24.0%
through the year 2002 in order to obtain a present value stock price of $36.59.
SierraWest's estimated stand-alone earnings growth rate through the year 2002 is
14.5%.
 
     The summary set forth above is not a complete description of the
presentation by NationsBanc Montgomery to SierraWest's board or of the analyses
performed by NationsBanc Montgomery. The preparation of a fairness opinion is
not necessarily susceptible to partial analysis or summary description.
NationsBanc Montgomery believes that its analyses and the summary set forth
above must be considered as a whole and that selecting a portion of its analyses
and factors, without considering all analyses and factors, would create an
incomplete view of the process underlying the analyses set forth in its
presentation to SierraWest's board. In addition, NationsBanc Montgomery may have
given various analyses more or less weight than other analyses, and may have
deemed various assumptions more or less probable than other assumptions, so that
the ranges of valuations resulting from any particular analysis described above
should not be taken to be NationsBanc Montgomery's view of the actual value of
SierraWest or the combined company. The fact that any specific analysis has been
referred to in the summary above is not meant to indicate that such analysis was
given greater weight than any other analysis.
 
     In performing its analyses, NationsBanc Montgomery made numerous
assumptions with respect to industry performance, general business and economic
conditions and other matters, many of which are beyond the control of SierraWest
or BancWest. The analyses performed by NationsBanc Montgomery are not
necessarily indicative of actual values or actual future results, which may be
significantly more or less favorable than suggested by such analyses. Such
analyses were prepared solely as part of NationsBanc Montgomery's analysis of
the fairness of the consideration to be received by you in the merger and were
provided to SierraWest's board in connection with the delivery of NationsBanc
Montgomery's opinion. The analyses do not purport to be appraisals or to reflect
the prices at which a company might actually be sold or the prices at which any
securities may trade at the present time or any time in the future. The
forecasts used by NationsBanc Montgomery in certain of its analyses are based on
numerous variables and assumptions, which are inherently unpredictable and must
be considered not certain of occurrence as projected. Accordingly, actual
results could vary significantly from those contemplated in such forecasts.
 
                                       25
<PAGE>   31
 
     As described above under "THE MERGER -- Reasons for the Merger;
Recommendation of the Board of Directors," NationsBanc Montgomery's opinion and
presentation to SierraWest's board were among the many factors taken into
consideration by SierraWest's board in making its determination to approve the
merger agreement.
 
     Pursuant to its engagement letter, SierraWest paid NationsBanc Montgomery a
fee of approximately $600,000 upon the signing of a definitive agreement. Based
on the transaction value at the time of the announcement of the transaction,
NationsBanc Montgomery will receive an additional fee of approximately
$1,930,000 upon the closing of the merger. Accordingly, a significant portion of
NationsBanc Montgomery's fee is contingent upon the closing of the merger.
SierraWest has also agreed to reimburse NationsBanc Montgomery for its
reasonable out-of-pocket expenses, including reasonable fees and disbursements
for NationsBanc Montgomery's legal counsel and other experts retained by
NationsBanc Montgomery. SierraWest has agreed to indemnify NationsBanc
Montgomery, its affiliates, and their respective partners, directors, officers,
agents, consultants, employees and controlling persons against certain
liabilities, including liabilities under the federal securities laws.
 
REGULATORY APPROVALS REQUIRED
 
     The merger and the contemplated merger of SierraWest Bank and Bank of the
West (the "bank merger") are subject to the approval of the FDIC under Section
18(c) of the Federal Deposit Insurance Act and the approval of the Commissioner
of Financial Institutions of the State of California (the "California
Commissioner") under the California Financial Code. Aspects of the merger and
the bank merger will require notifications to, and/or approvals from, certain
other federal authorities and banking authorities in certain other states.
 
     Under the Federal Deposit Insurance Act, the FDIC must withhold approval of
the merger or the bank merger, as the case may be, if it finds that the
transaction would tend to create a monopoly or would in any other manner be in
restraint of trade, unless it finds that any such anti-competitive effects of
the merger or the bank merger are clearly outweighed in the public interest by
the probable effects of the merger or the bank merger in meeting the convenience
and needs of the communities to be served. Also, the merger and the bank merger
may not be consummated for 30 days from the date of approval by the FDIC, during
which time it could be challenged by the United States Department of Justice on
antitrust grounds. With the approval of the Department of Justice, however, this
waiting period may be reduced to no less than 15 days. The commencement of an
antitrust action by the Department of Justice would stay the effectiveness of
FDIC approval of the merger or the bank merger unless a court specifically
ordered otherwise.
 
     In deciding whether to approve the merger and the bank merger, the FDIC
will also take into consideration the financial and managerial resources and
future prospects of the banking subsidiaries following the transactions. The
FDIC has indicated that it will not approve a significant acquisition unless the
resulting institution has sufficient capitalization, taking into account, among
other things, asset quality.
 
     In addition, under the Community Reinvestment Act of 1977, the FDIC must
take into account the record of performance of each insured depository
institution subsidiary of BancWest, including Bank of the West, in meeting the
credit needs of the entire community, including low- and moderate-income
neighborhoods, served by each company. As part of the review process for the
merger and the bank merger, the FDIC will solicit public comments regarding the
applications. The FDIC frequently receives, in merger
 
                                       26
<PAGE>   32
 
transactions, protests from community groups and others regarding various
aspects of the proposal and, in particular, the extent to which the applicants
are complying with Community Reinvestment Act and fair lending laws. All of the
banking subsidiaries of BancWest have received either an "outstanding" or a
"satisfactory" Community Reinvestment Act rating in their most recent Community
Reinvestment Act examinations by their respective federal regulators.
 
     The FDIC is also authorized (but generally not required) to hold a public
hearing or meeting in connection with an application for approval of a merger
under the Bank Merger Act if it determines that such a hearing or meeting would
be appropriate. A decision by the FDIC to hold a public hearing or meeting
regarding any such application could prolong the period during which that
application is subject to review by the FDIC.
 
     BancWest and Bank of the West filed an application seeking approval of the
merger and the bank merger with the FDIC on April 2, 1999. The parties
anticipate that the FDIC will act on and approve the application during the
second or third quarter of 1999.
 
     The merger and bank merger will also be subject to the prior approval of
the California Commissioner. The factors that the California Commissioner will
consider in determining whether to grant its approval include the competitive
effects of the merger and the bank merger, the principles of sound banking and
the public interest and the needs of the communities served by Bank of the West.
 
     BancWest and Bank of the West filed an application seeking approval of the
merger and the bank merger with the California Commissioner on April 2, 1999.
The parties anticipate that the California Commissioner will act on and approve
the application during the second or third quarter of 1999.
 
     There can be no assurance as to whether or when any of the above-described
regulatory approvals required for consummation of the merger and the bank merger
will be obtained or as to any conditions that may be imposed in connection with
the granting of such approvals. See "THE MERGER AGREEMENT -- Conditions to the
Completion of the Merger."
 
STOCK EXCHANGE LISTING
 
     The shares of BancWest common stock to be issued in the merger will be
listed on the New York Stock Exchange.
 
INTEREST OF CERTAIN OFFICERS AND DIRECTORS IN THE MERGER
 
     In considering the recommendation of the board of directors of SierraWest
to approve the merger agreement, you should be aware that certain officers and
directors of SierraWest have certain interests in, and will receive benefits as
a consequence of, the merger that are different from the benefits to SierraWest
shareholders generally.
 
     STOCK, OPTIONS AND DEFERRED COMPENSATION PLAN.  As of March 15, 1999,
SierraWest's directors and executive officers owned an aggregate of 373,649
shares of SierraWest stock (including 118,507 shares subject to options
exercisable within 60 days of March 15, 1999) which, if owned at the time of the
merger, will be converted into shares of BancWest stock with an approximate
aggregate market value of $12,448,714, based on the $40.63 closing price of
BancWest stock on April 13, 1999. All outstanding options to purchase SierraWest
stock will vest at the time of the merger. Based on a policy adopted by the
board of directors of SierraWest, any current director of SierraWest who
exercises nonqualified stock options after the merger will be entitled to
receive a payment in the
 
                                       27
<PAGE>   33
 
amount equal to the estimated imputed value of the tax benefits realized or
estimated to be realized by BancWest as a result of the exercise.
 
     SierraWest's outside directors (i.e., directors who are not employees of
SierraWest or SierraWest Bank) participate in a Deferred Compensation and Stock
Award Plan. Under this plan, outside directors are required to defer receipt of
one-third of their directors' fees for regular board meetings in the form of
promised shares of SierraWest stock. The director may also voluntarily defer all
or a portion of the remaining amount of director's fees. The plan provides that,
at the time of the merger, the outside directors will receive cash payments
equal to the value of their deferred SierraWest stock at that date or an
aggregate of $327,735, based on $40.63, the closing price of BancWest stock on
April 13, 1999. Under the plan the directors are not entitled to receive shares
of SierraWest common stock.
 
     PAYMENT CONTINUATION AGREEMENTS.  In the interest of retaining qualified
directors and senior management, SierraWest has entered into agreements with
some of its directors and officers providing salary continuation benefits upon
their retirement or earlier death. Under these agreements, the vesting of
benefits accelerate and become payable upon a change in control, such as the
merger.
 
     SierraWest has entered into Amended and Restated Payment Continuation
Agreements with the following directors: Jerrold Henley, Richard Gaston, Jack
Leonesio, William McClintock, Tom Watson, A. Morgan Jones, Ralph Coppola, John
Johnson, Ron Johnson and Dave Clark. These agreements provide for payments to
each participating director of $4,000 per year for 15 years, beginning 15 years
after the date of their respective agreement. Accrued benefits vest 20% per year
over a five-year period from the date of association with SierraWest. Upon a
change of control, such as the merger, the director will receive a lump sum cash
payment equal to the present value of the maximum benefits under the agreement.
The estimated present value for each participant is $31,018. In addition, two of
SierraWest's directors will have the right to purchase health insurance from
BancWest after the merger. SierraWest has entered into agreements with executive
officers William T. Fike and David Broadley providing salary continuation
benefits. These benefits fully vest and become payable upon the officer's
retirement at age 65, the officer's death or a change of control of SierraWest.
At the time of the merger, Mr. Fike will become entitled to receive annual
payments of $50,000 for 20 years and Mr. Broadley will become entitled to
receive annual payments of $40,000 for 20 years.
 
     See "EXECUTIVE COMPENSATION -- Salary Continuation Plan" for a more
complete description of these agreements.
 
     EMPLOYMENT AGREEMENTS.  Mr. Fike's employment agreement with SierraWest
provides for a payment upon a change of control of SierraWest. Under the terms
of his employment agreement, Mr. Fike will receive a lump-sum severance payment
equal to $393,750, the automobile SierraWest provides for his use and a club
membership that SierraWest purchased for his use.
 
     SierraWest has also entered into Senior Manager Separation Benefits
Agreement with officers, Rick Belstock, David Broadley, Pat Day, Mary Jane
Posnien and Bob Silver. Under the terms of these agreements, if the officer is
terminated, other than for cause, or (in the case of Mr. Broadley) chooses to
resign within nine months after a change in control, the officer will be
entitled to receive six, nine or 12 months' salary, depending on the officer,
either in a lump sum or in installments, at the officer's option. Under the
terms of his Senior Manager Separation Benefits Agreement and based on his
current salary,
 
                                       28
<PAGE>   34
 
Mr. Belstock will be entitled to six months' salary or $44,031 if he is
terminated or chooses to resign after the merger. Messrs. Broadley and Day will
be entitled to receive 12 months' salary or $160,000 and $135,200, respectively,
based on their current salaries. Ms. Posnien and Mr. Silver will be entitled to
receive nine months' salary, or $63,750 and $75,750, respectively. In addition,
Mr. Broadley will be entitled to receive the automobile SierraWest provides for
his use.
 
     See "EXECUTIVE COMPENSATION -- Employment Agreements" for a more complete
description of these agreements.
 
     INDEMNIFICATION COVENANT.  In the merger agreement, BancWest has agreed to
indemnify the directors and officers of SierraWest and SierraWest Bank against
claims and expenses paid in settlement of or in connection with any claim, suit
or investigation based, in whole or in part, on the fact that the person was or
is a director or officer of SierraWest or SierraWest Bank, the merger agreement
or the transactions described in the merger agreement.
 
EFFECT ON SIERRAWEST'S EMPLOYEE BENEFITS PLANS
 
     Under the merger agreement, SierraWest and SierraWest Bank have agreed to
modify their present incentive compensation and bonus plans to provide for the
settlement promptly after the effective time of the merger of the accrued
entitlement of the participants in such plans as of the closing date. BancWest
and Bank of the West will pay such entitlements to the respective participants
promptly after the effective time of the merger subject to required
withholdings. For the period from the closing date to and including December 31,
1999, BancWest and Bank of the West will either continue the existing incentive
compensation and bonus plans of SierraWest and SierraWest Bank or will make
comparable plans available to the participants in these plans.
 
     After the closing date, BancWest and Bank of the West will honor the
arrangements regarding health and life insurance coverage, continuation of
directors' fees and deferral of directors' fees for certain individuals and
SierraWest will have performed all necessary actions so that the modification or
termination of the various plans or arrangements will be effective at the
effective time of the merger.
 
ACCOUNTING TREATMENT
 
     BancWest expects to account for this merger under the pooling of interests
method of accounting. It is a condition to completion of the merger that both
BancWest and SierraWest receive a letter from their respective independent
accountants stating that the merger will qualify as a pooling of interests
transaction under generally accepted accounting principles.
 
     Under the pooling of interests method of accounting, BancWest will restate
its consolidated financial statements as follows:
 
     - BancWest will carry forward on its books the assets and liabilities of
       SierraWest at their historical recorded values,
 
     - income of the combined BancWest will include income of BancWest and
       SierraWest for the entire fiscal year in which the combination occurs,
 
     - the reported income, assets, liabilities and shareholders' equity of the
       separate companies for previous periods will be combined and restated as
       income, assets, liabilities and shareholders' equity of BancWest, and
 
                                       29
<PAGE>   35
 
     - all intercompany accounts will be eliminated.
 
BancWest has been informed by its independent accountants that the merger will
likely qualify as a pooling of interests for financial accounting and financial
reporting purposes.
 
CERTAIN FEDERAL INCOME TAX CONSEQUENCES
 
     The following discussion summarizes the material United States federal
income tax consequences of the merger. The discussion does not address all
aspects of United States federal taxation that may be relevant to you, and it
may not be applicable to SierraWest shareholders who, for United States federal
income tax purposes, are nonresident alien individuals, foreign corporations,
foreign partnerships, foreign trusts or foreign estates, or who acquired their
SierraWest common stock pursuant to the exercise of SierraWest stock options or
otherwise as compensation. YOU SHOULD CONSULT YOUR OWN TAX ADVISOR AS TO THE
PARTICULAR TAX CONSEQUENCES OF THE MERGER TO YOU.
 
     This discussion is based on the Internal Revenue Code of 1986, regulations
and rulings now in effect or proposed thereunder, current administrative rulings
and practice, and judicial precedent, all of which are subject to change. Any
such change, which may or may not be retroactive, could alter the tax
consequences to you as discussed in this proxy statement-prospectus. This
discussion assumes that you hold your SierraWest common stock as a capital asset
within the meaning of Section 1221 of the Internal Revenue Code.
 
     None of the parties is required to complete the merger unless BancWest
shall have received an opinion of Pillsbury Madison & Sutro LLP, counsel to
BancWest, and SierraWest shall have received an opinion of McCutchen, Doyle,
Brown & Enersen, LLP, counsel to SierraWest, based upon certain customary
assumptions and certain representations made by BancWest and SierraWest, to the
effect that, for United States federal income tax purposes, under currently
applicable law,
 
     - the merger will constitute a reorganization within the meaning of Section
       368(a) of the Internal Revenue Code, and
 
     - BancWest and SierraWest will each be party to that reorganization within
       the meaning of Section 368(b) of the Internal Revenue Code.
 
     Based upon such opinions, the following will be the material federal income
tax consequences of the merger:
 
     (1) You will recognize no gain or loss if you receive solely BancWest
         common stock in exchange for shares of SierraWest common stock you
         hold, except with respect to cash received instead of fractional shares
         of BancWest common stock.
 
     (2) The aggregate adjusted tax basis of the shares of BancWest common stock
         you receive in the merger (including any fractional share of BancWest
         common stock deemed to be received by you, as described in paragraph
         (4) below), will be equal to the aggregate adjusted tax basis of the
         shares of SierraWest common stock surrendered by you for the shares of
         the BancWest common stock.
 
     (3) The holding period of the shares of BancWest common stock you receive
         in the merger (including any fractional share of BancWest common stock
         deemed to be received by you, as described in paragraph (4) below) will
         include the holding period of the shares of SierraWest common stock you
         exchange for the shares of BancWest common stock.
 
                                       30
<PAGE>   36
 
     (4) If you receive cash in the merger instead of a fractional share of
         BancWest common stock it will be treated as if you received the
         fractional share in the merger and then BancWest redeemed such share in
         return for the cash. You will generally have to recognize capital gain
         or loss equal to the difference between the amount of cash received and
         the portion of your adjusted tax basis in the shares of BancWest common
         stock allocable to the fractional share.
 
     BancWest is not required to complete the merger if any SierraWest
shareholders perfect dissenters' rights. The following discussion applies only
if BancWest waives this condition. A shareholder who perfects dissenters' rights
and receives payment for his or her SierraWest shares will be treated as if such
shares were redeemed. In general, if the shares are held as a capital asset at
the date the merger is consummated, the dissenting shareholder will recognize a
capital gain or loss measured by the difference between the amount of cash
received and the basis of the shares in the hands of the dissenting shareholder.
However, if such dissenting shareholder owns, directly or indirectly through the
application of Section 318 of the Internal Revenue Code, any shares of common
stock as to which dissenters' rights are not exercised and perfected and which
are therefore exchanged for SierraWest common stock in the merger, such
shareholder may be treated as having received a dividend in the amount of cash
paid to the shareholder in exchange for the shares as to which dissenter's
rights were perfected. Under Section 318 of the Internal Revenue Code, you are
deemed to own stock that is actually owned (or deemed to be owned) by certain
members of your family (spouse, children, grandchildren and parents, with
certain exceptions) and other related parties, including, for example, certain
entities in which you have a direct or indirect interest (including
partnerships, estates, trusts and corporations), as well as stock that you have
(or a related person has) the right to acquire upon exercise of an option or
conversion right held by you (or a related person). If you intend to dissent
from the merger, you should consult your own tax advisor with respect to the
application of the constructive ownership rules to your particular
circumstances. See "-- Dissenters' Rights of Appraisal."
 
     THE FOREGOING IS A GENERAL DISCUSSION OF THE MATERIAL UNITED STATES FEDERAL
INCOME TAX CONSEQUENCES OF THE MERGER AND IS INCLUDED FOR GENERAL INFORMATION
ONLY. THE FOREGOING DISCUSSION DOES NOT TAKE INTO ACCOUNT THE PARTICULAR FACTS
AND CIRCUMSTANCES OF YOUR STATUS AND ATTRIBUTES. AS A RESULT, THE UNITED STATES
FEDERAL INCOME TAX CONSEQUENCES ADDRESSED IN THE FOREGOING DISCUSSION MAY NOT
APPLY TO YOU. IN VIEW OF THE INDIVIDUAL NATURE OF INCOME TAX CONSEQUENCES, YOU
ARE URGED TO CONSULT YOUR OWN TAX ADVISOR TO DETERMINE THE SPECIFIC TAX
CONSEQUENCES OF THE MERGER TO YOU, INCLUDING THE APPLICATION AND EFFECT OF
UNITED STATES FEDERAL, STATE, LOCAL AND OTHER TAX LAWS AND THE POSSIBLE EFFECTS
OF CHANGES IN UNITED STATES FEDERAL AND OTHER TAX LAWS.
 
DISSENTERS' RIGHTS OF APPRAISAL
 
     Because SierraWest common stock is traded on the Nasdaq National Market
System, dissenters' rights will be available to the shareholders of SierraWest
only if the holders of five percent or more of SierraWest common stock make a
written demand upon SierraWest for the purchase of dissenting shares in
accordance with Chapter 13 of California General Corporation Law. BANCWEST IS
NOT REQUIRED TO COMPLETE THE MERGER IF ANY HOLDERS OF SIERRAWEST COMMON STOCK
ARE ENTITLED TO HAVE THEIR SHARES TREATED AS "DISSENTING SHARES." THEREFORE, IF
YOU EXERCISE YOUR DISSENTERS' RIGHTS, THE MERGER MAY NOT BE COMPLETED AND YOU
MAY RETAIN YOUR SIERRAWEST SHARES. However, if holders are entitled to have
their shares treated as "dissenting shares" and BancWest has waived the
condition described above and the merger is completed, then shareholders of
SierraWest who dissent
 
                                       31
<PAGE>   37
 
from the merger by complying with the procedures set forth in Chapter 13 of the
California law would be entitled to receive an amount equal to the fair market
value of their shares as of February 25, 1999, the day before the public
announcement of the merger. The closing sales price for SierraWest common stock
on February 25, 1999 was $28.875. A copy of Chapter 13 of the California General
Corporation Law is attached hereto as Appendix D and should be read for more
complete information concerning dissenters' rights. You must follow the required
procedure set forth in Chapter 13 of the California General Corporation Law
exactly or you will lose any dissenters' rights. The information set forth below
is a general summary of dissenters' rights as they apply to SierraWest
shareholders and is qualified in its entirety by reference to Appendix D.
 
     In order to be entitled to exercise dissenters' rights, you must vote
"AGAINST" the merger. Thus, if you wish to dissent and execute and return a
proxy in the accompanying form, you must specify that your shares are to be
voted "AGAINST" the merger. If you return a proxy without voting instructions or
with instructions to vote "FOR" the merger, your shares will automatically be
voted in favor of the merger and you will lose any dissenters' rights. If you do
not return a proxy and you attend the annual meeting, you must vote "AGAINST"
the merger at the meeting to preserve your dissenter's rights. In addition, if
you abstain from voting your shares, you will lose your dissenter's rights.
 
     Furthermore, in order to preserve your dissenters' rights, you must make a
written demand upon SierraWest for the purchase of dissenting shares and payment
to you of their fair market value, specifying the number of shares held of
record by you and a statement of what you claim to be the fair market value of
those shares as of February 25, 1999. You must address your demand to SierraWest
Bancorp, P.O. Box 61000, 10181 Truckee-Tahoe Airport Road, Truckee, California
96161, Attn: David Broadley, and we must receive it not later than the date of
the SierraWest annual meeting. A vote "AGAINST" the merger does not constitute
such written demand.
 
     WE CANNOT ASSURE YOU THAT THE MERGER WILL BE COMPLETED IF ANY HOLDERS OF
SIERRAWEST COMMON STOCK ARE ENTITLED TO EXERCISE DISSENTERS' RIGHTS OF
APPRAISAL. BANCWEST IS NOT REQUIRED TO COMPLETE THE MERGER IF ANY HOLDERS OF
SIERRAWEST COMMON STOCK HAVE SUCH RIGHTS. SEE "THE MERGER
AGREEMENT -- CONDITIONS TO THE COMPLETION OF THE MERGER." AS A RESULT, THE
FOLLOWING DISCUSSION WILL ONLY APPLY IF BANCWEST WAIVES THAT CONDITION TO
CLOSING. BANCWEST DOES NOT INTEND TO WAIVE THAT CONDITION.
 
     If the holders of five percent or more of the outstanding shares of
SierraWest common stock have submitted a written demand for SierraWest to
purchase their shares, and these demands are received by SierraWest on or before
the date of the SierraWest annual meeting and the merger is approved by the
shareholders, SierraWest will have 10 days after such approval to send to those
shareholders who have voted against the approval of the merger written notice of
such approval accompanied by a copy of Chapter 13 of the California General
Corporation Law, a statement of the price determined by SierraWest to represent
the fair market value of the dissenting shares as of February 25, 1999, and a
brief description of the procedure to be followed if you desire to exercise the
dissenters' rights. Within 30 days after the date on which the notice of the
approval of the merger is mailed, the dissenting shareholder must surrender to
SierraWest, at the office designated in the notice of approval, the certificate
representing the dissenting shares to be stamped or endorsed with a statement
that they are dissenting shares or to be exchanged for certificates for the
appropriate number of shares that are stamped or endorsed. Any shares
 
                                       32
<PAGE>   38
 
of SierraWest common stock that are transferred prior to their submission for
endorsement lose their status as dissenting shares.
 
     If SierraWest and the dissenting shareholder agree that the surrendered
shares are dissenting shares and agree upon the price of the shares, the
dissenting shareholder will be entitled to the agreed price with interest
thereon at the legal rate on judgments from the date of the agreement. Payment
of the fair market value of the dissenting shares shall be made within 30 days
after the amount thereof has been agreed upon or 30 days after any statutory or
contractual conditions to the merger have been satisfied, whichever is later,
subject to the surrender of the certificates therefor, unless provided otherwise
by agreement.
 
     If SierraWest denies that the shares surrendered are dissenting shares, or
SierraWest and the dissenting shareholder fail to agree upon a fair market value
of such shares of SierraWest common stock, then the dissenting shareholder of
SierraWest must, within six months after the notice of approval is mailed, file
a complaint in the Superior Court of the proper county requesting the court to
make such determinations or the dissenting shareholder must intervene in any
pending action brought by any other dissenting shareholder. If the complaint is
not filed or intervention in a pending action is not made within the specified
six-month period, the dissenters' rights are lost. If the fair market value of
the dissenting shares is at issue, the court or one or more court-appointed
appraisers will determine, such fair market value. A dissenting shareholder may
not withdraw his or her dissent or demand for payment unless SierraWest consents
to such withdrawal.
 
     The receipt of a cash payment for dissenting shares will result in
recognition of gain or loss for federal and California state income tax purposes
by dissenting shareholders. See "-- Certain Federal Income Tax Consequences."
 
SIERRAWEST'S RIGHTS AGREEMENT
 
     Under the merger agreement, SierraWest agreed to amend its rights agreement
between SierraWest and American Stock Transfer & Trust Co. dated January 16,
1996, as amended, so that the transactions contemplated by the merger agreement
do not result in the grant of any rights to any person under the rights
agreement. Accordingly, on February 25, 1999, SierraWest amended its rights
agreement to allow SierraWest to enter into the merger agreement. As amended,
the rights agreement provides that BancWest, Bank of the West and their
affiliates and associates will not be "acquiring persons" as a result of the
merger agreement or the transactions described in the merger agreement.
 
RESALES OF BANCWEST COMMON STOCK
 
     The shares of BancWest common stock to be issued to shareholders of
SierraWest under the merger agreement have been registered under the Securities
Act, so these shares may be freely traded without restriction by people who will
not be affiliates of BancWest after the merger or who were not affiliates of
SierraWest on the date of the annual meeting. All directors and certain officers
of SierraWest may be deemed to have been affiliates of SierraWest within the
meaning of such rules. Those people may resell shares of BancWest common stock
to be received by them in the merger only if the shares are registered for
resale under the Securities Act or an exemption from such registration under the
Securities Act is available. Those people may be permitted to resell the
BancWest shares under the safe harbor provisions of Rule 145 under the
Securities Act (or Rule 144 in the case of such persons who become affiliates of
BancWest) or as otherwise permitted
 
                                       33
<PAGE>   39
 
under the Securities Act. People who may be deemed affiliates of SierraWest or
BancWest generally include individuals or entities that control, are controlled
by, or are under common control with, SierraWest or BancWest, and may include
certain officers and directors of such entities as well as principal
shareholders of SierraWest or BancWest. We encourage any such person to obtain
advice of securities counsel before reselling any BancWest shares.
 
     The merger agreement provides that SierraWest will use its commercially
reasonable efforts to cause each director, executive officer and other person
who is an affiliate for purposes of Rule 145 under the Securities Act to execute
and deliver, as soon as practicable after the date of the merger agreement and
prior to the date of the annual meeting, a written agreement to the effect that
such person will not offer or sell or otherwise dispose of:
 
     - any BancWest common stock received in the merger in violation of the
       Securities Act or the rules and regulations thereunder, and
 
     - any shares of BancWest common stock or SierraWest common stock during in
       the period beginning 30 days before the effective time of the merger and
       ending at such time as results covering at least 30 days of combined
       operations of BancWest and SierraWest have been published by BancWest.
 
                                       34
<PAGE>   40
 
                              THE MERGER AGREEMENT
 
     The following is a summary of the material provisions of the merger
agreement, as amended which is attached to this proxy statement-prospectus as
Appendix A. The merger agreement is incorporated by reference into this proxy
statement-prospectus. You are urged to read the merger agreement in its
entirety.
 
STRUCTURE OF THE MERGER; EFFECTIVE TIME
 
     The merger agreement contemplates the merger of SierraWest with and into
Bank of the West. Bank of the West will be the surviving corporation in the
merger and will continue its corporate existence under California law. The
merger will become effective upon the filing of an agreement of merger with the
Secretary of State of the State of California, or at such time thereafter as is
provided in such agreement of merger. The closing of the merger will take place
at 10:00 a.m., California time, on a date to be specified by the parties, which
will be the fifth business day after satisfaction or waiver (subject to
applicable law) of the conditions set forth in the merger agreement, unless
another time or date is agreed to in writing by BancWest, Bank of the West and
SierraWest. The merger agreement may be terminated by either BancWest or
SierraWest if, among other reasons, the merger shall not have been consummated
on or before September 30, 1999. See "-- Conditions to the Completion of the
Merger" and "-- Termination" below.
 
     Immediately after the merger of SierraWest into Bank of the West,
SierraWest's wholly-owned subsidiary SierraWest Bank will merge with and into
Bank of the West, and Bank of the West will be the surviving corporation.
 
     The merger agreement provides that BancWest may change the structure of the
merger for any reasonable business, tax or regulatory purpose. The alternate
structure, however, may not materially and adversely affect the timing of the
merger, or adversely affect the economic benefits, the form of consideration or
the tax effect of the merger to you.
 
CONVERSION OF SIERRAWEST COMMON STOCK
 
     If you are a shareholder of SierraWest common stock as of the effective
time of the merger your shares of SierraWest common stock will be converted into
the right to receive 0.82 of a share of BancWest common stock. Your shares of
SierraWest common stock will no longer be outstanding and will be automatically
canceled and retired and will cease to exist. Your stock certificate previously
representing shares of SierraWest common stock will be exchanged for a
certificate representing whole shares of BancWest common stock.
 
     Under the merger agreement SierraWest has the option to terminate the
merger if the price per share of BancWest common stock declines significantly.
During the two days after the FDIC approves the transactions, the board of
directors of SierraWest may terminate the agreement, if:
 
     - the average closing price per share of BancWest common stock for the 20
       consecutive trading days ending on the date the FDIC approves the merger
       is less than 85% of $43.8375 (the average closing price of BancWest
       common stock for the 5 trading days before the public announcement of the
       merger), and
 
     - the ratio of the average closing price of BancWest common stock during
       such 20 day period to $43.8375 is less than 85% of the ratio of Standard
       & Poor's Mid-
 
                                       35
<PAGE>   41
 
       Cap Regional Bank Index during such 20 trading day period to $223.854
       (the average bank index for the 5 trading days before the public
       announcement of the merger).
 
SierraWest cannot terminate the merger agreement, if within 5 business days of
being notified by SierraWest of SierraWest's intention to terminate, BancWest
adjusts the exchange ratio so that the value of BancWest shares you receive in
the merger will not have declined more than 15% in relation to any decline in
the bank stock index. For more information see "-- Termination."
 
     You will not receive any fractional shares of BancWest common stock. If you
are entitled to a fraction of a share of BancWest common stock you will,
instead, receive an amount in cash. The cash amount will be equal to the closing
price as reported on the New York Stock Exchange for the BancWest common stock
on the trading day immediately preceding the closing date, multiplied by the
fraction of a share of BancWest common stock to which you would otherwise be
entitled. You will not be entitled to dividends, voting rights, interest on the
value of, or any other rights in respect of a fractional share. In the event
BancWest pays, declares or otherwise effects a stock split, reverse stock split,
reclassification or stock dividend or stock distribution with respect to
BancWest common stock between the date of the merger agreement and the effective
time of the merger, appropriate adjustments will be made to the average BancWest
closing price of BancWest common stock.
 
OPTIONS
 
     At the effective time of the merger, each option to acquire SierraWest
common stock which is outstanding and unexercised will be converted
automatically into an option to purchase shares of BancWest common stock. The
number of shares to be subject to the new option will be equal to the product of
the number of shares of SierraWest common stock subject to the original option
and the exchange ratio, rounded down to the nearest share. The exercise price
per share of BancWest common stock under the new option will be equal to the
exercise price per share of SierraWest common stock under the original option
divided by the exchange ratio. The exercise price will be rounded up to the
nearest cent. In the case of any options which are "incentive stock options," as
defined in Section 422 of the Internal Revenue Code, the exercise price, the
number of shares purchasable pursuant to such options and the terms and
conditions of such options will be determined in order to comply with Section
424(a) of the Internal Revenue Code. The duration and other terms of the new
options will be the same as those of the original option.
 
EXCHANGE AGENT; EXCHANGE PROCEDURE
 
     Under the merger agreement, BancWest has agreed to appoint American Stock
Transfer & Trust Company, New York, New York or its successor, or any other bank
or trust company (having capital of at least $50 million) mutually acceptable to
SierraWest and BancWest, as exchange agent for the purpose of exchanging
certificates representing the BancWest common stock which are to be issued
pursuant to the merger agreement. As soon as practicable after the effective
time of the merger, upon the surrender of your SierraWest shares certificate for
cancellation, you will be entitled to receive a certificate representing the
number of shares of BancWest common stock determined in accordance with the
merger agreement and a payment in cash with respect to any fractional shares.
 
                                       36
<PAGE>   42
 
Do not send in your certificates at this time. Please wait until you receive a
transmittal letter with more specific instructions on exchanging your
certificates.
 
     You will not receive any dividends or other distributions of any kind which
are declared payable to shareholders of record of the shares of BancWest common
stock after the effective time of the merger until you surrender your
certificate for shares of SierraWest common stock. Upon such surrender of your
SierraWest certificate, you will be paid, without interest, any dividends or
other distributions with respect to the shares of BancWest common stock as to
which the record date and payment date occurred on or after the effective time
of the merger and on or before the date on which you surrendered your
certificate for shares of SierraWest common stock.
 
     If you would like your certificate for shares of BancWest common stock to
be issued in a name other than the name or names in which your exchanged
SierraWest certificate is registered, you will have to pay to the exchange agent
any transfer costs, taxes or other expenses required by reason of the issuance
of certificates for such shares of BancWest common stock in a name other than
the registered holder of the exchanged SierraWest certificate.
 
     All dividends or distributions, and any cash to be paid instead of
fractional shares, if held by the exchange agent for payment or delivery to the
holders of unsurrendered SierraWest certificates representing shares of
SierraWest common stock and unclaimed at the end of one year from the effective
time of the merger, shall (together with any interest earned thereon) at such
time be paid or redelivered by the exchange agent to BancWest. After such time,
if you still have not surrendered your SierraWest certificate, you must, look as
a general creditor only to BancWest for payment or delivery of such dividends or
distributions or cash, as the case may be.
 
     Neither BancWest nor the surviving corporation shall be liable to you for
such shares (or dividends or distributions thereon) or cash payable instead of
fractional shares delivered to a public official pursuant to any applicable
abandoned property, escheat or similar law.
 
REPRESENTATIONS AND WARRANTIES
 
     The merger agreement contains customary mutual representations by each of
BancWest and SierraWest relating to, among other things, (1) corporate
organization and existence, (2) capitalization, (3) corporate power and
authority to enter into, and due authorization, execution, delivery, performance
and enforceability of, the merger agreement, (4) required governmental and third
party consents and approvals and that neither the merger agreement nor the
transactions contemplated thereby violate either party's organizational
documents, applicable law and certain material agreements, (5) financial
statements, (6) Securities and Exchange Commission documents, (7) the accuracy
of the information provided by each of BancWest and SierraWest for inclusion in
this proxy statement-prospectus, (8) compliance with applicable laws, (9) the
absence of material litigation, (10) subsidiaries, (11) certain bank regulatory
matters, (12) the absence of certain material changes or events since September
30, 1998, (13) the absence of undisclosed liabilities, (14) timely filing of all
material regulatory reports, (15) brokers' and finders' fees, (16) confirmation
that the merger will qualify for pooling of interests treatment for accounting
purposes, (17) compliance with Community Reinvestment Act, and (18) Year 2000
readiness.
 
                                       37
<PAGE>   43
 
     In the merger agreement SierraWest also makes representations and
warranties to BancWest and Bank of the West concerning, (1) ownership of
BancWest common stock, (2) filing of tax returns, payment of taxes and related
matters, (3) certain material contracts, (4) employee benefit plans and
agreements, (5) the absence of material environmental liability, (6) title to
properties, (7) transactions with affiliates, (8) intellectual property, (9)
receipt of a fairness opinion from its financial advisor, (10) SierraWest's
rights agreement, (11) anti-takeover provisions of the California General
Corporation Law, (12) dissenters' rights, (13) insurance, (14) validity and
enforceability of all loans, leases, other extensions of credit, commitments or
other interest-bearing assets and investments of SierraWest, (15) absence of
restrictions on investments, (16) absence of brokered deposits (except as
disclosed), (17) absence of high risk, highly interest sensitive derivative
contracts (except as disclosed), and (18) absence of collective bargaining
agreements.
 
     BancWest also makes the representation and warranty that no stockholder
approval is needed from its stockholders with respect to the approval of the
merger agreement.
 
     The representations and warranties of BancWest and SierraWest terminate as
of the effective time of the merger.
 
CONDUCT OF BUSINESS PENDING THE MERGER
 
     Pursuant to the merger agreement, BancWest and SierraWest have each agreed
that, during the period from the date of the merger agreement to the effective
time of the merger, it and each of its respective subsidiaries will:
 
     - carry on its business in the usual, regular and ordinary course in
       substantially the same manner as conducted prior to the execution of the
       merger agreement and use all reasonable efforts to preserve intact its
       present business organizations, and
 
     - maintain the rights and franchises of its business, and preserve the
       relationships with its customers, suppliers and others having business
       dealings with it to the end that its goodwill and ongoing businesses will
       not be impaired in any material respect at the effective time of the
       merger.
 
The merger agreement permits exceptions for conduct expressly contemplated or
permitted by the merger agreement or consented to in writing by the other party.
 
     In addition, SierraWest has agreed that it and its subsidiaries will not,
without the prior written consent of BancWest:
 
     - declare or pay any dividends on or make other distributions in respect of
       any of its capital stock, except (1) for dividends by a wholly-owned
       subsidiary of SierraWest to SierraWest, and (2) SierraWest may pay on
       March 31, 1999, a cash dividend in the amount of up to $0.26 per share on
       SierraWest common stock and, if the effective time of the merger occurs
       after the record date set by BancWest's board of directors for the cash
       dividend payable by BancWest on its outstanding common stock in respect
       to the third fiscal quarter of 1999, a further cash dividend of up to
       $0.26 per share on the SierraWest common stock,
 
     - split, combine or reclassify any of its capital stock or issue any other
       securities instead of shares of its capital stock,
 
     - issue or sell any shares of its capital stock, other than issuances of
       SierraWest common stock in connection with the exercise of SierraWest
       options, or repurchase, redeem or otherwise acquire any shares of its
       capital stock,
 
                                       38
<PAGE>   44
 
     - amend its articles of incorporation or its bylaws or other organizational
       documents or that of any subsidiary,
 
     - enter into any new material line of business or materially change its
       lending, investment, liability management and other material banking
       policies, except as required by law or by policies imposed by a banking
       regulatory authority or the U.S. Small Business Administration,
 
     - subject to certain exceptions, incur or commit to any capital
       expenditures or any obligations or liabilities in connection with capital
       expenditures other than in the ordinary course of business consistent
       with past practice but in no event for more than $50,000 as to any one
       such item or $200,000 as to all such items in the aggregate,
 
     - make any acquisition or enter into any merger, except as permitted by
       certain limited exceptions,
 
     - sell, lease, encumber or otherwise dispose of, any material assets, other
       than in the ordinary course of business consistent with past practice,
 
     - incur or guarantee any long-term indebtedness or issue or sell any
       long-term debt securities or guarantee any long-term debt securities of
       others other than (1) indebtedness of any subsidiary of SierraWest to
       SierraWest or to another subsidiary of SierraWest, (2) deposits taken in
       the ordinary course of business consistent with past practice, or (3)
       renewals or extensions of existing long-term indebtedness without any
       change in the material terms thereof,
 
     - intentionally take or fail to take any action that would, or reasonably
       might be expected to, result in any of the representations and warranties
       set forth in the merger agreement being or becoming untrue in any
       material respect, or in any of the conditions to the closing set forth in
       the merger agreement not being satisfied, or, unless such action is
       required by applicable law or sound banking practice, which would
       adversely affect the ability of BancWest or SierraWest to obtain any of
       the requisite regulatory approvals,
 
     - subject to certain exceptions, change its methods of accounting,
 
     - subject to certain exceptions, enter into or amend any employee benefit
       plans or agreements or increase the compensation or fringe benefits of
       any director, officer and employee,
 
     - except in the ordinary course of business consistent with past practice,
       make or acquire any loan or issue a commitment for any loan or agree to
       issue any letters of credit or otherwise guarantee the obligations of any
       other persons,
 
     - engage or participate in any material transaction or incur or sustain any
       material obligation not in the ordinary course of business consistent
       with past practice,
 
     - settle any claim involving money damages involving a payment in excess of
       $50,000 as to any such matter, or settle any other matter not involving
       money damages which is material to SierraWest,
 
     - change or make any tax elections or settle or compromise any material tax
       liability,
 
     - subject to certain exceptions, make an application for the opening,
       relocation or closing of, or open, relocate or close any branch or loan
       production office,
 
                                       39
<PAGE>   45
 
     - amend or waive any provision of, or terminate, the SierraWest rights
       plan, or redeem any of the rights outstanding thereunder,
 
     - except as permitted by the merger agreement, enter into any
       securitization or similar transactions with respect to any loans, leases
       or other assets, or
 
     - agree to, or make any commitment to, take any of the actions described
       above.
 
     Additionally, BancWest has agreed not to:
 
     - amend its certificate of incorporation or its bylaws in a manner that
       would materially and adversely affect (1) its ability to perform it
       obligations under the merger agreement or complete the merger, or (2) the
       rights, powers and privileges of the shares of BancWest common stock to
       be issued in the merger,
 
     - intentionally take or fail to take any action that would, or reasonably
       might be expected to, result in (1) any of its representations and
       warranties set forth in the merger agreement being or becoming untrue,
       subject to such exceptions as do not have, and would not reasonably be
       expected to have, individually or in the aggregate, a material adverse
       effect on BancWest following the effective time of the merger, (2) any of
       the conditions to the closing set forth in the merger agreement not being
       satisfied, or unless such action is required by applicable law or sound
       banking practice, that would adversely affect the ability of BancWest or
       SierraWest to obtain any of the requisite regulatory approvals without
       imposition of a burdensome condition, or
 
     - agree to, or make any commitment to, take any of the actions described
       above.
 
ADDITIONAL AGREEMENTS
 
     SIERRAWEST SHAREHOLDERS' MEETING.  In the merger agreement, SierraWest has
agreed to call a meeting of its shareholders to be held as promptly as
practicable for the purpose of voting on the merger. SierraWest is required
through its board of directors to recommend to its shareholders approval of the
merger agreement unless the board of directors of SierraWest determines in good
faith, based upon the written advice of outside counsel, that making such
recommendation, or failing to withdraw, modify or amend any previously made
recommendation, would constitute a breach of fiduciary duty by SierraWest's
board of directors under applicable law.
 
     NO SOLICITATIONS.  From February 25, 1999, until the earlier of the
effective time of the merger or the termination of the merger agreement,
SierraWest has agreed that neither it, nor any of its subsidiaries, affiliates
or agents, will enter into discussions or negotiations with or provide
information to any person or group of persons (other than BancWest and its
subsidiaries) concerning:
 
     - the acquisition of a 15% or greater equity interest in, or a merger,
       consolidation, liquidation, dissolution or other disposition of 15% or
       more of the assets of, SierraWest or any significant subsidiary of
       SierraWest, or
 
     - any tender offer or exchange offer that if consummated would result in
       any person beneficially owning 15% or more of any class of equity
       securities of SierraWest or any significant subsidiary of SierraWest
       (other than pursuant to the transactions contemplated by the merger
       agreement and the stock option agreement).
 
                                       40
<PAGE>   46
 
     FILINGS AND OTHER ACTIONS.  In the merger agreement, BancWest and
SierraWest have each agreed to use all reasonable efforts:
 
     - to take all actions necessary to comply promptly with all legal
       requirements which may be imposed on such party or its subsidiaries with
       respect to the transactions contemplated by the merger agreement, and
 
     - to obtain (and to cooperate with the other party to obtain) any
       governmental or private consent, authorization, order, exemption or
       approval which is required to be obtained or made by such party or any of
       its subsidiaries in connection with the merger and the other transactions
       contemplated by the merger agreement.
 
     In addition, each of SierraWest and BancWest has agreed to use all
reasonable best efforts to take all actions, and to do, all things necessary and
proper or advisable to complete, as soon as practicable, the transactions
contemplated by the merger agreement, including using all reasonable best
efforts to:
 
     - lift or rescind any injunction or restraining order or other order
       adversely affecting the ability of the parties to complete the merger
       according to the merger agreement,
 
     - defend any litigation seeking to enjoin, prevent or delay the completion
       of the merger according to the merger agreement or seeking material
       damages, and
 
     - provide to counsel to the other party representations and certifications
       as to such matters as such counsel may reasonably request in order to
       render the respective tax opinions described under "-- Conditions to the
       Completion of the Merger" and to obtain the letters of their independent
       accountants regarding pooling of interests accounting treatment.
 
     EMPLOYEE BENEFIT PLANS.  For a description of the effect of the merger on
the SierraWest employee benefit plans, see "THE MERGER -- Effect on SierraWest's
Employee Benefits Plans."
 
     INDEMNIFICATION; DIRECTORS' AND OFFICERS' INSURANCE.  For a period of four
years after the effective time of the merger, BancWest has agreed to maintain in
effect the current policies of directors' and officers' liability insurance
maintained by SierraWest and its subsidiaries (provided that BancWest may
substitute policies of at least the same coverage and amounts containing terms
and conditions which are not materially less advantageous in the aggregate) with
respect to claims arising from facts or events which occurred before the
effective time of the merger. However, BancWest will not be obligated to make
annual premium payments for such insurance to the extent such premiums exceed
150% of the premiums paid by SierraWest and SierraWest Bank in respect of 1998
for such insurance, as previously disclosed to BancWest. If the premiums for
such insurance would at any time exceed 150% of the premiums paid by SierraWest
and SierraWest Bank in respect of 1998 for such insurance, then BancWest will
maintain policies of insurance which, in BancWest's good faith determination,
provide the maximum coverage available at an annual premium equal to 150% of the
premiums paid by SierraWest and SierraWest Bank in respect of 1998 for such
insurance.
 
                                       41
<PAGE>   47
 
CONDITIONS TO THE COMPLETION OF THE MERGER
 
     Each party's obligation to complete the merger is subject to various
conditions which include, in addition to the other customary closing conditions,
the following:
 
     - SierraWest shareholders shall have approved the merger agreement,
 
     - all necessary governmental approvals for the merger shall been obtained
       and any waiting periods required by any governmental entity with respect
       to the merger shall have expired,
 
     - there shall not be any injunction or restraining order preventing the
       completion of the merger or the transactions contemplated by the merger
       agreement, stock option agreement and agreement of merger, nor shall the
       merger or the transactions contemplated by the merger agreement, stock
       option agreement and agreement of merger be illegal under any applicable
       law,
 
     - the shares of BancWest common stock to be issued to the shareholders of
       SierraWest in the merger shall have been approved for listing upon
       official notice of issuance on the New York Stock Exchange,
 
     - no stop orders suspending the effectiveness of the registration statement
       filed by BancWest to register the shares to be issued to SierraWest
       shareholders shall have been issued, and
 
     - receipt of reasonably satisfactory opinions of independent public
       accountants to both BancWest and SierraWest to the effect that the merger
       will qualify for "pooling of interests" accounting treatment.
 
     In addition, each party's obligation to complete the merger is subject to
the following additional conditions, any of which may be waived by such party:
 
     - the representations and warranties of the other party set forth in the
       merger agreement shall be true and correct in all material respects as of
       the date of the merger agreement (except to the extent such
       representations and warranties speak as of an earlier date) and as of the
       closing date,
 
     - the other party shall have performed in all material respects all of its
       obligations under the merger agreement,
 
     - such party shall have received a copy of the resolution or resolutions
       duly approved by the board of directors (or a duly authorized committee
       thereof) of the other party and in the case of BancWest, of the holders
       of the SierraWest common stock, authorizing the execution, delivery and
       performance by such party of the merger agreement,
 
     - such party shall have received an opinion of its counsel, dated the
       closing date, that the merger will be treated for federal income tax
       purposes as a reorganization within the meaning of Section 368(a) of the
       Internal Revenue Code and each party will be a party to that
       reorganization within the meaning of Section 368(b) of the Internal
       Revenue Code, and
 
     - the rights issued under the SierraWest rights agreement shall not have
       become nonredeemable, exercisable, distributed or triggered pursuant to
       the terms of such agreement.
 
                                       42
<PAGE>   48
 
     In addition, BancWest's obligation to complete the merger is subject to the
following additional conditions, any of which may be waived by BancWest:
 
     - in approving the merger agreement, no governmental authority shall have
       imposed a burdensome condition or restriction upon BancWest or its
       subsidiaries or any affiliate which would reasonably be expected to (1)
       have a material adverse effect after the effective time of the merger on
       the present or prospective consolidated financial condition, business,
       operating results or prospects of BancWest or the surviving corporation,
       including, without limitation, any requirement to dispose of any material
       assets or businesses or restrict in any significant way any material
       operations or activities, (2) prevent BancWest or its subsidiaries from
       realizing all or a substantial portion of the economic benefits of the
       transactions contemplated by the merger agreement, or (3) materially
       impair BancWest's ability to exercise and enforce its rights under the
       merger agreement, stock option agreement and agreement of merger,
 
     - SierraWest shall have provided to BancWest a review report prepared in
       accordance with the provisions of Statement on Auditing Standards No. 71,
       Interim Financial Information, by SierraWest's independent accountants
       covering SierraWest's quarterly financial report for the most recent
       quarter ending at least 45 days prior to the closing date,
 
     - all conditions, including required regulatory approvals, required to be
       satisfied prior to the completion of the merger of SierraWest Bank with
       and into Bank of the West shall have been satisfied and such merger shall
       be able to be consummated immediately following the effective time of the
       merger of SierraWest with and into Bank of the West, and
 
     - BancWest shall have determined to its satisfaction that no holders of
       SierraWest common stock are entitled to have their shares treated as
       "dissenting shares."
 
TERMINATION
 
     The merger agreement may be terminated at any time before the effective
time of the merger. If the merger agreement is terminated, the merger will not
occur. The merger agreement may be terminated by action taken or authorized by
the board of directors of the terminating party or parties whether before or
after the approval of the merger agreement by the shareholders of SierraWest:
 
     - by mutual consent of BancWest and SierraWest in a written instrument,
 
     - by either party upon written notice to the other party if any bank
       regulator shall have issued an order denying approval of the merger and
       the other material aspects of the transactions contemplated by the merger
       agreement or if any governmental entity with authority to do so shall
       have issued a final permanent order enjoining or otherwise prohibiting
       the completion of the merger in accordance with the merger agreement, and
       the time for appeal or petition for reconsideration of any such order
       shall have expired without such appeal or petition being granted,
 
     - by either party if the merger shall not have been completed on or before
       September 30, 1999, except that if the merger shall not been completed on
       or before such date due to the act or omission of BancWest or SierraWest,
       then that party may not terminate the merger agreement under this
       provision,
 
                                       43
<PAGE>   49
 
     - by either party if the approval of the merger agreement by the SierraWest
       shareholders is not obtained,
 
     - by BancWest if any governmental entity with authority to do so shall have
       issued an order in connection with the transactions under the merger
       agreement imposing a burdensome condition on BancWest or the surviving
       corporation, and the time for appeal or petition for reconsideration of
       any such order shall have expired without such appeal or petition being
       granted,
 
     - by BancWest, in the event of a breach by SierraWest of any
       representation, warranty or covenant contained in the merger agreement,
       which breach (1) either is not cured within 45 days after the giving of
       written notice to SierraWest, or is of a nature which cannot be cured
       prior to the closing and (2) would entitle the non-breaching party to
       elect not to complete the merger in accordance with the merger agreement
       under the provisions described under "-- Conditions to the Completion of
       the Merger;" provided, however, that BancWest may immediately terminate
       the merger agreement upon notice to SierraWest in the event that
       SierraWest shall breach the covenant described under "-- Additional
       Agreements -- No Solicitations,"
 
     - by BancWest, if the board of directors of SierraWest (1) fails to
       recommend approval of the merger agreement by the shareholders of
       SierraWest, (2) amends or modifies such recommendation in a manner
       materially adverse to BancWest, or (3) withdraws such recommendation to
       the shareholders of SierraWest,
 
     - by SierraWest, in the event of a breach by BancWest of any
       representation, warranty or covenant contained in the merger agreement,
       which breach (1) either is not cured within 45 days after the giving of
       written notice to SierraWest, or is of a nature which cannot be cured
       prior to the closing and (2) would entitle the non-breaching party to
       elect not to complete the merger in accordance with the merger agreement
       under the provisions described under "-- Conditions to the Completion of
       the Merger," or
 
     - by SierraWest, if SierraWest's board of directors so determines by a vote
       of a majority of the members of its entire board, at any time during the
       two business day period beginning on the first business day after the
       date on which FDIC approves the merger, if:
 
        (1) the average of the closing prices of BancWest common stock on the
            New York Stock Exchange for the 20 consecutive trading days ending
            on the date on which the FDIC approves the merger (the "Average
            BancWest Closing Price") is less than 85% of the average of closing
            prices of BancWest common stock on the New York Stock Exchange for
            the 5 consecutive trading days immediately before the public
            announcement of the merger (or 85% of $43.8375), and
 
        (2) the ratio of the Average BancWest Closing Price to $43.8375 (the
            "BancWest Ratio") is less than 85% of the ratio of the average of
            the Standard & Poor's Mid-Cap Regional Bank Index as published by
            Bloomberg Financial Markets for the 20 consecutive trading days
            ending on the date on which the FDIC approves the merger (the "Final
            Index Price") to the average of the bank index on the 5 consecutive
            trading days immediately before the public announcement of the
            merger (or $223.854) (the "Index Ratio").
 
                                       44
<PAGE>   50
 
However, during the five business day period beginning on the day after receipt
of notice of election by SierraWest to terminate under this provision, BancWest
will have the option to adjust the exchange ratio to equal the lesser of:
 
<TABLE>
    <S>  <C>
    -        0.85 X $43.8375 X 0.82
         ------------------------------
         Average BancWest Closing Price
                       or
    -          Index Ratio X 0.82
             ---------------------
                 BancWest Ratio
</TABLE>
 
     If BancWest elects to so adjust the exchange ratio, then the right of
SierraWest to terminate the merger will end.
 
     In other words, SierraWest will not have to option to terminate the merger
agreement unless, for the applicable period, the BancWest stock price falls
below $37.2619, AND the BancWest stock price declines more than 15% in relation
to the bank stock index. If BancWest chooses to adjust the exchange ratio, then
the value of the BancWest shares you receive in the merger will not have
declined more than 15% in relation to any decline in the bank stock index. In
the absence of an adjustment to the exchange ratio, the bank stock index will
have no effect on the value of the BancWest shares you receive.
 
     For example, if the Average BancWest Closing Price is $34.00 and the Final
Index Price is $215.00, the ratio of $34 to $43.8375 (.7756) is less than 85% of
the ratio of $215.00 to $223.854 (.8164). As a result, SierraWest would have the
right to terminate the merger agreement. In such situation, BancWest would have
the option to adjust the exchange ratio to equal the lesser of:
 
<TABLE>
    <S>  <C>                     <C>
    -    0.85 X $43.8375 X 0.82
         ----------------------  = 0.8987
                  $34
                   or
    -        0.8164 X 0.82
            ---------------      = 0.8631
                 0.7756
</TABLE>
 
     In this example, the new exchange ratio would be 0.8631.
 
FEES AND EXPENSES
 
     Whether or not the merger is completed in accordance with the merger
agreement, all costs and expenses incurred in connection with the merger
agreement and the transactions covered by the merger agreement will be paid by
the party incurring such expense.
 
AMENDMENT
 
     The merger agreement may be amended by the parties at any time before or
after approval of the merger agreement by the shareholders of SierraWest.
However, after the approval by the shareholders of SierraWest, no amendment
shall be made which by law requires further approval by such shareholders
without such further approval. The merger agreement may not be amended except by
an instrument in writing signed on behalf of each of the parties.
 
                                       45
<PAGE>   51
 
EXTENSION; WAIVER
 
     At any time prior to the closing of the merger, the parties, by action
taken or authorized by their respective board of directors, may, to the extent
legally allowed, (1) extend the time for the performance of any of the
obligations or other acts of the other parties, (2) waive any inaccuracies in
the representations and warranties contained in the merger agreement or in any
document delivered pursuant to it, and (3) waive compliance with any of the
agreements or conditions contained in the merger agreement. To "waive" means to
give up rights.
 
     Any agreement on the part of a party to the merger agreement to any such
extension or waiver shall be valid only if set forth in a written instrument
signed on behalf of such party.
 
                                       46
<PAGE>   52
 
                             STOCK OPTION AGREEMENT
                        BETWEEN SIERRAWEST AND BANCWEST
 
     Concurrently with the execution and delivery of the merger agreement,
SierraWest and BancWest entered into a stock option agreement, under which
SierraWest granted BancWest an option to purchase up to 1,059,490 shares of
SierraWest common stock representing approximately 19.9% of the outstanding
shares of SierraWest common stock at a per share price equal to $28.875. The
option will only become exercisable upon the occurrence of certain events
described below.
 
     The following is a summary of the material provisions of the stock option
agreement, which is attached as Appendix B to this to proxy statement-prospectus
and incorporated herein by reference. You are urged to read the stock option
agreement in its entirety.
 
EXERCISE OF STOCK OPTION
 
     BancWest may elect to exercise the option in whole or in part only after
the occurrence of one of the following events (each a "Purchase Event"):
 
     - SierraWest or any of its significant subsidiaries, without BancWest's
       prior written consent, recommends, publicly announces an intention to
       recommend, or enters into an agreement with any person (other than
       BancWest or any subsidiary of BancWest) to effect (1) a merger,
       consolidation or similar transaction involving SierraWest or any of its
       significant subsidiaries, (2) the disposition, by sale, lease, exchange
       or otherwise, of assets or deposits of SierraWest or any of its
       significant subsidiaries representing 15% or more of the consolidated
       assets or deposits of SierraWest and its subsidiaries, or (3) the
       issuance, sale, or other disposition by SierraWest of securities
       representing 15% or more of the voting power of SierraWest or any of its
       significant subsidiaries (any of the foregoing is defined as an
       "Acquisition Transaction"),
 
     - any person or group of persons (other than BancWest or any subsidiary of
       BancWest) acquires beneficial ownership (as such term is defined in Rule
       13d-3 under the Securities Exchange Act) or the right to acquire
       beneficial ownership of 15% or more of the voting power of SierraWest or
       any of its significant subsidiaries,
 
     - any person commences a tender offer or files a registration statement
       under the Securities Act with respect to an exchange offer for SierraWest
       common stock such that, upon completion of that offer, that person will
       own or control 15% or more of the then outstanding shares of SierraWest
       common stock, or
 
     - the SierraWest shareholders have not approved the merger agreement, the
       annual meeting of shareholders of SierraWest has not been held or has
       been canceled or the SierraWest board has withdrawn or modified in a
       manner adverse to BancWest its recommendation with respect to the merger
       agreement, in each case after any person (other than BancWest or any
       subsidiary of BancWest) has (1) made, or disclosed an intention to make,
       a bona fide proposal, to engage in an Acquisition Transaction, (2)
       commenced a tender offer or filed a registration statement under the
       Securities Act with respect to an exchange offer, or (3) filed an
       application (or given a notice) with a bank regulatory authority to
       engage in an Acquisition Transaction.
 
                                       47
<PAGE>   53
 
TERMINATION OF STOCK OPTION
 
     The option will terminate and be of no further force or effect upon the
earliest to occur of:
 
     - the effective time of the merger,
 
     - termination of the merger agreement in accordance with its terms before a
       Purchase Event or a Preliminary Purchase Event (as defined below) occurs,
       or
 
     - 12 months after the termination of the merger agreement after a Purchase
       Event or a Preliminary Purchase Event occurs.
 
Even if the option terminates, BancWest will still be entitled to purchase those
SierraWest shares with respect to which it exercised the option before the
termination of the option.
 
     A "Preliminary Purchase Event" means the occurrence of one of the following
events:
 
     - any person (other than BancWest or any subsidiary of BancWest) makes a
       bona fide proposal to SierraWest or its shareholders by public
       announcement, or written communication that is or becomes the subject of
       public disclosure, to engage in an Acquisition Transaction,
 
     - after a proposal is made by a third party to SierraWest or its
       shareholders to engage in an Acquisition Transaction, or a third party
       states its intention to SierraWest to make such a proposal if the merger
       agreement terminates, SierraWest breaches any representation, warranty,
       covenant or agreement contained in the merger agreement, which breach
       would entitle BancWest to terminate the merger agreement,
 
     - any person (other than BancWest or any subsidiary of BancWest) other than
       in connection with a transaction to which BancWest has given its prior
       written consent, shall have filed an application or notice with any
       governmental entity for approval to engage in an Acquisition Transaction,
       or
 
     - SierraWest's board of directors (1) fails to recommend approval of the
       merger agreement, (2) amends or modifies its recommendation in a manner
       materially adverse to BancWest, or (3) withdraws it recommendation to the
       shareholders of SierraWest.
 
ADJUSTMENT OF NUMBER OF SHARES SUBJECT TO OPTION
 
     The number and type of securities subject to the option and the purchase
price of shares will be adjusted for any stock split, reverse split, dividend,
exchange of shares or similar transaction relating to the SierraWest common
stock, so that BancWest will receive upon exercise of the option the same number
and type of securities as if the option had been exercised immediately before
the change in SierraWest common stock. The number of shares of SierraWest common
stock subject to the option will also be adjusted if SierraWest issues
additional shares of SierraWest common stock, so that the number of shares of
SierraWest common stock subject to the option represents 19.9% of issued and
outstanding SierraWest common stock. In the event of a capital reorganization,
merger or consolidation of SierraWest with or into another corporation, or the
sale of all or substantially all of SierraWest's assets to any other person,
then, as a part of any such transaction, provision will be made so that BancWest
will be entitled to receive an option of the succeeding corporation, any person
that controls the succeeding corporation or SierraWest, at the election of
BancWest.
 
                                       48
<PAGE>   54
 
REPURCHASE OF OPTION SHARES
 
     During the 18-month period after a Repurchase Event (as defined below)
occurs, BancWest can require SierraWest to repurchase (1) the option, and (2)
the shares of SierraWest common stock received by BancWest upon exercise of the
option by BancWest. In addition, BancWest has the same right of repurchase at
its request for the 30 days after (1) BancWest receives official notice that an
approval of a governmental entity required for the purchase of the option shares
will not be granted, or (2) the closing date of its purchase of option shares
does not occur within 18 months after the date on which BancWest delivered
notice to exercise to SierraWest.
 
     The shares will be repurchased at a price equal to the sum of:
 
     - the aggregate purchase price paid by BancWest for any shares of
       SierraWest common stock acquired under the option with respect to which
       BancWest then has beneficial ownership;
 
     - the excess, if any, of (x) the Applicable Price (as defined below) for
       each share of SierraWest common stock over (y) $28.875 (subject to any
       adjustments), multiplied by the number of shares of SierraWest common
       stock with respect to which the option has not been exercised; and
 
     - the excess, if any, of the Applicable Price over $28.875 (subject to any
       adjustments) paid (or, in the case of option shares with respect to which
       the option has been exercised but the closing date has not occurred,
       payable) by BancWest for each share of SierraWest common stock with
       respect to which the option has been exercised and with respect to which
       BancWest then has beneficial ownership, multiplied by the number of such
       shares.
 
     A "Repurchase Event" will occur if:
 
     - any person (other than BancWest or any subsidiary of BancWest) or group
       of persons acting in concert acquires beneficial ownership of, 25% or
       more of the then outstanding shares of SierraWest common stock, or
 
     - SierraWest enters into an agreement in connection with any capital
       reorganization, merger or consolidation of SierraWest with or into
       another corporation, or the sale of all or substantially all of
       SierraWest's assets to any other person.
 
     The term "Applicable Price" means the highest of:
 
     (1) the highest price per share of SierraWest common stock paid by any
         person in connection with a Repurchase Event, or
 
     (2) the highest closing sales price per share of SierraWest common stock
         quoted on the Nasdaq National Market System during the 40 business days
         before the request date,
 
provided, however, that in the event of a sale of less than all of SierraWest's
assets, the Applicable Price will be the sum of the price paid in such sale for
such assets and the current market value of the remaining assets of SierraWest
as determined by a nationally recognized investment banking firm selected by
BancWest, divided by the number of shares of the SierraWest common stock
outstanding at the time of such sale.
 
                                       49
<PAGE>   55
 
REGISTRATION RIGHTS
 
     BancWest has certain rights to require registration of any shares of
SierraWest common stock purchased pursuant to the stock option agreement under
the securities laws if necessary to enable BancWest to sell such shares.
 
EFFECT OF STOCK OPTION AGREEMENT
 
     The stock option agreement is intended to increase the likelihood that the
merger will be completed on the terms set forth in the merger agreement. As a
result, certain aspects of the stock option agreement may have the effect of
discouraging persons who might now or before the effective time of the merger be
interested in acquiring all of or a significant interest in SierraWest from
considering or proposing such an acquisition, even if they were prepared to
offer higher consideration per share for SierraWest common stock than the
consideration set forth in the merger agreement.
 
                                       50
<PAGE>   56
 
                        OPERATIONS FOLLOWING THE MERGER
 
     Although we cannot assure you that any specific level of cost savings will
be achieved or as to the timing thereof, BancWest currently expects the
surviving corporation to achieve approximately $8.75 million and $12.25 million
in pre-tax annual cost savings in the first and second 12-month periods
following the completion of the merger, respectively, as a result of the merger.
It is also estimated that one-time, after-tax restructuring and other merger
related costs of approximately $6.8 million will be incurred upon completion of
the merger, principally as a result of severance costs, consolidation of
branches and investment banking, legal and accounting fees.
 
                                       51
<PAGE>   57
 
                              BANCWEST CORPORATION
 
     BancWest, a Delaware corporation, is a registered bank holding company
under the Bank Holding Company Act of 1956. As a bank holding company, BancWest
is allowed to acquire or invest in the securities of companies that are engaged
in banking or in activities closely related to banking as authorized by the
Board of Governors of the Federal Reserve System (the "Federal Reserve Board").
BancWest, through its subsidiaries, operates a general commercial banking
business and other businesses related to banking. Its principal assets are its
investments in:
 
     - First Hawaiian Bank, a State of Hawaii chartered bank,
 
     - Bank of the West, a State of California chartered bank with authority to
       operate interstate branches in Oregon, Washington and Idaho, and
 
     - First Hawaiian Capital I (the "Trust"), a Delaware business trust.
 
First Hawaiian, Bank of the West, and the Trust are wholly-owned subsidiaries of
BancWest. At December 31, 1998, BancWest had consolidated total assets of $15.0
billion, total deposits of $11.3 billion and total stockholders' equity of $1.7
billion.
 
FIRST HAWAIIAN, INC./BANCWEST CORPORATION MERGER
 
     On November 1, 1998, the former BancWest Corporation ("Old BancWest"),
parent company of Bank of the West, merged with and into First Hawaiian, Inc.
Upon completion of the merger, First Hawaiian, Inc., the surviving corporation,
changed its name to "BancWest Corporation." Prior to the completion of the
merger, Old BancWest was wholly-owned by Banque Nationale de Paris ("BNP"). BNP
(and its affiliate, French American Banking Corporation) received approximately
25.8 million shares of BancWest's newly-authorized Class A Common Stock,
representing approximately 45% of the then outstanding total voting stock of
BancWest, after the merger (a purchase price of approximately $905.7 million).
As a result of the merger, Bank of the West is now a wholly-owned subsidiary of
BancWest.
 
     The merger of Old BancWest into First Hawaiian, Inc. represented a unique
opportunity for First Hawaiian. The merger transformed First Hawaiian into a
diversified regional financial services institution large enough to compete in
the modern banking environment, and yet able to retain its community banking
focus on individual service.
 
FIRST HAWAIIAN BANK
 
     First Hawaiian Bank, the oldest financial institution in Hawaii, was
established as Bishop & Co. in 1858 in Honolulu. First Hawaiian is a State of
Hawaii-chartered bank that is not a member of the Federal Reserve System. The
deposits of First Hawaiian are insured by the Bank Insurance Fund and the
Savings Association Insurance Fund of the FDIC to the extent and subject to the
limitations set forth in the Federal Deposit Insurance Act.
 
     First Hawaiian Bank, is a full-service bank conducting a general commercial
and consumer banking business and offering trust and insurance services. Its
banking activities include receiving demand, savings and time deposits for
personal and commercial accounts, making commercial, agricultural, real estate
and consumer loans, acting as a United States tax depository facility, providing
money transfer and cash management services, selling cash management services,
insurance products, mutual funds and annuities, traveler's checks and personal
money orders, issuing letters of credit, handling domestic and foreign
 
                                       52
<PAGE>   58
 
collections, providing safe deposit and night depository facilities, offering
lease financing, and investing in U.S. Treasury securities and securities of
other U.S. government agencies and corporations and state and municipal
securities.
 
     At December 31, 1998, First Hawaiian Bank had total assets of $7.2 billion
and total deposits of $5.5 billion, making it the second largest bank in Hawaii.
 
     On June 19, 1998, First Hawaiian Creditcorp, Inc., a former wholly-owned
subsidiary of BancWest, was merged with and into First Hawaiian Bank. As a
result of the merger, all 13 Creditcorp branches were closed.
 
DOMESTIC SERVICES
 
     The domestic operations of First Hawaiian are carried out through its main
banking office located in Honolulu, Hawaii, with 55 other banking offices
located throughout the State of Hawaii. All but one of the banking offices are
equipped with automatic teller machines that provide 24-hour service to
customers wishing to make withdrawals from and deposits to their personal
checking and savings accounts, to make balance inquiries, to obtain interim bank
statements and to make utility and loan payments. Sixty-three automatic teller
machines at nonbranch locations provide balance inquiry, withdrawal transaction
and account transfer services. At selected non-branch locations, interim bank
statements are also available. First Hawaiian is a member of the
CIRRUS(R)/MasterCard(R), Plus(R)/VISA(R) and Star System(R), AFFN(R), American
Express(R), Discover(R) and JCB(R) automatic teller machine networks which
provide First Hawaiian's customers with access to their funds nationwide and in
selected foreign countries.
 
LENDING ACTIVITIES
 
     First Hawaiian engages in a broad range of lending activities, including
making real estate, commercial and consumer loans. At December 31, 1998, First
Hawaiian's loans totaled $5.6 billion, representing 76.7% of total assets. At
that date, 47.2% of the loans were construction, commercial and residential real
estate loans, 27.6% were commercial loans, 13.0% were consumer loans, 6.8% were
foreign loans and 5.4% were leases.
 
     REAL ESTATE LENDING -- CONSTRUCTION.  First Hawaiian provides construction
financing for a variety of commercial and residential single-family subdivision
and multi-family developments. At December 31, 1998, 3.4% of First Hawaiian's
total real estate loans were collateralized by properties under construction.
 
     REAL ESTATE LENDING -- COMMERCIAL.  First Hawaiian provides permanent
financing for a variety of commercial developments, such as various retail
facilities, warehouses and office buildings. At December 31, 1998, 36.1% of
First Hawaiian's total real estate loans were collateralized by commercial
properties.
 
     REAL ESTATE LENDING -- RESIDENTIAL.  First Hawaiian makes residential real
estate loans, including home equity loans to enable borrowers to purchase,
refinance or improve residential real property. The loans are collateralized by
mortgage liens on the related property, substantially all of which is located in
Hawaii. At December 31, 1998, 60.5% of First Hawaiian's total real estate loans
were collateralized by single-family and multi-family residences.
 
     COMMERCIAL LENDING.  First Hawaiian is a major lender to primarily
small-and medium-sized businesses in Hawaii. First Hawaiian also participates in
syndication lending to good credit quality large corporate entities and to the
media and telecommunications industry located on the mainland United States.
 
                                       53
<PAGE>   59
 
     CONSUMER LENDING.  First Hawaiian offers many types of loans and credits to
consumers. First Hawaiian provides lines of credit, uncollateralized or
collateralized, and provides various types of personal and automobile loans.
First Hawaiian also provides indirect consumer automobile financing on new and
used autos by purchasing finance contracts from dealers. First Hawaiian's Dealer
Center is the largest commercial bank automobile lender in the State of Hawaii.
First Hawaiian is the largest issuer of MasterCard(R) credit cards and the
second largest issuer of VISA(R) credit cards in Hawaii.
 
AFFORDABLE HOUSING INITIATIVES
 
     In an effort to support affordable housing and as part of First Hawaiian's
community reinvestment program, First Hawaiian is a member of the Hawaii
Community Reinvestment Corporation (the "HCRC"). The HCRC is a consortium of
local financial institutions that provides $50 million in permanent long-term
financing for affordable housing rental projects throughout Hawaii for low- and
moderate-income residents.
 
     The $50 million loan pool is funded by the member financial institutions
which participate pro rata (based on deposit size) in each HCRC loan. First
Hawaiian's participations in these HCRC loans are included in its loan
portfolio.
 
     To further enhance First Hawaiian's community reinvestment program and
provide support for the development of additional affordable housing rental
units in Hawaii, First Hawaiian, and other HCRC member institutions, have
subscribed to a $19.7 million tax credit equity fund ("Hawaii Affordable Housing
Fund I"). First Hawaiian and other HCRC members have also subscribed to a $20.0
million tax credit equity fund ("Hawaii Affordable Housing Fund II").
 
     Hawaii Affordable Housing Fund I and Hawaii Affordable Housing Fund II have
been established to invest in qualified low-income housing tax credit rental
projects and to ensure that these projects are maintained as low-income housing
throughout the required compliance period. First Hawaiian's investments in the
Funds are included in its investment portfolio.
 
INTERNATIONAL BANKING SERVICES
 
     First Hawaiian maintains an International Banking Division which provides
international banking products and services through First Hawaiian's branch
system, international banking headquarters in Honolulu, a Grand Cayman branch,
two Guam branches, a branch in Saipan and a representative office in Tokyo,
Japan. First Hawaiian maintains a network of correspondent banking relationships
throughout the world.
 
     First Hawaiian's international banking activities are primarily
trade-related and are concentrated in the Asia-Pacific area.
 
TRUST, INVESTMENT AND INSURANCE SERVICES
 
     TRUST SERVICES.  First Hawaiian's Trust and Investments Division offers a
full range of trust and investment management services. The Trust and
Investments Division provides asset management, advisory and administrative
services for estates, trusts and individuals. It also acts as trustee and
custodian of retirement and other employee benefit plans. At December 31, 1998,
the Trust and Investments Division had 5,696 accounts with a market value of
$9.7 billion. Of this total, $7.0 billion represented assets in nonmanaged
accounts and $2.7 billion were managed assets.
 
                                       54
<PAGE>   60
 
     The Trust and Investments Division maintains custodial accounts pursuant to
which it acts as agent for customers in rendering a variety of services,
including dividend and interest collection, collection under installment
obligations and rent collection.
 
     INVESTMENT AND INSURANCE SERVICES.  First Hawaiian, through a wholly-owned
subsidiary, offers insurance needs analysis for individuals, families and
businesses as well as insurance products such as life, disability and long-term
care. In association with an independent registered broker-dealer, First
Hawaiian offers mutual funds, annuities and other securities in its branches.
 
BANK OF THE WEST
 
     Bank of the West is a State of California-chartered bank that is not a
member of the Federal Reserve System. The deposits of the Bank of the West are
insured by the Bank Insurance Fund of the FDIC to the extent and subject to the
limitations set forth in the Federal Deposit Insurance Act. The predecessor of
Bank of the West, "The Farmers National Gold Bank", was chartered as a national
banking association in 1874 in San Jose, California.
 
     On November 1, 1998, Pacific One Bank, a former wholly-owned subsidiary of
First Hawaiian, Inc., was merged with and into Bank of the West. As a result of
the merger, 40 Pacific One Bank branches in Oregon, Washington and Idaho became
branches of Bank of the West.
 
     Bank of the West is the fifth largest commercial bank in the State of
California, with total assets of approximately $7.7 billion and total deposits
of approximately $5.8 billion at December 31, 1998. Bank of the West conducts a
general commercial banking business, providing retail and corporate banking and
trust services to individuals, institutions, businesses and governments through
146 branches and other commercial banking offices located primarily in the San
Francisco Bay area and elsewhere in the Northern and Central Valley regions of
California and in Oregon, Washington and Idaho. Bank of the West also generates
indirect automobile loans and leases, recreational vehicle loans, recreational
marine vessel loans, equipment leases and deeds of trust on single family
residences through a network of manufacturers, dealers, representatives and
brokers in all 50 states. Bank of the West's principal subsidiary is Essex
Credit Corporation ("Essex"), a Connecticut corporation. Essex is engaged
primarily in the business of originating and selling consumer loans on a
nationwide basis, such loans being made for the purpose of acquiring or
refinancing pleasure boats or recreational vehicles. Essex has a network of 11
regional direct lending offices located in the following states: California,
Connecticut, Florida, Maryland, Massachusetts, New Jersey, New York, North
Carolina, Texas and Washington.
 
OPERATING DIVISIONS
 
     COMMUNITY BANKING.  The focus of Bank of the West's community banking
strategy has been Northern California and now, with the merger with Pacific One
Bank, the Pacific Northwest region. The Northern California market region is
comprised of the San Francisco Bay area and the Central Valley area of
California. This market area includes a population of approximately 14 million
residents, with approximately 7 million in the San Francisco Bay area and
approximately 4 million in the Central Valley as of December 31, 1998. The San
Francisco Bay area is one of the State of California's wealthiest regions, and
the Central Valley of California is an area which has been experiencing rapid
transition from a largely agricultural base to a mix of agricultural and
commercial
 
                                       55
<PAGE>   61
 
enterprises. The Pacific Northwest region includes the states of Oregon,
Washington and Idaho.
 
     Bank of the West utilizes its 146 branch network as its principal funding
source. A key element of Bank of the West's community banking strategy is to
seek to distinguish itself as the provider of the "best value" in community
banking services. To this end, Bank of the West seeks to position itself within
its markets as an alternative to both the higher priced, smaller "boutique"
commercial banks as well as the larger commercial banks, which may be perceived
as offering lower service and lower prices on a "mass market" basis.
 
     In pursuing the Northern California and Pacific Northwest community banking
markets, Bank of the West seeks to serve a broad customer base by furnishing a
range of retail and commercial banking products. Through its branch network,
Bank of the West generates a variety of consumer loans, including direct vehicle
loans, consumer lines of credit and second mortgages. In addition, Bank of the
West generates and holds a small portfolio of first mortgage loans on
one-to-four family residences. Through its commercial banking operations
conducted from its branch network, Bank of the West offers a wide range of basic
commercial banking products that are intended to serve the needs of smaller
community-based businesses. These loan products include in-branch originations
of standardized products for businesses with relatively simple banking and
financing needs. More complex and customized commercial banking services are
offered through Bank of the West's regional banking centers which serve clusters
of branches and provide lending, deposit and cash management services to
companies operating in the relevant market areas. Bank of the West also provides
a number of fee-based products and services such as annuities, insurance and
securities brokerage.
 
     The Professional Banking and Trust & Investment Services areas within the
Community Banking division provide a wide range of products to targeted markets.
Professional Banking, located in San Francisco, serves the banking needs of
attorneys, doctors and other working professionals. The Trust & Investment
Services area, headquartered in San Jose, and with offices in San Francisco,
provides a full range of individual and corporate trust services.
 
     Another important element in Bank of the West's strategic plan for its
community banking operations is an effective promotion strategy which seeks to
position Bank of the West as "the Northern California Community Bank."
Television and radio advertising is an important component of developing Bank of
the West's community banking image and customer acceptance. In furtherance of
this objective, Bank of the West sponsors special events such as the "Bank of
the West Classic" professional women's tennis tournament held annually in Palo
Alto, California.
 
     COMMERCIAL BANKING.  Bank of the West's Business Banking division supports
commercial lending activities for larger business customers through ten regional
lending centers, six of which are located in Northern and Central California,
two of which are located in Oregon, and one each in Washington and Idaho. Each
regional office provides a wide range of loan and deposit services to mid-sized
companies with borrowing needs of $500,000 to $25 million. Lending services
include receivable and inventory financing, equipment term loans, letters of
credit, agricultural loans and trade finance. Other banking services include
cash management, insurance products, trust, investment, foreign exchange and
various international banking services.
 
                                       56
<PAGE>   62
 
     The Specialty Lending division seeks to provide focused banking services
and products to specifically targeted markets where Bank of the West's
resources, experience and technical expertise give it a competitive advantage.
Through operations conducted in this division, Bank of the West has established
a significant national market niche among those commercial banks which are
lenders to religious organizations. In addition, leasing operations within
Specialty Lending have made Bank of the West a significant provider of equipment
leasing financing, including both standard and tax-oriented products, to a wide
array of clients. To support the cash management needs of both Bank of the
West's corporate banking customers and large private and public deposit
relationships maintained with Bank of the West, the Specialty Lending division
operates a Cash Management group which provides a full range of innovative and
relationship-focused cash management services.
 
     The Real Estate Industries division, whose primary markets are Northern and
Central California and Nevada, originates and services construction, short-term
and permanent loans to residential developers, commercial builders and
investors. The division is particularly active in financing the construction of
detached residential subdivisions in Northern California. Other construction
lending activities include low-income housing, industrial development,
apartment, retail and office projects. The division also originates and services
single-family home loans sourced through Bank of the West's Community Bank
branch network.
 
     CONSUMER FINANCE.  The Consumer Finance division targets the production of
auto loans and leases in the Western United States, and recreational vehicle and
marine loans nationwide, with a business emphasis on originating credits at the
high end of the credit spectrum. The Consumer Finance division originates
recreational vehicle and marine credits on a nationwide basis through sales
representatives located throughout the country servicing a network of over 1,900
recreational vehicle and marine dealers and brokers. During the fourth quarter
of 1997, Bank of the West acquired Essex to complement its dealer marine and
recreational vehicle presence. Essex primarily focuses on the origination and
sale of loans in the broker marine market and also originates and sells loans to
finance the acquisition of recreational vehicles.
 
     The division's auto lending activity is primarily focused in the Western
United States. Bank of the West originates loans and leases to finance the
purchase of new and used autos, light trucks and vans through a network of more
than 2,000 dealers and brokers in California, Nevada and Arizona.
 
FIRST HAWAIIAN CAPITAL I
 
     The Trust is a Delaware business trust which was formed in 1997. The Trust
issued $100,000,000 of its Capital Securities (the "Capital Securities") and
used the proceeds therefrom to purchase junior subordinated deferrable interest
debentures of BancWest. The Capital Securities qualify as Tier 1 Capital of
BancWest and are fully and unconditionally guaranteed by BancWest. All of the
common securities of the Trust are owned by BancWest.
 
EMPLOYEES
 
     At December 31, 1998, BancWest had 4,851 full-time equivalent employees.
First Hawaiian and Bank of the West employed 2,470 and 2,381 persons,
respectively. None are represented by any collective bargaining agreements and
relations with employees are considered excellent.
 
                                       57
<PAGE>   63
 
MONETARY POLICY AND ECONOMIC CONDITIONS
 
     The earnings and business of BancWest are affected not only by general
economic conditions (both domestic and international), but also by the monetary
policies of various governmental regulatory authorities of (1) the United States
and foreign governments and (2) international agencies. In particular,
BancWest's earnings and growth may be affected by actions of the Federal Reserve
Board in connection with its implementation of national monetary policy through
its open market operations in United States Government securities, control of
the discount rate and establishment of reserve requirements against both member
and non-member financial institutions' deposits. These actions have a
significant effect on the overall growth and distribution of loans, investments
and deposits as well as on the rates earned on loans or paid on deposits. It is
not possible to predict the effect of future changes in monetary policies upon
the operating results of BancWest.
 
COMPETITION
 
     The financial services industry is highly competitive. BancWest's
subsidiaries compete with financial services providers, such as banks, savings
and loan associations, credit unions, finance companies, mortgage banking
companies and money market and mutual fund companies.
 
     They also face increased competition from non-banking institutions such as
brokerage houses and insurance companies, as well as from financial services
subsidiaries of commercial and manufacturing companies. Many of these
competitors enjoy the benefits of advanced technology, fewer regulatory
constraints and lower cost structures.
 
     The financial services industry is likely to become even more competitive
as further technological advances enable more companies to provide financial
services. These technological advances may diminish the importance of depository
institutions and other financial intermediaries in the transfer of funds between
parties.
 
SUPERVISION AND REGULATION
 
     BancWest and its banking subsidiaries are subject to extensive regulation
by federal and state agencies. The regulation of bank holding companies and
their subsidiaries is intended primarily for the protection of depositors,
federal deposit insurance funds and the banking system as a whole and is not in
place to protect shareholders or other investors.
 
BANK HOLDING COMPANY ACTIVITIES; INTERSTATE BANKING
 
     BancWest is a bank holding company subject to the Bank Holding Company Act
and, as a result, it reports to, is registered with, and is subject to
examination by, the Federal Reserve Board which also has authority to examine
the subsidiaries of BancWest, including Bank of the West and First Hawaiian
Bank.
 
     A bank holding company is generally prohibited under the Bank Holding
Company Act from engaging in nonbanking (i.e., commercial or industrial)
activities, subject to certain exceptions. Specifically, the activities of a
bank holding company, and those companies that it controls or in which it holds
more than 5% of the voting stock, are limited to banking or managing or
controlling banks, furnishing services to its subsidiaries and such other
activities that the Federal Reserve Board determines to be so closely related to
banking as to be a "proper incident thereto." In determining whether an activity
is sufficiently related to banking, the Federal Reserve Board will consider
whether the performance of such activity by the bank holding company can
reasonably be expected to
 
                                       58
<PAGE>   64
 
produce benefits to the public (e.g., greater convenience, increased competition
or gains in efficiency) that outweigh possible adverse effects (e.g., undue
concentration of resources, decreased or unfair competition, conflicts of
interest or unsound banking practices). For additional information regarding the
Bank Holding Company Act and the powers of the Federal Reserve Board thereunder,
you should refer to BancWest's Annual Report on Form 10-K for the year ended
December 31, 1998 "Item 1. Business -- Supervision and Regulation -- Holding
Company Structure," which is incorporated by reference herein. See "WHERE YOU
CAN FIND MORE INFORMATION."
 
     Under the Riegle-Neal Interstate Banking and Branching Act (the "Interstate
Banking Act"), which became effective on September 29, 1995, a bank holding
company may acquire banks in states other than its home state, subject to any
state requirement that the bank has been organized and operating for a minimum
period of time, not to exceed five years, and the requirement that the bank
holding company not control, prior to or following the proposed acquisition,
more than 10% of the total amount of deposits of insured depository institutions
nationwide or, unless the acquisition is the bank holding company's initial
entry into the state, more than 30% of such deposits in the state (or such
lesser or greater amount set by the state).
 
     The Interstate Banking Act also authorizes banks to merge across state
lines, thereby creating interstate branches, effective June 1, 1997. States may
opt out of the Interstate Banking Act, thereby prohibiting interstate mergers in
the state or opt in early, thereby allowing interstate mergers prior to June 1,
1997. BancWest will be unable to consolidate its banking operations in one state
with those of another state if either state in question has opted out of the
Interstate Banking Act. California and Hawaii "opted in" to certain provisions
of the Interstate Banking Act.
 
     BancWest's acquisitions of banking institutions and other companies
generally are subject to the prior approval of the Federal Reserve Board and
other applicable federal or state regulatory authorities. In determining whether
to approve a proposed bank acquisition, federal banking regulators will
consider, among other factors:
 
     - the effect of the acquisition on competition,
 
     - the public benefits expected to be received from the consummation of the
       acquisition,
 
     - the projected capital ratios and levels on a post-acquisition basis, and
 
     - the acquiring institution's record of addressing the credit needs of the
       communities it serves, including the needs of low-and moderate-income
       neighborhoods, consistent with the safe and sound operation of the bank,
       under the Community Reinvestment Act.
 
     BancWest is also a bank holding company within the meaning of Section 3700
of the California Financial Code and accordingly, is subject to examination by,
and may be required to file reports with, the California Commissioner of
Financial Institutions and the California Department of Financial Institutions.
 
FEDERAL DEPOSIT INSURANCE CORPORATION
 
     Bank of the West and First Hawaiian Bank are federally insured depository
institutions and accordingly are regulated by the FDIC. For information
regarding the FDIC's powers of supervision and examination with respect to
insured banks, you should refer to BancWest's Annual Report on Form 10-K for the
year ended December 31, 1998,
 
                                       59
<PAGE>   65
 
"Item 1. Business -- Supervision and Regulation," which is incorporated by
reference herein. See "WHERE YOU CAN FIND MORE INFORMATION."
 
DIVIDENDS
 
     The power of the board of directors of insured depository institutions,
such as Bank of the West and First Hawaiian Bank, to declare a cash dividend or
other distribution with respect to capital is subject to statutory and
regulatory restrictions which limit the amount available for such distribution
depending upon the earnings, financial condition and cash needs of the
institution. The Federal Deposit Insurance Act prohibits insured depository
institutions from paying management fees to any controlling persons or, with
certain limited exceptions, making capital distributions, including dividends,
if, after such transaction, the institution would be undercapitalized.
 
     The FDIC also has the authority to prohibit an insured depository
institution from engaging in business practices which are considered to be
unsafe or unsound, including the payment of dividends under certain
circumstances even if such payments are not expressly prohibited by statute.
 
     Bank of the West and First Hawaiian Bank are also subject to dividend
restrictions under applicable state law.
 
CAPITAL STANDARDS
 
     BancWest is subject to risk-based capital adequacy guidelines promulgated
by the federal banking agencies. For additional information regarding such
capital adequacy guidelines, please refer to BancWest's Annual Report on Form
10-K for the year ended December 31, 1998, "Item 1. Business -- Supervision and
Regulation -- Capital Requirements," which is incorporated by reference herein.
See "WHERE YOU CAN FIND MORE INFORMATION."
 
     The following tables present the capital ratios for BancWest, compared to
the standards promulgated by the federal banking agencies for well-capitalized
depository institutions, as of December 31, 1998 (amounts in thousands except
percentage amounts).
 
<TABLE>
<CAPTION>
                                                       BANCWEST
                                   -------------------------------------------------
                                         ACTUAL              WELL          MINIMUM
                                   -------------------    CAPITALIZED      CAPITAL
                                    CAPITAL      RATIO       RATIO       REQUIREMENT
                                   ----------    -----    -----------    -----------
<S>                                <C>           <C>      <C>            <C>
Leverage.........................  $1,080,353     9.16%       5.00%         3.00%
Tier 1 Risk-Based................  $1,080,353     8.17%       6.00%         4.00%
Total Risk-Based.................  $1,329,938    10.06%      10.00%         8.00%
</TABLE>
 
REGULATION BY STATE BANKING AUTHORITIES
 
     First Hawaiian Bank and Bank of the West, as state-chartered banks that are
not members of the Federal Reserve System, are subject to supervision and
regulation by the FDIC and by the Hawaii Division of Financial Institutions or
the California Department of Financial Institutions, respectively. The
regulations of these agencies affect most aspects of both banks' business and
prescribe permissible types of loans and investments, the amount of required
reserves, requirements for branch offices, the permissible scope of the banks'
activities and various other requirements. While the banks are not members of
the Federal Reserve System, they are subject to certain regulations of the
Federal Reserve Board
 
                                       60
<PAGE>   66
 
dealing primarily with check clearing activities, establishment of banking
reserves, Truth-in-Lending (Regulation Z), Truth-in-Savings (Regulation DD), and
Equal Credit Opportunity (Regulation B). The banks are also subject to the
requirements of the Community Reinvestment Act.
 
     Under Hawaii and California law, both banks are subject to various
restrictions on, and requirements regarding, this operation and administration,
including the maintenance of branch offices and automated teller machines,
capital and reserve requirements, deposits and borrowings, and investment and
lending activities.
 
     Under Hawaii law, a receiver or conservator may be appointed to take
possession and control of a Hawaii state chartered bank if it becomes insolvent,
fails to correct an impairment of capital required by law, is not likely to be
able to meet the demands of its depositors or in other specified circumstances.
Appointment would be by the Hawaii Commissioner of Financial Institutions (the
"Hawaii Commissioner"), subject to judicial review.
 
     Whenever it appears to the California Commissioner that certain conditions
exist with respect to a California state-chartered bank, such as when the
tangible shareholders' equity of such bank is less than the greater of three
percent of such bank's total assets or $1,000,000, the California Commissioner
may take possession of the property and business of such bank until such bank
resumes business upon such conditions as may be prescribed by the California
Commissioner or its affairs are finally liquidated as provided by statute.
 
     Hawaii law permits a Hawaii state chartered bank to invest in the equity
securities of other corporations engaged in certain specified activities related
to banking or, with the approval of the Hawaii Commissioner, in subsidiary
corporations that are engaged in activities authorized for a bank or which are
usual to incidental to the business of a bank. California law permits a state
chartered bank to invest in the stock and securities of other corporations,
subject to receiving either general authorization or, depending on the amount of
the proposed investment, specific authorization from the California
Commissioner.
 
     The Federal Deposit Insurance Corporation Improvement Act of 1991
("FDICIA"), however, imposes limitations upon the activities and equity
investments of state chartered, federally insured banks. The FDIC rules on
investments prohibit a state bank from acquiring equity investments of a type,
or in an amount, not permissible for a national bank. FDICIA also prohibits a
state bank from engaging as a principal in any activity that is not permissible
for a national bank, unless the bank is adequately capitalized and the FDIC
approves the activity after determining that such activity does not pose a
significant risk to the deposit insurance fund. The FDIC rules on activities of
state-chartered banks generally permit subsidiaries of such banks, without prior
FDIC authorization, to engage in those activities that have been approved by the
Federal Reserve Board for bank holding companies. The subsidiaries of both banks
engage only in such permissible activities. Other activities which may be
conducted by state-chartered banks generally require specific FDIC prior
approval, and the FDIC may impose additional restrictions on such activities on
a case-by-case basis in approving applications to engage in otherwise
impermissible activities.
 
     California "opted in" to certain provisions of the Interstate Banking Act
regarding interstate branching. Under the California Interstate Banking and
Branching Act, California opted into the portion of the federal law allowing
early interstate merger transactions. However, California opted out of the
provisions of the federal law allowing interstate branching through the
acquisition of a branch located in California (without
 
                                       61
<PAGE>   67
 
acquisition of the whole business unit of the California bank) and also opted
out of the provision of the federal law allowing interstate branching through de
novo establishment of California branch offices. Further, no out of state bank
that does not already maintain a California branch office may merge as the
surviving bank with a California bank or purchase the whole of the business of
the California bank unless the California bank has been in existence for at
least five years. The California Commissioner is also authorized to approve an
interstate merger transaction involving a California bank which would result in
a deposit concentration exceeding 30% of the deposits in California if the
California Commissioner finds that the transaction is consistent with the public
convenience and advantage in California.
 
     Hawaii also permits interstate banking and branching transactions. Similar
to California, Hawaii also opted out of provisions of the federal law which
allowed banks not already operating a branch in the state to acquire a branch or
branches located in Hawaii (without acquisition of the entire bank) and which
allowed such banks to establish de novo branches in Hawaii. Out of state banks
not already operating branches in Hawaii may not merge with or acquire Hawaii
banks unless the Hawaii bank has been in existence for at least five years. The
Hawaii interstate banking and branching law also permits the Hawaii Commissioner
to waive, on a case by case basis, the federal 30% deposit concentration limit
under circumstances similar to those provided in the California law.
 
                                       62
<PAGE>   68
 
                               SIERRAWEST BANCORP
 
     SierraWest was incorporated under the name Sierra Tahoe Bancorp under the
laws of the State of California on December 5, 1985, as a bank holding company.
Pursuant to a plan of reorganization, SierraWest acquired 100% of the
outstanding shares of common stock of SierraWest Bank, then named Truckee River
Bank, in a one-for-one exchange of its stock for the stock of SierraWest Bank on
July 31, 1986. The activities of SierraWest are subject to the supervision of
the Federal Reserve Board. SierraWest may engage, directly or through subsidiary
corporations, in those activities closely related to banking which are
specifically permitted under the Bank Holding Company Act. SierraWest's
principal executive office is located at 10181 Truckee-Tahoe Airport Road,
Truckee, California 96161 and its telephone number is (530) 582-3000.
 
     SierraWest Bank was incorporated under the laws of the State of California
as Truckee River Bank on March 19, 1980, and, with the approval of the
California Commissioner, opened for business on January 20, 1981. Truckee River
Bank commenced operations in Truckee, California, a small tourist-based town
located in the County of Nevada and situated in the High Sierra about 12 miles
north of Lake Tahoe. Truckee River Bank changed its name to SierraWest Bank in
early 1996. SierraWest Bank currently maintains 20 branch offices in the
following communities: Truckee (two branches), South Lake Tahoe, Tahoe City,
Kings Beach, Grass Valley (two branches), Auburn and Sacramento (two branches),
Vacaville (two branches), Fairfield (two branches), Vallejo (two branches),
Benicia and Concord, California, and in Reno and Carson City, Nevada. In
addition, SierraWest Bank maintains lending offices and agency relationships
primarily for its Small Business Administration lending activities in
California, Nevada, Arizona, Oregon, Washington, Colorado, Texas, Tennessee,
Georgia, Alabama and Florida. SierraWest Bank's deposits are insured by the FDIC
up to applicable limits.
 
     In June 1997, SierraWest acquired Mercantile Bank, a state-chartered
commercial bank with its principal office formerly in Sacramento, California,
through a merger of Mercantile Bank with and into SierraWest Bank. On the
acquisition date, Mercantile Bank had assets of $42.8 million, deposits of $37.7
million and shareholders' equity of $4.9 million. The consideration for the
acquisition was a combination of cash and shares of SierraWest common stock with
an aggregate value of approximately $6.6 million. The merger was accounted for
by the purchase method of accounting.
 
     On April 15, 1998, SierraWest acquired California Community Bancshares
Corporation and its wholly owned banking subsidiary Continental Pacific Bank.
Continental Pacific Bank was a state chartered bank with its principal office in
Vacaville, California. Continental Pacific Bank engaged in general commercial
banking, in Solano and Contra Costa counties in the State of California. On the
acquisition date, California Community Bancshares had assets of $206 million,
deposits of $184 million and shareholders' equity of $15.4 million. The
consideration for the acquisition was shares of SierraWest common stock with a
value of approximately $44.7 million. The merger was accounted for by the
pooling of interests method of accounting.
 
                                       63
<PAGE>   69
 
                     MARKET PRICE AND DIVIDEND INFORMATION
 
     BancWest common stock is traded on the New York Stock Exchange under the
symbol "BWE." SierraWest is quoted on the Nasdaq National Market under the
symbol "SWBS." The following table sets forth, for the calendar quarters
indicated, the reported high and low sales prices for BancWest common stock as
reported on the New York Stock Exchange composite transactions tape and the
reported high and low sales prices for SierraWest common stock as reported on
the Nasdaq National Market. In addition, the table sets forth for the calendar
quarters indicated, the per share cash dividend declared by BancWest and by
SierraWest.
 
<TABLE>
<CAPTION>
                                       BANCWEST                      SIERRAWEST
                                     COMMON STOCK                   COMMON STOCK
                              ---------------------------   -----------------------------
                                                 COMMON                         COMMON
                               MARKET PRICE       STOCK      MARKET PRICE        STOCK
                              ---------------   DIVIDENDS   ---------------    DIVIDENDS
                               HIGH     LOW     DECLARED     HIGH     LOW     DECLARED(1)
                              ------   ------   ---------   ------   ------   -----------
<S>                           <C>      <C>      <C>         <C>      <C>      <C>
1997
  First Quarter.............  $36.00   $30.50     $0.31     $18.69   $14.64      $0.15
  Second Quarter............   35.75    28.63      0.31      20.12    16.68         --
  Third Quarter.............   40.75    33.63      0.31      25.75    18.81       0.16
  Fourth Quarter............   43.88    36.00      0.31      36.00    24.75         --
1998
  First Quarter.............   42.00    34.63      0.31      39.00    30.00         --
  Second Quarter............   41.00    34.31      0.31      39.00    29.75       0.20
  Third Quarter.............   38.00    27.63      0.31      35.75    20.50         --
  Fourth Quarter............   48.00    31.25      0.31      28.00    18.00       0.20
1999
  First Quarter.............   48.50    38.88      0.31      32.56    23.50       0.26
  Second Quarter (through
     April 13)..............   42.44    39.56        --      32.38    30.50         --
</TABLE>
 
- -------------------------
 
(1) Dividends shown exclude dividends paid by California Community Bankshares
    Corporation of $.124 each quarter of 1997 and the first quarter of 1998. In
    April 1998, each share of California Community Bankshares Corporation common
    stock was converted to 0.8283 shares of SierraWest common stock.
 
DIVIDEND POLICY
 
     BancWest expects no changes in its dividend policies after the merger. The
current quarterly dividend rate on BancWest common stock is $0.31 per share.
 
                                       64
<PAGE>   70
 
                     DESCRIPTION OF BANCWEST CAPITAL STOCK
 
     BancWest is authorized to issue 325,000,000 shares of capital stock. The
shares are divided into three classes:
 
     - 200,000,000 shares of common stock, $1.00 par value per share,
 
     - 75,000,000 shares of Class A Common Stock, $1.00 par value per share, and
 
     - 50,000,000 shares of preferred stock, $1.00 par value per share.
 
As of April 9, 1999, there were 31,576,886 shares of BancWest common stock
outstanding and 1,655,711 shares of stock were available for issuance upon
exercise of outstanding stock options or awards under incentive plans. As of
April 9, 1999, 25,814,768 shares of BancWest Class A Common Stock were
outstanding (all of which were owned by BNP and its affiliate, French American
Banking Corporation) and no shares of preferred stock were issued and
outstanding.
 
COMMON STOCK
 
     DIVIDEND RIGHTS.  Holders of BancWest common stock are entitled to receive
ratably such dividends as may be legally declared by BancWest's board of
directors. Holders of Class A Common Stock are entitled to receive ratably any
dividends paid to holders of BancWest common stock. If BancWest pays any
dividends on the Class A Common Stock in shares of Class A Common Stock or in
options, warrants or other securities exercisable for or convertible into shares
of Class A Common Stock, then BancWest must declare and pay an equivalent
dividend per share on the BancWest common stock. In addition, if BancWest
declares a stock-split that results in a greater or lesser number of Class A
shares being issued and outstanding, then BancWest will declare an equivalent
stock-split on the BancWest common stock.
 
     LIQUIDATION RIGHTS.  In the event of any voluntary or involuntary
liquidation, dissolution or winding-up of BancWest, the holders of BancWest
common stock will be entitled, together with the holders of Class A Common
Stock, to share ratably all the remaining assets of BancWest available for
distribution to stockholders.
 
     VOTING RIGHTS; CLASSIFICATION OF DIRECTORS.  Each holder of BancWest common
stock is entitled to one vote in respect of each share of BancWest common stock
held by such holder on each matter voted upon by the stockholders. Holders of
BancWest common stock, however, are not entitled to vote on the election of
Class A directors. On all matters other than the election of directors and
certain fundamental corporate actions under certain circumstances, holders of
BancWest common stock will vote together with the holders of the Class A Common
Stock as a single class. Approval of any matter voted upon at any stockholders'
meeting requires the affirmative vote of a majority of the shares of BancWest
stock which are present in person or by proxy and entitled to vote thereon,
except:
 
     - election of directors, who are elected by plurality vote,
 
     - amendment or repeal of the provisions of BancWest's certificate of
       incorporation establishing a classified board of directors, which will
       require the affirmative vote of three-fourths of the shares of BancWest
       common stock and Class A Common Stock at the time outstanding, voting
       together as a class, and
 
     - with respect to any other actions (such as a merger or sale of
       substantially all of BancWest assets) where higher percentage is required
       by applicable law or the rules and regulations of any applicable stock
       exchange.
 
                                       65
<PAGE>   71
 
     As long as the Class A shares are outstanding, unless the prior approval of
two-thirds of BancWest's entire board is obtained, BancWest may not take certain
fundamental corporate actions without the stockholder votes describe in this
paragraph (even if such approval not required by Delaware law), including but
not limited to:
 
     (1) any merger, consolidation or other business combination resulting in a
         change of control of BancWest,
 
     (2) any merger, consolidation or other business combination which would not
         result in a change of control of BancWest but in which either:
 
        - any person, other than a holder of shares of Class A Common Stock,
          would become the beneficial owner of 25% or more of the total voting
          power of all voting securities of BancWest outstanding after such
          transaction, or
 
        - any three persons, other than a holder of shares of Class A Common
          Stock, would become the beneficial owners of 45% or more of the total
          voting power of all BancWest voting securities outstanding after such
          transaction,
 
     (3) the amendment of our certificate of incorporation or our bylaws which
         materially and adversely affect the rights of the holders of Class A
         Common Stock,
 
     (4) (A) the issuance of any series or class of capital stock having either:
 
        - more than one vote per share or
 
        - a class vote on any matter, except to the extent such class vote is
          required by Delaware law or to the extent that holders of any series
          of preferred stock may have the right, voting separately as a class,
          to elect a number of directors of the corporation upon the occurrence
          of a default in payment of dividends or redemption price or
 
        (B) the adoption of any stockholder rights plan, or
 
     (5) the issuance of any series of preferred stock which at the time of such
         issuance would not be "non-voting shares" as defined under federal law.
 
In the case of the transaction described in clause (1), the holders of a
majority of the outstanding shares of Class A Common Stock and BancWest common
stock, voting together as a class, must consent to or approve such transaction.
In the case of the actions described in clauses (2), (3), (4) and (5) the
holders of a majority of the outstanding shares of Class A Common Stock and
BancWest common stock, voting as separate classes, must approve or consent to
such transaction.
 
     BancWest's certificate of incorporation provides that the directors of
BancWest will be divided into three classes, each class to consist as nearly as
practicable of one-third of the number of directors then constituting the
authorized number of directors, and that each director shall be elected for a
term of three years. At each meeting of stockholders held for the purpose of
electing directors, the holders of BancWest common stock will have the right to
elect that number of directors equal to the excess of:
 
     (1) the total number of directors then constituting the authorized number
         of directors over
 
     (2) the sum of (x) the number of directors which the holders of the shares
         of Class A Common Stock are entitled to elect, (y) the number of
         directors elected by the stockholders of BancWest, other than the
         holders of shares of Class A Common Stock or preferred stock, in each
         of the other two classes and (z) the
 
                                       66
<PAGE>   72
 
         number of directors, if any, that the holders of the preferred stock,
         voting separately by class or series, are entitled to elect.
 
The holders of shares of Class A Common Stock will not be entitled to vote for
the directors who are elected by the holders of the BancWest common stock as
described above.
 
     The holders of the Class A Common Stock will have the right, voting
separately as a class, to elect that number of directors of BancWest equal to
the product, rounded to the nearest whole number, of:
 
     (1) the Class A multiplier (as defined below), and
 
     (2) the total number of directors constituting the authorized number of
         directors; provided that the number of directors entitled to be elected
         by holders of Class A Common Stock cannot constitute a majority of the
         total number of directors constituting the authorized number of
         directors.
 
     The Class A multiplier is equal to:
 
     - .45, if the percentage of the outstanding BancWest common stock and Class
       A Common Stock represented by shares of Class A Common Stock, determined
       in accordance with the certificate of incorporation, is greater than or
       equal to 40% and less than or equal to 45%,
 
     - .35, if the percentage of the outstanding BancWest common stock and Class
       A Common Stock represented by shares of Class A Common Stock, determined
       in accordance with the certificate of incorporation, is greater than or
       equal to 35% and less than 40%,
 
     - .30, if the percentage of the outstanding BancWest common stock and Class
       A Common Stock represented by shares of Class A Common Stock, determined
       in accordance with the certificate of incorporation, is greater than or
       equal to 30% and less than 35%,
 
     - .25, if the percentage of the outstanding BancWest common stock and Class
       A Common Stock represented by shares of Class A Common Stock, determined
       in accordance with the certificate of incorporation, is greater than or
       equal to 25% and less than 30%,
 
     - .20, if the percentage of the outstanding BancWest common stock and Class
       A Common Stock represented by shares of Class A Common Stock, determined
       in accordance with the certificate of incorporation, is greater than or
       equal to 20% and less than 25%,
 
     - .15, if the percentage of the outstanding BancWest common stock and Class
       A Common Stock represented by shares of Class A Common Stock, determined
       in accordance with the certificate of incorporation, is greater than or
       equal to 15% and less than 20%, and
 
     - .10, if the percentage of the outstanding BancWest common stock and Class
       A Common Stock represented by shares of Class A Common Stock, determined
       in accordance with the certificate of incorporation, is greater than or
       equal to 10% and less than 15%.
 
     PREEMPTIVE RIGHTS.  No holders of any class of BancWest's capital stock or
holder of any security or obligation convertible into shares of BancWest's
capital stock will have any
 
                                       67
<PAGE>   73
 
preemptive right to subscribe for, purchase or otherwise acquire additional
shares of capital stock of any class of BancWest.
 
     CONVERSION AND REDEMPTION PROVISIONS.  Shares of BancWest common stock are
not convertible into shares of any other class of capital stock, nor do they
have any redemption provisions.
 
     OTHER.  The rights and percentage ownership of the holders of BancWest
common stock may be affected by the conversion rights of the holders of Class A
Common Stock. BancWest's certificate of incorporation provides that if
beneficial ownership of Class A Common Stock is sold, transferred, pledged, or
otherwise disposed of to any person other than:
 
     - an affiliate of the transferring holder, or
 
     - certain permitted transferees or permitted pledgees,
 
then each such share of Class A Common Stock will automatically be converted
into one share of BancWest common stock.
 
     In addition, if the number of outstanding shares of Class A Common Stock as
a percentage of the sum of the total number of outstanding shares of:
 
     (1) BancWest common stock,
 
     (2) Class A Common Stock, and the number of shares of BancWest common stock
         or Class A Common Stock that are issuable upon conversion, exchange or
         exercise of any shares of the outstanding shares of Class A Common
         Stock decreases to less than 10%,
 
then each outstanding share of Class A Common Stock will automatically be
converted into one share of BancWest common stock. Shares of Class A Common
Stock may also be converted into BancWest common stock at the option of BNP
under certain limited circumstances.
 
     STANDSTILL AGREEMENT WITH BANQUE NATIONALE DE PARIS.  The standstill
agreement, dated as of November 1, 1998, between BancWest and BNP restricts BNP
and its affiliates, during a four year period, from acquiring additional shares
of BancWest capital stock, subject to certain exceptions, including the right to
increase its ownership level up to the then applicable permitted ownership level
(presently 45%) under the standstill agreement if BNP's percentage ownership is
diluted by issuances of common stock by BancWest, including the issuance of
shares to SierraWest shareholders in the merger. The standstill agreement also
restricts BNP's and its affiliates' ability to freely transfer any equity
securities of BancWest, including Class A Common Stock and BancWest common
stock, owned by BNP or such affiliates and gives BancWest a right of first
refusal with respect to certain transfers.
 
     TRANSFER AGENT AND REGISTRAR.  The transfer agent and registrar for
BancWest common stock is American Stock Transfer & Trust Company.
 
                                       68
<PAGE>   74
 
             CERTAIN DIFFERENCES IN RIGHTS OF HOLDERS OF SIERRAWEST
                     COMMON STOCK AND BANCWEST COMMON STOCK
 
     SierraWest is incorporated under the laws of California. The rights of
SierraWest's shareholders are governed by the California General Corporation
Law. BancWest is incorporated under the laws of the state of Delaware. The
rights of BancWest stockholders are governed by Delaware General Corporation Law
and BancWest's restated certificate of incorporation and amended and restated
bylaws. Upon completion of the merger, SierraWest shareholders will become
stockholders of BancWest. As a result, their rights will be governed by Delaware
Corporation Law and BancWest's governing documents.
 
     The following discussion compares certain rights of holders of SierraWest
common stock to the rights of the holders of BancWest common stock. You can find
additional information concerning the rights of BancWest stockholders in
BancWest's restated certificate of incorporation and amended and restated
bylaws, which are filed as exhibits to the registration statement in which this
document is included, and in BancWest's registration statement on Form 8-A dated
October 30, 1998, containing a description of BancWest's common stock.
 
DIVIDENDS
 
     BANCWEST.  Delaware corporations may pay dividends out of surplus or, if
there is no surplus, out of net profits for the fiscal year in which declared
and for the preceding fiscal year. Section 170 of Delaware Corporation Law also
provides that dividends may not be paid out of net profits if, after the payment
of the dividend, capital is less than the capital represented by the outstanding
stock of all classes having a preference upon the distribution of assets.
 
     SIERRAWEST.  Under California law, the directors of SierraWest may declare
and pay dividends upon the shares of its capital stock either (a) out of its
retained earnings, or (b) out of capital, provided the company would, after
making the distribution, meet two conditions, which generally stated are as
follows: (1) the corporation's assets must equal at least 125% of its
liabilities; and (2) the corporation's current assets must equal at least its
current liabilities or, if the average of the corporation's earnings before
taxes on income and before interest expense for the two preceding fiscal years
was less than the average of the corporation's interest expense for such fiscal
years, then the corporation's current assets must equal at least 125% of its
current liabilities.
 
INDEMNIFICATION OF DIRECTORS AND EXECUTIVE OFFICERS
 
     BANCWEST.  BancWest's amended and restated bylaws provide that BancWest
must indemnify, to the extent permitted by Delaware law, each person who is
party to any threatened, pending or completed action, suit or proceeding by
reason of the fact that such person is or was a director, officer, employee or
agent of BancWest against all expenses, judgments, fines and amounts paid in
settlement or incurred in connection with such action. BancWest can only
eliminate or limit the liability of a director, officer, employee, or agent if:
 
     - such person acted in good faith and in a manner that such person
       reasonably believed to be in the best interests of BancWest, and
 
     - with respect to any criminal action, such person had no reasonable cause
       to believe his/her conduct was unlawful.
 
                                       69
<PAGE>   75
 
BancWest cannot indemnify any person against expenses incurred in defense of an
action or suit if such person shall have been adjudged to be liable to BancWest.
 
     BancWest may maintain insurance, at its expense, to protect any directors,
officers, employees or agents of BancWest or another entity against any expense,
liability or loss, regardless of whether BancWest has the power or obligation to
indemnify that person against such expense, liability or loss under Delaware
law.
 
     SIERRAWEST.  SierraWest's articles of incorporation provide that the
liability of directors for monetary damages is eliminated to the fullest extent
permissible under the California General Corporation Law. This provision
relieves directors of SierraWest of liability to SierraWest for simple
negligence but not for liability where the director was either grossly negligent
or guilty of a willful breach of his or her loyalty to SierraWest. The
SierraWest articles of incorporation do not, and under the California General
Corporation Law cannot, eliminate or limit the liability of a director resulting
from the following actions:
 
     - acts or omissions that involve intentional conduct or a knowing and
       culpable violation of law,
 
     - acts or omissions that a director believes to be contrary to the best
       interests of the corporation or its shareholders or that involve the
       absence of good faith on part of the director,
 
     - any transaction from which a director derived an improper personal
       benefit,
 
     - acts or omissions that show a reckless disregard for the director's duty
       to the corporation or its shareholders in circumstances in which the
       director was aware, or should have been aware, in the ordinary course of
       performing a director's duties, of a risk of serious injury to the
       corporation or its shareholders,
 
     - acts or omissions that constitute an unexcused pattern of inattention
       that amounts to an abdication of the director's duty to the corporation
       or its shareholders,
 
     - any transaction between the corporation and (1) a director, or (2) a
       corporation, firm or association in which the director has a material
       financial interest, or
 
     - any distribution to shareholders, and for any loan or guaranty to
       officers or directors, that violates specified provisions of California
       law.
 
CUMULATIVE VOTING
 
     BANCWEST.  Stockholders of BancWest common stock may not cumulate their
votes for the elections of directors. Cumulative voting allows a stockholder to
cast a number of votes equal to the number of directors to be elected multiplied
by the number of shares held in the stockholder's name on the record date. This
total number of votes may be cast for one nominee or may be distributed among as
many candidates as the stockholder desires. The candidates, up to the number of
directors to be elected, receiving the highest number of votes are elected.
 
     Under the Delaware General Corporation Law, stockholders of a Delaware
corporation do not have cumulative voting rights unless the certificate of
incorporation is amended to provide for cumulative voting.
 
     SIERRAWEST.  Shareholders of SierraWest are entitled to cumulate their
votes for the election of directors.
 
                                       70
<PAGE>   76
 
CLASSIFIED BOARD OF DIRECTORS
 
     BANCWEST.  BancWest's certificate of incorporation provides for three
classes of directors, each class to consist as nearly as practicable of
one-third of the number of directors then constituting the authorized number of
directors. Each director is elected for a term of three years. For more
information regarding the mechanics for voting for directors see "DESCRIPTION OF
BANCWEST CAPITAL STOCK."
 
     SIERRAWEST.  The SierraWest bylaws provide that directors will be elected
for a one-year term at each annual meeting of shareholders.
 
DISSENTERS' RIGHTS IN MERGERS AND OTHER REORGANIZATIONS
 
     BANCWEST.  Under the Delaware General Corporation Law, stockholders do not
have dissenters' rights in connection with a business combination if:
 
     - the shares of the corporation were either listed on a national securities
       exchange or designated as a national market security by Nasdaq on the
       record date for the stockholders meeting relating to such business
       combination,
 
     - held of record by 2,000 or more stockholders, or
 
     - the transaction does not require approval of the corporation's
       stockholders.
 
Notwithstanding the foregoing, stockholders have dissenters' rights if (1)
stockholder approval is required and (2) the holders would be required to accept
for their shares any consideration other than:
 
     - shares of stock of the surviving corporation,
 
     - shares of another corporation if such shares are listed on a national
       exchange or designated as a national market system security by Nasdaq, or
 
     - cash in lieu of fractional shares.
 
     SIERRAWEST.  Under the California General Corporation Law, a dissenting
shareholder of a corporation participating in certain business combinations may,
under varying circumstances, receive cash in the amount of the fair market value
of his or her shares in lieu of the consideration he or she would otherwise
receive under the terms of the transaction. The California General Corporation
Law generally does not require dissenters' rights of appraisal with respect to
shares which, immediately prior to the merger, are:
 
     - listed on any national securities exchange certified by the California
       Commissioner of Corporations, or
 
     - listed on the list of over-the-counter margin stock issued by the Federal
       Reserve Board.
 
SierraWest common stock is listed on the list of over-the-counter margin stocks
issued by the Federal Reserve Board. However, dissenters' rights are available
to the shareholders of SierraWest if the holders of five percent or more of
SierraWest's outstanding shares make a written demand upon SierraWest not later
than the date of the SierraWest shareholders' meeting for the purchase of
dissenting shares in accordance with Section 13 of the California General
Corporation Law.
 
                                       71
<PAGE>   77
 
ANTI-TAKEOVER STATUTES
 
     BANCWEST.  Section 203 of Delaware General Corporation Law would prohibit a
"business combination" defined generally to include mergers, sales and leases of
assets, issuances of securities and similar transactions, by BancWest or a
subsidiary with an "Interested Shareholder" (as defined in the Delaware General
Corporation Law) within three years after the person or entity becomes an
Interested Shareholder, unless:
 
     - prior to the person or entity becoming an Interested Shareholder, the
       business combination or the transaction pursuant to which such person or
       entity became an Interested Shareholder shall have been approved by the
       board of directors of BancWest,
 
     - upon the consummation of the transaction in which the person or entity
       became an Interested Shareholder, the Interested Shareholder holds at
       least 85% of the voting stock of BancWest (excluding shares held by
       persons who are both officers and directors of BancWest and shares held
       by certain employee benefit plans), or
 
     - the business combination is approved by the board of directors of
       BancWest and by the holders of at least two-thirds of the outstanding
       voting stock of BancWest, excluding shares held by the Interested
       Shareholder.
 
     SIERRAWEST.  California law requires that in certain transactions involving
tender offers or acquisition proposals made to a target corporation's
shareholders by a person who:
 
     - controls the target corporation,
 
     - is an officer or director of the target or is controlled by an officer or
       director of the target, or
 
     - is an entity in which a director or executive officer of the target has a
       material interest.
 
A written opinion of an independent expert be provided as to the fairness of the
consideration to the shareholders of the target corporation.
 
     The statute also provides that if a competing proposal is made at least ten
days before shareholders are to vote or shares are to be purchased under the
pending offer by the affiliated party, the latter offer must be communicated to
shareholders and they must be given a reasonable opportunity to revoke their
vote or withdraw their shares, as the case may be.
 
SHAREHOLDER VOTE FOR MERGERS AND ASSET SALES
 
     BANCWEST.  Under Delaware law, unless a greater vote of stockholders is
required by a corporation's certificate of incorporation or unless the
provisions of Delaware law relating to "business combinations" discussed above
are applicable, a sale, lease or exchange of all or substantially all of the
corporation's assets, or a merger or consolidation of the corporation with
another corporation (except in certain limited circumstances) plus, with certain
exceptions, the affirmative vote of a majority of the outstanding stock entitled
to vote thereon. The foregoing provisions apply to BancWest and the holders of
its common stock and Class A Common Stock.
 
     In addition, as long as the Class A shares are outstanding, BancWest's
certificate of incorporation provides that unless the prior approval of
two-thirds of BancWest's entire board is obtained, BancWest may not take the
following actions without the approval of the holders of a majority of the
outstanding shares of BancWest common stock and
 
                                       72
<PAGE>   78
 
Class A Common Stock, voting together as a class (except in the case of the
matters described in the fifth bullet point, in which case the holders of the
Class A Common Stock will vote as a separate class). This stockholder approval
will be required even if no such approval is required under Delaware law.
 
     - any merger, consolidation or other business combination in which BancWest
       is a constituent company and (1) BancWest is not the surviving or
       resulting entity in such transaction, or (2) if BancWest is the surviving
       or resulting entity and such transaction results in a "Change of
       Control," as defined in the certificate of incorporation,
 
     - the sale, exchange, lease or mortgage of all or substantially all of
       BancWest's assets in one or a series of related transactions,
 
     - any acquisition by BancWest or any of its subsidiaries of any assets or
       business, in one transaction or a series of related transactions in any
       twelve-month period, in which the consideration paid by BancWest (1) if
       in BancWest common stock, will exceed 20% of the aggregate voting power
       of the outstanding voting securities, as defined in the certificate of
       incorporation, as of the date that BancWest or any such subsidiary enters
       into a definitive agreement to effect such transaction or, in the case of
       a series of related transactions, as of the date that the corporation or
       any such subsidiary enters into a definitive agreement to effect the last
       of such related transactions, or (2) if in cash, property or other
       securities of the BancWest, has a fair market value, as defined in the
       certificate of incorporation, at the time of the execution by BancWest or
       such subsidiary of a definitive agreement to effect such transaction or,
       in the case of a series of related transactions, at the time of the
       execution by BancWest or such subsidiary of a definitive agreement to
       effect the last of such related transactions, which will exceed
       one-fourth of the market capitalization, as defined in the certificate of
       incorporation, of BancWest at such time,
 
     - any disposition, directly or indirectly, by BancWest or any of its
       subsidiaries, except to BancWest or a subsidiary of BancWest, of any
       assets or businesses, in one transaction or a series of related
       transactions in any twelve-month period, whether by merger, tender or
       exchange offer, asset purchase or otherwise, in which the book value of
       the assets disposed of, as shown on the most recently available financial
       statements of BancWest, exceed one-sixth of the total consolidated
       assets, as defined in the certificate of incorporation, of the
       corporation at the time of the execution by BancWest or such subsidiary
       of a definitive agreement to effect such disposition or, in the case of a
       series of related transactions, at the time of the execution by BancWest
       or such subsidiary of a definitive agreement to effect the last of such
       dispositions, or
 
     - any merger, consolidation, recapitalization, reorganization, sale,
       acquisition, other business combination or other transaction to which
       BancWest is a party involving the issuance of voting securities of
       BancWest that does not result in a Change of Control of BancWest if, as a
       result of such transaction, any person, other than a holder of shares of
       Class A Common Stock, would become the beneficial owner of 25% or more of
       the total voting power of all voting securities of BancWest outstanding
       after such transaction or any three persons, other than holders of shares
       of Class A Common Stock, would become the beneficial owners of 45% or
       more of the total voting power of all voting securities of BancWest
       outstanding after such transaction. In this case, the holders of each
       class vote separately as a class.
 
                                       73
<PAGE>   79
 
     SIERRAWEST.  The California General Corporation Law requires shareholder
vote for mergers and reorganizations by:
 
     - shareholders of each California corporation where two California
       corporations are to merge,
 
     - shareholders of a corporation selling all or substantially all of its
       assets,
 
     - shareholders of an acquiring corporation in either a share for share
       exchange or a sale of assets reorganization, and
 
     - in certain circumstances, shareholders of a parent corporation, even
       though it is not a California corporation whose securities are being
       issued in connection with a corporate reorganization such as a triangular
       merger.
 
INSPECTION OF STOCKHOLDER LISTS
 
     BANCWEST.  The Delaware Corporation Law permits any stockholder of record
to inspect the stockholder list for any purpose reasonably related to such
person's interest as a stockholder and, for a ten-day period preceding a
stockholders' meeting, for any purpose germane to the meeting.
 
     SIERRAWEST.  The California General Corporation Law provides an absolute
right of inspection of the corporation's shareholder list to any shareholder
holding five percent or more of a corporation's voting shares or a shareholder
holding one percent or more of a corporation's shares who has filed a Schedule
14B with the Securities and Exchange Commission. Schedule 14B is filed in
connection with certain proxy contests relating to the election of directors. In
addition, the California General Corporation Law provides a right of inspection
of shareholder lists by any shareholder for a purpose reasonably related to such
holder's interest as a shareholder.
 
NOMINATION OF DIRECTORS
 
     BANCWEST.  Under BancWest's certificate of incorporation, each candidate
for a directorship of BancWest, other than a directorship to be filled with a
Class A director, shall be nominated by a majority vote of a committee composed
of all non-Class A directors in office.
 
     SIERRAWEST.  Under SierraWest's bylaws, nominations for election of members
of the board of directors of SierraWest may be made by the board of directors or
by any holder of any outstanding class of capital stock of SierraWest entitled
to vote for the election of directors.
 
AMENDMENT OF CERTIFICATE OF INCORPORATION AND BYLAWS
 
     BANCWEST.  Under Delaware law, BancWest's certificate of incorporation may
be amended with the approval of the board of directors and a majority of the
stockholders (except the provision providing for a classified board which
requires the approval of three-fourths of the stockholders). In addition, any
amendment that would materially and adversely affect the rights of holders of
Class A Common Stock would require the vote of a majority of the holders of the
Class A shares, voting separately as a class. BancWest's certificate of
incorporation provides that the board of directors has the authority to make,
 
                                       74
<PAGE>   80
 
alter or repeal the by-laws, but that the vote of two-thirds of the authorized
number of directors is necessary to repeal or alter the following provisions of
the by-laws:
 
     - the notice provision for board meetings,
 
     - the definition of quorum of the board,
 
     - board vote required for certain transactions with interested
       stockholders,
 
     - board vote required for (1) distributions or dividends if the fair market
       value exceeds a certain amount, (2) repurchases or redemptions of
       BancWest equity securities where the consideration paid exceeds a certain
       amount, or (3) a decision by the board to consent to enter into any cease
       and desist order or formal agreement with any bank regulatory authority,
 
     - board vote required for appointment of the Chief Executive Officer and
       the Chief Operating Officer,
 
     - provisions relating to the executive committee, and
 
     - indemnification of directors, executive officers, employees, and agents
       of BancWest.
 
     SIERRAWEST.  SierraWest's articles may be amended with the approval of the
board of directors and a majority of the outstanding shares of SierraWest common
stock. Under certain circumstances the articles of incorporation of a California
corporation may be amended without shareholder approval in connection with stock
splits. SierraWest's articles provide that the board of directors has the
authority to make, alter or repeal the bylaws.
 
                                       75
<PAGE>   81
 
                                  PROPOSAL TWO
 
                             ELECTION OF DIRECTORS
 
     This section of the proxy statement-prospectus provides information related
to the election of directors at the annual meeting. The shareholders of
SierraWest will be asked to vote upon the election the following thirteen
nominees to serve as directors until the next annual meeting and until their
successors are elected and have been qualified, or until completion of the
merger, if earlier:
 
     The following table lists the nominees, all of whom are currently directors
of SierraWest, and provides certain information about their ages, tenures and
principal occupations.
 
<TABLE>
<CAPTION>
                                             YEAR FIRST
                                             APPOINTED     POSITION AND PRINCIPAL OCCUPATION
NAME                                   AGE    DIRECTOR        DURING THE PAST FIVE YEARS
- ----                                   ---   ----------   -----------------------------------
<S>                                    <C>   <C>          <C>
CURRENT DIRECTORS
David W. Clark                         61       1990      Chairman/CEO of Clark and Sullivan
                                                          Constructors, Inc. since January
                                                          1977.
Ralph J. Coppola                       64       1996      Self-employed physician and auto
                                                          dealer.
William T. Fike                        51       1992      President/CEO and Director of
                                                          SierraWest since July 1992.
                                                          President/ CEO of SierraWest Bank
                                                          since October 1996. Executive Vice
                                                          President and Chief Operating
                                                          Officer of SierraWest from May 1991
                                                          to July 1992.
Richard S. Gaston                      65       1995      Chairman and Director of Gaston &
                                                          Wilkerson Management Group, real
                                                          estate investments and management
                                                          companies. Until 1998, President
                                                          and Director of GAC Corporation,
                                                          real estate management.
Jerrold T. Henley                      61       1986      Chairman of SierraWest since July
                                                          1992. President/CEO of SierraWest
                                                          from its inception to June 1992.
John J. Johnson                        65       1996      Retired. Owner, Johnson's Sporting
                                                          World, Reno, Nevada until April
                                                          1992.
Ronald A. Johnson                      58       1996      Self-employed CPA and financial
                                                          consultant.
A. Morgan Jones                        67       1986      Attorney. President and director of
                                                          Truckee River Associates,
                                                          (commercial real estate management,
                                                          development and sales).
Jack V. Leonesio                       55       1986      Owner of a restaurant/bar in
                                                          Truckee, California since 1973 and
                                                          co-owner of a bar in Reno, Nevada
                                                          since April 1994.
</TABLE>
 
                                       76
<PAGE>   82
 
<TABLE>
<CAPTION>
                                             YEAR FIRST
                                             APPOINTED     POSITION AND PRINCIPAL OCCUPATION
NAME                                   AGE    DIRECTOR        DURING THE PAST FIVE YEARS
- ----                                   ---   ----------   -----------------------------------
<S>                                    <C>   <C>          <C>
William W. McClintock                  53       1986      Self-employed CPA and financial
                                                          consultant.
Bernard E. Moore                       69       1998      President of Bernard Moore, Inc.,
                                                          d.b.a. Moore Tractor, Inc., which
                                                          sells farm and industrial
                                                          equipment. Between 1983 and April
                                                          1998, Director of Continental
                                                          Pacific Bank. Continental Pacific
                                                          Bank's Chairman of the Board from
                                                          1986 until it merged with
                                                          SierraWest Bank in April 1998.
                                                          Between 1995 and April 1998,
                                                          Chairman of the Board of Directors
                                                          of California Community Bancshares
                                                          Corporation, Continental Pacific
                                                          Bank's holding company,
Gary E. Stein                          53       1998      Physician in Vacaville, California,
                                                          formerly with University of
                                                          California, Davis, Davis Medical
                                                          Group, now retired. Director of
                                                          California Community Bancshares
                                                          Corporation since its inception in
                                                          1995 and a Director of Continental
                                                          Pacific Bank since 1983 until its
                                                          merger with SierraWest Bank in
                                                          April 1998.
Thomas M. Watson                       55       1986      Managing Officer, Truckee River
                                                          Associates, (commercial real estate
                                                          management, development and sales).
EXECUTIVE OFFICERS
William T. Fike                        51                 President/CEO and Director of
                                                          SierraWest since July 1992.
                                                          President/ CEO of SierraWest Bank
                                                          since October, 1996. Executive Vice
                                                          President and Chief Operating
                                                          Officer of SierraWest, from May
                                                          1991 to July 1992.
David C. Broadley                      55                 Executive Vice President and Chief
                                                          Financial Officer of SierraWest
                                                          since February 1994. Executive Vice
                                                          President and Chief Financial
                                                          Officer of SierraWest Bank since
                                                          February 1995. Senior Vice
                                                          President and Chief Financial
                                                          Officer of SierraWest, from 1985 to
                                                          1994.
</TABLE>
 
                                       77
<PAGE>   83
 
<TABLE>
<CAPTION>
                                             YEAR FIRST
                                             APPOINTED     POSITION AND PRINCIPAL OCCUPATION
NAME                                   AGE    DIRECTOR        DURING THE PAST FIVE YEARS
- ----                                   ---   ----------   -----------------------------------
<S>                                    <C>   <C>          <C>
Patrick S. Day                         49                 Executive Vice President and Chief
                                                          Credit Officer of SierraWest and
                                                          SierraWest Bank since July 1995.
                                                          Executive Vice President and Chief
                                                          Operating Officer of Business &
                                                          Professional Bank from January
                                                          through June 1995. Principal of PSD
                                                          Associates, a bank consulting
                                                          company, from 1993 to 1995.
                                                          Executive Vice President and Chief
                                                          Credit Officer of Bank of San
                                                          Francisco from 1991 to 1993.
Robert C. Silver                       56                 Senior Vice President, manager of
                                                          Administration Division for
                                                          SierraWest Bank since November,
                                                          1995. Senior Vice President and
                                                          Director of Human Resources for
                                                          SierraWest Bank from July, 1991
                                                          through October, 1995.
Richard L. Belstock                    42                 Senior Vice President, Controller
                                                          and Chief Accounting Officer for
                                                          SierraWest since August 1997.
                                                          Senior Vice President and
                                                          Controller of SierraWest from July
                                                          1994 through September 1996. Senior
                                                          Vice President and Controller of
                                                          SierraWest Bank since June, 1994.
                                                          Vice President and Assistant
                                                          Controller for the Company from
                                                          August, 1990 through June, 1994.
Mary Jane Posnien                      55                 Senior Vice President of Operations
                                                          for SierraWest Bank since March
                                                          1998. Senior Vice President of
                                                          Operations for SierraWest Bank from
                                                          November 1995 to August 1997.
                                                          Senior Vice President of Operations
                                                          for Sierra Bank of Nevada from
                                                          March 1995 to November 1995. Vice
                                                          President of Operations for Sierra
                                                          Bank of Nevada from December 1993
                                                          to March 1995. Manager of Gotcha
                                                          Covered, a carpet/window covering
                                                          store from 1991 through 1993.
</TABLE>
 
     Messrs. Moore and Stein were elected as directors in accordance with the
terms of the merger agreement between SierraWest and California Community
Bancshares. Except for Messrs. Moore and Stein, none of the directors or
nominees were selected pursuant to any arrangement or understanding other than
with the directors of SierraWest acting
 
                                       78
<PAGE>   84
 
within their capacities as such. There are no family relationships between any
of the directors and executive officers of SierraWest.
 
THE BOARD OF DIRECTORS AND COMMITTEES
 
     SierraWest's board of directors met 14 times during 1998. None of the
directors or executive officers attended less than 75 percent of the aggregate
of all board of directors meetings and committee meetings, of which they were
members, held during 1998. SierraWest has a standing Audit/Ethics, Nominating
and Personnel/Compensation Committee.
 
     The Audit/Ethics Committee reviews audits of SierraWest and its
subsidiaries, and considers the adequacy of auditing procedures. The
Audit/Ethics Committee consists of Messrs. Coppola, John J. Johnson, Moore,
Stein, McClintock, Gaston and Mr. Henley as a member-at-large. The Audit/Ethics
Committee met 12 times in 1998. The Nominating Committee consists of Messrs.
Henley, Watson, Clark, Fike and Jones. The Nominating Committee met once in
1998. The Nominating Committee recommends the nominees for director positions on
SierraWest's Board of Directors for review and approval by the Board. SierraWest
has a Personnel/Compensation Committee which consists of Messrs. Coppola, Moore,
Watson, Clark, Gaston, McClintock, Fike and Mr. Henley as a member-at-large. The
Personnel/Compensation Committee met 12 times in 1998. The Personnel/
Compensation Committee determines the salaries of executive officers of
SierraWest and also reviews and approves salary recommendations for all Senior
Vice Presidents and above. The Committee reviews and approves all benefit
program changes recommended by management and works with management in the
development of all company-wide incentive compensation programs.
 
COMPENSATION OF DIRECTORS
 
     Directors' fees for board and committee meetings are as follows:
 
<TABLE>
<CAPTION>
                              BOARD MEETING              COMMITTEE MEETINGS
                        -------------------------   ----------------------------
                          RETAINER     ATTENDANCE    RETAINER      ATTENDANCE
                        ------------   ----------   ----------   ---------------
<S>                     <C>            <C>          <C>          <C>
Chairman of the
  Board...............  $3,383/month     $0             $0             $0
Director..............  $1,500/month     $0  (1)        $0       $150/meeting(2)
Committee Chairman....      N/A         N/A         $100/month   $150/meeting(2)
</TABLE>
 
- -------------------------
 
(1) Compensation for attendance at special board meetings is $150 per director
    per meeting.
 
(2) Fee for attendance at Directors' Loan Committee is $250 per meeting.
 
     In addition to the above fees, an educational allowance is determined
annually by the board. The Chairman of the Board allocates funds for educational
expenses pursuant to requests submitted by each director until the allowance is
exhausted.
 
     SierraWest's Deferred Compensation and Stock Award plan is provided to
members of the board of directors who are not employees of SierraWest or of its
subsidiary ("Outside Directors"). Under this plan Outside Directors are required
to take on a deferred basis one-third of their directors' fees for regular board
meetings in the form of promised shares of SierraWest common stock. The
remaining amount of director fees for regular board meetings may also be
deferred at the election of the director. The purpose of
 
                                       79
<PAGE>   85
 
this plan is to enable Outside Directors to defer receipt of compensation for
their services to later years and to provide part of the compensation for their
services in the cash value of the promised shares of SierraWest common stock in
order to better align the interest of Outside Directors with those of
SierraWest's shareholders. Under the plan the directors are not entitled to
receive shares of SierraWest common stock.
 
     Expenses for the directors and their spouses related to attendance at
SierraWest's annual weekend directors' retreat are paid for by SierraWest.
Directors are eligible for coverage under SierraWest's group health insurance
plan. Premiums for health insurance coverage are shared between the director and
SierraWest on the same basis as that for SierraWest employees. Additionally,
SierraWest pays for premiums covering the first $25,000 of accidental death
benefits and the administration of Keogh plans for directors, if they elect to
participate.
 
     SierraWest maintains a salary continuation plan (see "Salary Continuation
Plan" on page 83) for its executive officers, certain senior officers and its
directors. As of December 31, 1998, SierraWest's non-employee directors were
credited with $201,097 in accrued benefits under the directors' salary
continuation plan. SierraWest allocated $113,075 to the Salary Continuation Plan
in 1998 on behalf of its non-employee directors.
 
                                       80
<PAGE>   86
 
                             EXECUTIVE COMPENSATION
 
SUMMARY COMPENSATION TABLE
 
<TABLE>
<CAPTION>
                                                                     ANNUAL COMPENSATION
                                                             -----------------------------------
                              ANNUAL COMPENSATION                     AWARDS             PAYOUTS
                              -------------------            -------------------------   -------
                                                             # OF SHARES
                                                    OTHER    RESTRICTED    # OF SHARES
                                                    ANNUAL      STOCK       OPTIONS/      LTIP     ALL OTHER
                       YEAR    SALARY     BONUS     COMP.      AWARDS        SARS(2)     PAYOUTS     COMP.
                       ----   --------   --------   ------   -----------   -----------   -------   ---------
<S>                    <C>    <C>        <C>        <C>      <C>           <C>           <C>       <C>
William T. Fike......  1998   $260,417   $225,000   $4,428        0               0         0       $21,709
  President/CEO of     1997   $247,760   $135,000   $3,394        0               0         0       $21,005
  SierraWest and       1996   $230,384(1) $      0  $4,643        0          52,500         0       $17,351
  SierraWest Bank
David C. Broadley....  1998   $156,917   $ 60,000   $4,721        0               0         0       $23,594
  Executive Vice       1997   $140,376   $ 73,870   $3,668        0               0         0       $20,902
  President/CFO of     1996   $131,256   $      0   $  106        0               0         0       $20,154
  SierraWest and
  SierraWest Bank
Patrick S. Day.......  1998   $134,333   $ 47,726   $5,399        0               0         0       $ 5,376
  Executive Vice       1997   $129,167   $ 67,972   $3,305        0               0         0       $ 4,391
  President of the     1996   $126,519   $      0   $3,858        0               0         0       $ 2,245
  SierraWest and
  SierraWest Bank
Robert C. Silver.....  1998   $100,347   $ 35,653   $  138        0               0         0       $ 8,422
  Senior Vice          1997   $ 98,078   $ 49,780   $    0        0           2,750         0       $ 6,333
  Senior Vice          1996   $ 87,369   $      0   $    0        0               0         0       $ 4,423
  President of
  SierraWest Bank
Richard L.             1998   $ 87,362   $ 31,086   $    0        0               0         0       $ 6,052
  Belstock...........
  Senior Vice          1997   $ 83,202   $ 43,783   $    0        0               0         0       $ 4,440
                       1996   $ 81,101   $ 15,000   $    0        0               0         0       $ 3,114
 President/Controller
  of SierraWest and
  SierraWest Bank
</TABLE>
 
- -------------------------
 
NOTES:
 
(1) Includes payment of accrued vacation pay of $30,384.
 
(2) Adjusted for 5% stock dividend paid August 29, 1997.
 
     BONUS -- Bonuses are generally paid in the year after they are earned. For
purposes of this table, bonuses have been reflected in the year earned, not the
year paid.
 
     OTHER ANNUAL COMPENSATION -- Includes value of personal use of automobiles
provided by SierraWest and reimbursements for the personal portion of club dues
and spousal travel expenses.
 
                                       81
<PAGE>   87
 
     ALL OTHER COMPENSATION -- Includes the following:
 
<TABLE>
<CAPTION>
                                                    1998       1997       1996
                                                   -------    -------    -------
<S>                                                <C>        <C>        <C>
SIERRAWEST CONTRIBUTION TO 401(k) PLAN FOR:
  Mr. Fike.......................................  $ 5,000    $ 4,750    $ 4,652
  Mr. Broadley...................................  $ 5,000    $ 4,145    $ 3,896
  Mr. Day........................................  $ 3,035    $ 1,938    $   938
  Mr. Silver.....................................  $ 4,504    $ 2,779    $ 2,566
  Mr. Belstock...................................  $ 3,934    $ 2,946    $ 2,383
SIERRAWEST CONTRIBUTIONS TO ESOP PLAN FOR:
  Mr. Fike.......................................  $ 2,028(1) $ 2,715    $ 1,240
  Mr. Broadley...................................  $ 2,028(1) $ 2,382    $ 1,085
  Mr. Day........................................  $ 2,028(1) $ 2,192    $ 1,046
  Mr. Silver.....................................  $ 2,028(1) $ 1,664    $   722
  Mr. Belstock...................................  $ 2,028(1) $ 1,412    $   671
</TABLE>
 
- -------------------------
 
(1) Amount estimated for 1998, pending final plan accounting for the 1998 plan
    year.
 
<TABLE>
<S>                                                <C>        <C>        <C>
ALLOCATIONS TO SALARY CONTINUATION PLAN FOR:
  Mr. Fike.......................................  $12,031    $10,890    $ 9,858
  Mr. Broadley...................................  $14,226    $12,877    $13,675
</TABLE>
 
<TABLE>
<S>                                                <C>        <C>        <C>
COST OF LIFE INSURANCE PROVIDED BY SIERRAWEST OF
  WHICH THE BENEFIT EXCEEDED $50,000 FOR:
  Mr. Fike.......................................  $ 2,650    $ 2,650    $ 1,601
  Mr. Broadley...................................  $ 2,340    $ 1,498    $ 1,498
  Mr. Day........................................  $   313    $   261    $   261
  Mr. Silver.....................................  $ 1,890    $ 1,890    $ 1,135
  Mr. Belstock...................................  $    90    $    82    $    60
</TABLE>
 
     In 1998, SierraWest did not grant any options to any of the five executive
officers listed above.
 
                                       82
<PAGE>   88
 
            AGGREGATED OPTION/SAR EXERCISES IN LAST FISCAL YEAR AND
                            FY-END OPTION/SAR VALUE
 
<TABLE>
<CAPTION>
                                                        NUMBER OF         VALUE OF
                                                       UNEXERCISED       UNEXERCISED
                                                      OPTIONS/SARS      IN-THE-MONEY
                                                       AT FY-END-       Options/SARs
                                                        # SHARES         AT FY-END-$
                       SHARES ACQUIRED     VALUE      EXERCISABLE/      EXERCISABLE/
NAME                     ON EXERCISE      REALIZED    UNEXERCISABLE     UNEXERCISABLE
- ----                   ---------------    --------    -------------    ---------------
<S>                    <C>                <C>         <C>              <C>
Mr. Fike.............      45,987         $926,560    47,200/6,300     $589,530/92,712
Mr. Broadley.........       5,280         $127,900     2,835/2,520     $ 40,146/35,685
Mr. Day..............           0         $      0     6,720/5,880     $103,160/91,265
Mr. Silver...........       2,362         $ 58,488     1,180/3,460     $  9,331/19,481
Mr. Belstock.........       2,257         $ 49,332     1,680/3,360     $ 23,290/46,580
</TABLE>
 
The value of unexercised in-the-money options and the value realized on exercise
of stock options is calculated by subtracting the exercise price from the fair
market value at December 31, 1998 or the date of exercise, respectively, of the
securities underlying the options. All share amounts have been adjusted for the
5% stock dividend paid August 29, 1997.
 
SALARY CONTINUATION PLAN
 
     SierraWest has entered into agreements with certain directors of SierraWest
and SierraWest Bank and certain executive officers of SierraWest Bank, to
provide for salary continuation benefits upon the retirement or earlier death of
the directors and executive officers. The benefits pursuant to this plan are:
$50,000 per year for Mr. Fike and $40,000 per year for Mr. Broadley payable for
a period of 20 years following retirement at age 65 or earlier death. Benefits
for the participating directors are $4,000 per year for 15 years, beginning 15
years after their respective plan commencement dates.
 
     In the event of earlier death, the benefits are payable to the officer's or
director's designated beneficiary. SierraWest has secured life insurance
policies for the purpose of protecting it from loss in the event of earlier
death. In the event of earlier retirement or early termination of office or
employment of the officer or director, a reduced benefit is payable. At the
option of the officer or director a reduced benefit may be received in a lump
sum based on a discounting formula. Accrued benefits for both officers and
directors vest 20% per year over a five-year period from the date of association
with SierraWest. Additionally, there are restrictions on the covered individual
from engaging in any competing occupation upon retirement and provisions
requiring the covered individual to perform advisory services, for compensation,
for a period of five (5) years following retirement or early termination of
office or employment.
 
     During 1996 the agreements of Messrs. Fike and Broadley and certain
directors of SierraWest were modified to provide for an acceleration of benefits
such that the full amount due under the agreement would become payable in the
case of a change of control of SierraWest. For the Directors' plan this would be
in the form of a lump sum payment based on a discounting formula. The plan for
Messrs. Fike and Broadley provided for this payment in the form of 240 equal
monthly installments. The agreements were further modified to eliminate the
restrictions described above related to engaging in a competing occupation and
the performance of advisory services upon a change in control.
 
                                       83
<PAGE>   89
 
     As of December 31, 1998, executive officers were credited with the
following accrued benefits under this Plan:
 
<TABLE>
<S>                                                           <C>
David C. Broadley...........................................  $120,491
William T. Fike.............................................    67,772
</TABLE>
 
EMPLOYMENT AGREEMENTS
 
     Effective October 1, 1994, SierraWest entered into an employment agreement
with Mr. Fike covering the terms of his employment, compensation, and conditions
of termination. Unless employment is terminated or the agreement is extended,
Mr. Fike's employment will continue until December 31, 2000. His base salary was
set initially at $200,000 per year and he is eligible for bonuses and
participation in all employee benefit programs. He will be considered for
periodic increases in base salary at the discretion of the board of directors.
He will continue to participate in the Salary Continuation Plan and be provided
with a company car and a country club membership. In the event of termination
without cause, Mr. Fike will receive all amounts owing to him at the date of
termination, a lump-sum severance payment equal to eighteen months' base salary,
direct payment of the premiums necessary to continue then existing medical
coverage for 18 months at the rate equivalent to SierraWest employees and
retention of his company-provided automobile and country club membership. During
the month of February 1997 Mr. Fike's base salary was increased to $250,000 per
year and during February 1998 Mr. Fike's base salary was increased to $262,500
per year.
 
     In January 1999, Mr. Fike's employment agreement was amended to allow for a
scale back of payments to be made to Mr. Fike if any payment to be made, or
benefit to be provided, pursuant to the agreement would constitute a "parachute
payment" as defined in Section 280G of the Internal Revenue Code. The payments
to be made, or benefits to be provided, shall be reduced so that the aggregate
present value of all parachute payments does not exceed 299% of Mr. Fike's
"annualized includible compensation for the base period" (as such term is
defined in Section 280G(d)(1) of the Code Internal Revenue).
 
     In 1996, Messrs. Broadley, Day, Silver and Belstock entered into Senior
Manager Separation Benefits Agreements. Under the terms of these agreements as
amended during 1997 and 1998, certain benefits would become payable to the
manager in the event of the termination of employment for any reason, other than
a material violation of SierraWest's personnel policies and procedures. Ms.
Posnien entered into her agreement in 1998. The benefit includes one year's base
salary (as to Messrs. Broadley and Day) or nine months' base salary (as to Mr.
Silver and Ms. Posnien) or six months' base salary (as to Mr. Belstock) paid as
a lump sum or in 24 equal semi-monthly payments (as to Messrs. Broadley and Day)
or 18 equal semi-monthly payments (as to Mr. Silver and Ms. Posnien) or twelve
equal semi-monthly payments (as to Mr. Belstock), at the election of the
executive officer. If the semi-monthly payments are chosen, health benefits
continue to be provided on the same terms as during active employment.
 
     For Mr. Broadley, in the event of a change in control or reorganization of
SierraWest, the executive officer may, within a nine month period, resign from
SierraWest and receive the same benefits as would be payable upon involuntary
termination. Additionally, as to Mr. Broadley, upon termination of service for
any reason, except for cause, he is entitled to receive SierraWest-provided
automobile in use by him at the time of his termination. For Messrs. Day and
Silver and Ms. Posnien, resignation in response to and reasonably promptly
following a material reduction in job duties and responsibilities and/or
material
 
                                       84
<PAGE>   90
 
reduction in compensation which reduction in job duties, responsibilities and/or
compensation occurs within six months following a change in control would result
in benefits the same as would be payable upon involuntary termination.
 
PERSONNEL/COMPENSATION COMMITTEE REPORT ON EXECUTIVE COMPENSATION
 
COMPENSATION POLICIES FOR EXECUTIVE OFFICERS
 
     The members of the Personnel/Compensation Committee collectively represent
a wide range of business and professional occupations, including business owners
and operators, accountants, and retired community leaders. They have available
to them various surveys reflecting executive compensation practices in banking
and other related industries. These sources are used by the Committee in
reviewing compensation. Once each year the Committee reviews the total
compensation of the executive officers listed in this proxy
statement-prospectus. The Committee has adopted a practice of keeping the base
salaries of these executives at, or slightly below, industry norms for
comparable positions within similarly sized and located institutions. The
Committee then establishes cash incentive bonus plans and stock option grants to
bring the executives' total compensation to, or above, industry norms only if
certain performance criteria are met. The performance criteria and resulting
cash incentive payments are approved annually by the Committee, and reflect
those elements that will most closely affect earnings and the growth of
shareholder equity.
 
     In the Committee's opinion the named executives are properly compensated at
the present time when compared with all others in similar positions in companies
of similar size. In the Committee's opinion, they are not overcompensated and
never have been during the Committee's tenure.
 
CHIEF EXECUTIVE OFFICER COMPENSATION
 
     In 1998, Mr. Fike received a salary of $260,417. This salary reflected a 5%
merit increase awarded on March 1, 1998. Mr. Fike's base salary was paid in
accordance with an employment agreement discussed herein. The Committee
considered this salary appropriate in light of Mr. Fike's leadership of one of
the stronger bank holding companies in California. Mr. Fike's total cash
compensation was also based on his contributions to the overall long-term
strategy and financial success of SierraWest.
 
     The employment agreement was executed in 1994 and is effective through
1999. The Committee retained the consulting services of the Wyatt Company, a
leading compensation and benefits consulting firm, to research and recommends a
total compensation package for Mr. Fike, which is reflected in the employment
agreement. The only instructions given to the Wyatt Company were to recommend a
package based on comparable bank holding companies in California. During the
course of the Wyatt Company's engagement, the Committee independently reviewed
the following peer group survey studies: The Findley Reports "1994 Senior
Management Compensation Survey Analysis of California Banks;" Deloitte & Touche
LLP "California Banks 1994 Compensation Survey;" BAI Foundation "The Bank Key
Executive Compensation Survey -- 1994 Results;" Wyatt Data Services "1994
Community Bank Compensation Report;" and Sheshunoff "Bank Executive and Director
Compensation Survey." Based on this data, the Committee established Mr. Fike's
annual salary. The salary level thus established was then reviewed by Wyatt
Company's consultant and deemed to be competitive and within the appropriate
range.
 
                                       85
<PAGE>   91
 
     The board of directors awarded an incentive bonus of $225,000 to Mr. Fike
for his performance during 1998. In determining incentive bonus awards the
Compensation Committee and the board generally considers many factors, some of
which are: strategic focus, return on equity, improvement in shareholder value,
credit quality, plus the efficiency ratio of SierraWest. Additionally, study is
made of aggregate total compensation paid for other outstanding performing banks
of similar size. In light of Mr. Fike's leadership and consideration of the
above factors, an incentive bonus of $225,000 was awarded.
 
PERSONNEL/COMPENSATION COMMITTEE:
 
<TABLE>
<S>                                     <C>
       David W. Clark, Chairman                     William T. Fike
           Richard S. Gaston              Jerrold T. Henley (member-at-large)
         William W. McClintock
</TABLE>
 
PERSONNEL/COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION
 
     With the exception of Jerrold Henley and William Fike, no member of the
Personnel/ Compensation Committee is a former or current officer or employee of
SierraWest. Mr. Henley retired as President and CEO of SierraWest in June 1992.
Mr. Fike succeeded Mr. Henley as President and CEO of SierraWest. There are no
compensation committee interlocks between SierraWest and other entities
involving SierraWest executive officers and SierraWest directors.
 
     COMMON STOCK PERFORMANCE:  As part of the executive compensation
information presented in this proxy statement-prospectus, the Securities and
Exchange Commission requires a five-year comparison of stock performance for
SierraWest with stock performance of appropriate similar companies.
 
                                       86
<PAGE>   92
 
     The following graph compares SierraWest's performance with the total return
index for the Nasdaq Stock Market (US Companies) and the total return index for
Nasdaq traded banks:
 
                            COMPARISON OF FIVE YEAR
                            CUMULATIVE TOTAL RETURNS
 
                              [Graph appears here]

<TABLE>
<CAPTION>
                                                   SIERRA WEST BANCORP        CRSP INDEX FOR NASDAQ
                                                   -------------------          STOCK MARKET (US          CRSP INDEX FOR NASDAQ
                                                                                   COMPANIES)                  BANK STOCKS
                                                                              ---------------------       ---------------------
<S>                                             <C>                         <C>                         <C>
Dec 93                                                   100.00                      100.00                      100.00
Dec 94                                                   125.00                       97.75                       99.63
Dec 95                                                   155.63                       98.23                      148.38
Dec 96                                                   232.00                      170.02                      195.91
Dec 97                                                   538.39                      208.30                      328.02
Dec 98                                                   405.54                      293.52                      325.38
</TABLE>
 
- -------------------------
 
Note to Graph Above:  Assumes $100 invested on December 31, 1993, in SierraWest
Common Stock and an identical amount in the NASDAQ Indexes, includes the
reinvestment of dividends. The NASDAQ indexes were compiled by the Center for
Research in Securities Prices (CRSP), University of Chicago, Graduate School of
Business.
 
           SHAREHOLDINGS OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT
 
     Management of SierraWest knows of no person who owns, beneficially or of
record, either individually or together with associates, five percent (5%) or
more of the outstanding shares of SierraWest's common stock, except as set forth
in the table on the following page. This table sets forth, as of March 15, 1999
the number and percentage of shares of SierraWest's outstanding common stock
beneficially owned, directly or indirectly, by each of SierraWest's current
directors, chief executive officer, and four next most highly compensated
executive officers of SierraWest whose salary and bonus exceeded $100,000 during
1998 ("named executive officers") and principal shareholders, and by the
directors and executive officers of SierraWest as a group. The shares
"beneficially owned" are determined under Securities and Exchange Commission
rules, and do not necessarily indicate ownership for any other purpose. In
general, beneficial ownership includes shares over which a director, principal
shareholder, or executive officer has sole or shared voting or investment power
and shares which such person has the right to acquire within sixty (60) days of
March 15, 1999. Management is not aware of any arrangements which may, at a
subsequent date, result in a change of control of SierraWest.
 
                                       87
<PAGE>   93
 
<TABLE>
<CAPTION>
                               SHARES        SHARES
                             OWNED WITH    OWNED WITH
                             SOLE VOTING     SHARED       SHARES
                                 AND       VOTING AND   ACQUIRABLE
NAME AND ADDRESS OF          INVESTMENT    INVESTMENT   WITHIN 60               PERCENT OF
BENEFICIAL OWNER(1)             POWER        POWER      DAYS(2)(3)    TOTAL    TOTAL SHARES
- -------------------          -----------   ----------   ----------   -------   ------------
<S>                          <C>           <C>          <C>          <C>       <C>
Directors and Nominees and
  Named Executive Officers
Richard L. Belstock........      3,308            0        1,680       4,988         *
David C. Broadley(4).......     26,220        8,350        2,835      37,405         *
David W. Clark.............      9,985        5,864        7,095      22,944         *
Ralph J. Coppola...........      7,430            0        2,017       9,447         *
Patrick S. Day.............      1,914            0        6,720       8,634         *
William T. Fike(4).........     51,401        9,112       47,200     107,713       2.1%
Richard Gaston.............      3,866            0        2,335       6,201         *
Jerrold T. Henley..........     47,725            0        9,381      57,106       1.1%
John J. Johnson............      2,388        1,155        2,383       5,926         *
Ronald A. Johnson..........      3,931          100          671       4,702         *
A. Morgan Jones............      1,727          650        9,334      11,711         *
Jack V. Leonesio...........     16,688            0          671      17,359         *
William W. McClintock......      7,820          105        9,334      17,259         *
Bernard E. Moore...........      7,648            0          263       7,911         *
Robert C. Silver(4)........        754        8,350        1,180      10,284         *
Gary E. Stein..............     17,912        1,704          263      19,879         *
Thomas M. Watson...........      8,102            0       10,733      18,835         *
Total for Directors and
  Executive Officers
  (numbering 18)...........    219,752       35,390      118,507     373,649       7.0%
Principal Shareholders
Investors of America, L. P.
  39 Glen Eagles Drive
  St. Louis, MO 63124......    297,045                               297,045       5.6%
</TABLE>
 
- -------------------------
 
 *  Less than 1%
 
(1) The address for all Directors, Nominees and Named Executive Officers is c/o
    SierraWest, P. O. Box 61000, Truckee, CA 96161.
 
(2) Includes shares that can be purchased through SierraWest's stock option
    plan. For non-employee directors, includes 671 shares earned under the
    Directors Deferred Compensation and Stock Award Plan for all but Mr. Clark
    (688 shares), Mr. Henley (718 shares), Mr. Watson (2,070 shares), Mr. Stein
    (159 shares), Mr. Moore (159 shares) and Mr. Coppola (2,017 shares).
 
(3) Upon completion of the proposed merger, all unvested options will become
    vested. Unvested options held by the above listed persons total 27,609.
 
                                       88
<PAGE>   94
 
(4) Messrs. Fike, Broadley and Silver have voting authority for 8,350 shares of
    unallocated SierraWest common stock held by the SierraWest ESOP plan.
 
               SECTION 16(a) BENEFICIAL OWNERSHIP AND COMPLIANCE
 
     Section 16(a) of the Securities Exchange Act requires SierraWest's
directors, certain officers and persons who own more than ten percent of a
registered class of SierraWest's equity securities, to file reports of ownership
and changes in ownership with the Securities and Exchange Commission. Directors,
certain officers and greater than ten-percent shareholders ("Reporting Persons")
are required by Securities and Exchange Commission regulation to furnish
SierraWest with copies of all Section 16(a) forms they file.
 
     Based solely on its review of the copies of such forms received by it, or
written representations from certain reporting persons that no Forms 5 were
required for those persons, SierraWest believes that from January 1, 1998, to
December 31, 1998, all filing requirements applicable to its Reporting Persons
were complied with, except that Mr. Ronald Johnson was late in filing a Form 4
covering one transaction.
 
                 CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS
 
     Some of the directors of SierraWest and the companies with which they are
associated are customers of, or have had banking transactions with, SierraWest
Bank in the ordinary course of its business and SierraWest Bank expects to have
banking transactions with these persons in the future. In management's opinion,
since January 1, 1998, all loans and commitments to lend included in such
transactions were made in the ordinary course of business on substantially the
same terms, including interest rates and collateral, as those prevailing for
comparable transactions with other persons of similar credit worthiness and, in
the opinion of management, did not involve more than a normal risk of
collectability or present other unfavorable features.
 
                                 LEGAL MATTERS
 
     Certain legal matters with respect to BancWest, including the validity of
the shares of BancWest common stock to be issued in connection with the merger,
will be passed upon for BancWest by Simpson Thacher & Bartlett, New York, New
York. Certain federal tax matters will be passed upon by Pillsbury Madison &
Sutro LLP, San Francisco, California and McCutchen, Doyle, Brown & Enersen, LLP,
San Francisco, California.
 
                                    EXPERTS
 
     The consolidated financial statements incorporated in this proxy
statement-prospectus by reference from BancWest's annual report on Form 10-K for
the year ended December 31,1998 have been audited by PricewaterhouseCoopers LLP,
independent auditors, as stated in their report which is incorporated herein by
reference. Such statements have been so incorporated in reliance upon the report
of such firm given upon the authority as experts in accounting and auditing.
 
     The consolidated financial statements of BancWest Corporation and
subsidiaries (old BancWest) as of December 31, 1997 and 1996 and for the years
then ended incorporated by reference from BancWest Corporation's (new BancWest)
Form 8-K/A dated November 1, 1998 have been audited by PricewaterhouseCoopers
LLP, independent
 
                                       89
<PAGE>   95
 
auditors, as stated in their report which is incorporated herein by reference.
Such statements have been so incorporated in reliance upon the report of such
firm given upon their authority as experts in accounting and auditing.
 
     The consolidated financial statements of BancWest Corporation and
subsidiaries (old BancWest) for the year ended December 31, 1995 incorporated by
reference from BancWest Corporation's (new BancWest) Form 8-K/A dated November
1, 1998 have been audited by Deloitte & Touche LLP, independent auditors, as
stated in their report which is incorporated herein by reference. Such
statements have been so incorporated in reliance upon the report of such firm
given upon their authority as experts in accounting and auditing.
 
     The consolidated financial statements incorporated in this proxy
statement-prospectus by reference from SierraWest's annual report on Form 10-K
for the year ended December 31, 1998 have been audited by Deloitte & Touche LLP,
independent auditors, as stated in their report which is incorporated herein by
reference. Such statements have been so incorporated in reliance upon the report
of such firm given upon their authority as experts in accounting and auditing.
Representatives of Deloitte & Touche LLP are expected to be present at the
annual meeting and are expected to have an opportunity to make a statement if
they desire and to be available to respond to appropriate questions.
 
                     SUBMISSIONS OF SHAREHOLDERS' PROPOSALS
 
     If the merger is completed, you will become a stockholder of BancWest. If
you wish to submit a proposal for consideration at the BancWest 2000 Annual
Meeting of Stockholders you must submit a proposal to BancWest Corporation, 999
Bishop Street, Honolulu, Hawaii 96813, Attention: Corporate Secretary. Your
proposal must be received no later than November 1, 1999, for inclusion, if
appropriate, in BancWest's proxy statement and form of proxy relating to its
2000 Annual Meeting of Stockholders and is subject to the Securities and
Exchange Commission's rules regarding the inclusion of shareholder proposals.
 
     In the event the merger is not completed, you must submit proposals for
consideration at the SierraWest 2000 Annual Meeting of Shareholders to
SierraWest Bancorp, P.O. Box 61000, 10181 Truckee-Tahoe Airport Road, Truckee,
California 96161, Attention: Corporate Secretary. Such proposals must have been
received not later than December 20, 1999 for inclusion, if appropriate, in
SierraWest's proxy statement and form of proxy relating to its 2000 Annual
Meeting of Shareholders and are subject to the Securities and Exchange
Commission's rules regarding the inclusion of shareholder proposals.
 
                   INFORMATION CONCERNING BANCWEST MANAGEMENT
 
     Information concerning:
 
     - directors and executive officers,
 
     - executive compensation,
 
     - principal stockholders,
 
     - certain relationships and related transactions,
 
     - and other related matters concerning BancWest
 
                                       90
<PAGE>   96
 
is included or incorporated by reference in its annual report on Form 10-K for
the year ended December 31, 1998.
 
     BancWest's annual report on form 10-K is incorporated by reference into
this proxy statement-prospectus.
 
                      WHERE YOU CAN FIND MORE INFORMATION
 
     Both BancWest and SierraWest file annual, quarterly and current reports,
proxy statements and other information with the Securities and Exchange
Commission. You may read and copy any reports, statements or other information
that BancWest and SierraWest file at the Commission's public reference rooms in
Washington, D.C., New York, New York and Chicago, Illinois. Please call the
Commission at (800) SEC-0330 for further information on the public reference
rooms. The Commission also maintains an Internet World Wide Web site at
"http://www.sec.gov" at which reports, proxy and information statements and
other information regarding BancWest and SierraWest are available. In addition,
reports, proxy statements and other information concerning SierraWest also may
be inspected at the offices of The Nasdaq Stock Market, 1735 K Street,
Washington, D.C. 20006.
 
     BancWest has filed with the Securities and Exchange Commission a
registration statement on Form S-4 under the Securities Act relating to the
shares of BancWest common stock to be issued in connection with the merger. This
proxy statement-prospectus also constitutes the prospectus of BancWest filed as
part of the registration statement and does not contain all the information set
forth in the registration statement and exhibits thereto. You may copy and read
the registration statement and its exhibits at the public reference facilities
maintained by the Securities Exchange Commission at the address set forth above.
 
     The Commission allows both BancWest and SierraWest to "incorporate by
reference" information into this proxy statement-prospectus, which means that
BancWest and SierraWest can disclose important information to you by referring
you to another document filed separately with the Commission. The information
incorporated by reference is deemed to be part of this proxy
statement-prospectus, except for any information superseded by information
contained directly in this proxy statement-prospectus. This proxy
statement-prospectus incorporates by reference the documents set forth below
that either BancWest or SierraWest has previously filed with the Commission.
These documents contain important information about BancWest and SierraWest and
their respective financial conditions.
 
                                       91
<PAGE>   97
 
<TABLE>
<S>                                     <C>
BANCWEST COMMISSION FILINGS             PERIOD
(FILE NO. 0-7949)
Annual Report on Form 10-K............  Year ended December 31, 1998
Current Reports on Form 8-K...........  Dated November 1, 1998 (filed December
                                        30, 1998) and dated February 25, 1999
                                        (filed February 26, 1999)
Proxy Statement.......................  Dated March 1, 1999
Registration Statement on Form 8-A....  Dated October 30, 1998
SIERRAWEST COMMISSION FILINGS
(FILE NO. 0-15450)
Annual Report on Form 10-K............  Year ended December 31, 1998
Current Reports on Form 8-K...........  Dated February 25, 1999 (filed March
                                        3, 1999)
</TABLE>
 
     BancWest and SierraWest incorporate by reference any additional documents
that either may file with the Commission between the date of this proxy
statement-prospectus and the date of the annual meeting. These include periodic
reports, such as annual reports on Form 10-K, quarterly reports on Form 10-Q and
current reports on Form 8-K, as well as proxy statements.
 
     BancWest has supplied all information contained or incorporated by
reference in the proxy statement-prospectus relating to BancWest and SierraWest
has supplied all such information relating to SierraWest.
 
     This proxy statement-prospectus incorporates by reference documents
relating to BancWest and SierraWest which are not presented in this proxy
statement-prospectus or delivered herewith. Those documents relating to BancWest
are available from BancWest without charge, excluding all exhibits unless
specifically incorporated by reference in this proxy statement-prospectus, by
requesting them in writing or by telephone from BancWest Corporation, Herbert E.
Wolff, Secretary, 999 Bishop Street, Honolulu, Hawaii 96813, (808) 525-8144.
Those documents relating to SierraWest are available from SierraWest without
charge, excluding all exhibits unless specifically incorporated by reference in
this proxy statement-prospectus, by requesting them in writing or by telephone
from SierraWest Bancorp, David Broadley, Chief Financial Officer, 10181
Truckee-Tahoe Airport Road, Truckee, California 96161, (530) 582-3000. If you
would like to request documents from either BancWest or SierraWest, please do so
by May 20, 1999 to receive them before the annual meeting.
 
     In deciding how to vote on the merger, you should rely only on the
information contained or incorporated by reference in this proxy
statement-prospectus. Neither BancWest nor SierraWest has authorized any person
to provide you with any information that is different from what is contained in
this proxy statement-prospectus. This proxy statement-prospectus is dated April
  , 1999. You should not assume that the information contained in this proxy
statement-prospectus is accurate as of any date other than such date, and
neither the mailing to you of this proxy statement-prospectus nor the issuance
to you of shares of BancWest common stock will create any implication to the
contrary. This proxy statement-prospectus does not constitute an offer to sell
or a solicitation of any offer to buy any securities, or the solicitation of a
proxy in any jurisdiction in which, or to any person to whom, it is unlawful.
 
                                       92
<PAGE>   98
 
                                                                      APPENDIX A
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
 
                          AGREEMENT AND PLAN OF MERGER
 
                         DATED AS OF FEBRUARY 25, 1999*
 
                                     AMONG
 
                             BANCWEST CORPORATION,
 
                                BANK OF THE WEST
 
                                      AND
 
                               SIERRAWEST BANCORP
 
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
* RESTATED TO REFLECT AMENDMENT NO. 1 DATED MARCH 24, 1999
<PAGE>   99
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                         PAGE
                                         ----
<S>     <C>   <C>                        <C>
ARTICLE I  THE MERGER..................   A-1
  1.1   Effective Time of the Merger...   A-1
  1.2   Closing........................   A-2
  1.3   Effects of the Merger..........   A-2
  1.4   Alternative Structure..........   A-2
  1.5   Absence of Control.............   A-2
ARTICLE II  EFFECT OF THE MERGER ON THE
  CAPITAL STOCK OF THE CONSTITUENT
  CORPORATIONS; EXCHANGE OF
  CERTIFICATES.........................   A-3
  2.1   Effect on Capital Stock of the
          Constituent Corporations.....   A-3
        (a)   Cancellation of Treasury
              Stock....................   A-3
        (b)   Conversion of SierraWest
              Common Stock.............   A-3
        (c)   BW Common Stock..........   A-3
        (d)   BC Capital Stock.........   A-3
  2.2   No Further Ownership Rights in
          SierraWest Common Stock......   A-4
  2.3   Fractional Shares..............   A-4
  2.4   Surrender of Shares of
          SierraWest Common Stock......   A-4
  2.5   Adjustments....................   A-5
  2.6   Options........................   A-5
ARTICLE III  REPRESENTATIONS AND
  WARRANTIES...........................   A-6
  3.1   Representations and Warranties
          of SierraWest................   A-6
        (a)   Organization, Standing
              and Power................   A-6
        (b)   Capital Structure;
              Ownership of BC Common
              Stock....................   A-7
        (c)   Authority; No
              Violation................   A-8
        (d)   Financial Statements.....   A-9
        (e)   SierraWest SEC
              Documents................  A-10
        (f)   SierraWest Information
              Supplied.................  A-10
        (g)   Compliance with
              Applicable Laws..........  A-11
</TABLE>
 
<TABLE>
<CAPTION>
                                         PAGE
                                         ----
<S>     <C>   <C>                        <C>
        (h)   Litigation...............  A-11
        (i)   Taxes....................  A-12
        (j)   Certain Agreements.......  A-13
        (k)   Benefit Plans............  A-14
        (l)   Subsidiaries.............  A-16
        (m)   Agreements with Bank or
              Other Regulators.........  A-16
        (n)   Absence of Certain
              Changes or Events........  A-16
        (o)   Undisclosed
              Liabilities..............  A-16
        (p)   Governmental Reports.....  A-17
        (q)   Environmental
              Liability................  A-17
        (r)   Properties...............  A-18
        (s)   Transactions with
              Affiliates...............  A-18
        (t)   Brokers or Finders.......  A-18
        (u)   Intellectual Property....  A-19
        (v)   Pooling of Interests.....  A-19
        (w)   Opinion of Financial
              Advisor..................  A-19
        (x)   Rights Agreement; Anti-
              takeover Provisions......  A-19
        (y)   Community Reinvestment
              Act Compliance...........  A-19
        (z)   Year 2000 Readiness......  A-19
        (aa)  Dissenters' Rights.......  A-20
        (ab)  Insurance................  A-20
        (ac)  Loan and Other Assets....  A-21
        (ad)  Restrictions on
              Investments..............  A-22
        (ae)  No Brokered Deposits.....  A-22
        (af)  Derivatives Contracts;
              Structured Notes; Etc....  A-22
        (ag)  Labor Matters............  A-22
  3.2   Representations and Warranties
          of BC........................  A-23
        (a)   Organization, Standing
              and Power................  A-23
        (b)   Capital Structure........  A-23
</TABLE>
 
                                       A-i
<PAGE>   100
 
<TABLE>
<CAPTION>
                                         PAGE
                                         ----
<S>     <C>   <C>                        <C>
        (c)   Authority; No
              Violation................  A-24
        (d)   Financial Statements.....  A-25
        (e)   BC SEC Documents.........  A-25
        (f)   BC Information
              Supplied.................  A-26
        (g)   Compliance with
              Applicable Laws..........  A-26
        (h)   Litigation...............  A-26
        (i)   Subsidiaries.............  A-27
        (j)   Agreements with Bank
              Regulators...............  A-27
        (k)   Absence of Certain
              Changes or Events........  A-27
        (l)   Undisclosed
              Liabilities..............  A-27
        (m)   Governmental Reports.....  A-28
        (n)   Pooling of Interests.....  A-28
        (o)   Vote Required............  A-28
        (p)   Brokers or Finders.......  A-28
        (q)   Community Reinvestment
              Act Compliance...........  A-28
        (r)   Year 2000 Readiness......  A-28
ARTICLE IV  COVENANTS RELATING TO
  CONDUCT OF BUSINESS..................  A-29
  4.1   Covenants of SierraWest........  A-29
  4.2   Covenants of BC................  A-32
ARTICLE V  ADDITIONAL AGREEMENTS.......
                                         A-33
  5.1   Regulatory Matters.............  A-33
  5.2   Access to Information..........  A-34
  5.3   SierraWest Shareholders'
        Meeting........................  A-34
  5.4   No Solicitations...............  A-34
  5.5   Legal Conditions...............  A-35
  5.6   Employee Benefit Plans.........  A-35
  5.7   Indemnification; Directors' and
          Officers' Insurance..........  A-36
  5.8   Additional Agreements..........  A-38
  5.9   Fees and Expenses..............  A-38
  5.10  Cooperation....................  A-38
  5.11  Affiliates.....................  A-38
  5.12  Stock Exchange Listing.........  A-38
  5.13  Advice of Changes..............  A-38
</TABLE>
 
<TABLE>
<CAPTION>
                                         PAGE
                                         ----
<S>     <C>   <C>                        <C>
  5.14  Subsequent Interim and Annual
          Financial Statements; Certain
          Reports......................  A-39
  5.15  Dissenters' Rights.............  A-39
ARTICLE VI  CONDITIONS PRECEDENT.......  A-39
  6.1   Conditions to Each Party's
          Obligation...................  A-39
        (a)   Shareholder Approval.....  A-39
        (b)   Other Approvals..........  A-39
        (c)   No Injunctions or
              Restraints...............  A-39
        (d)   NYSE Listing.............  A-40
        (e)   S-4......................  A-40
        (f)   Pooling..................  A-40
  6.2   Conditions to Obligations of
        BC.............................  A-40
        (a)   Representations and
              Warranties...............  A-40
        (b)   Performance of
              Obligations..............  A-40
        (c)   Corporate Action.........  A-40
        (d)   Tax Opinion..............  A-40
        (e)   SierraWest Rights
              Agreement................  A-40
        (f)   Burdensome Condition.....  A-40
        (g)   SAS 71 Review Letters....  A-41
        (h)   Bank Merger..............  A-41
        (i)   No Dissenters' Rights....  A-41
  6.3   Conditions to Obligations of
          SierraWest...................  A-41
        (a)   Representations and
              Warranties...............  A-41
        (b)   Performance of
              Obligations..............  A-41
        (c)   Tax Opinion..............  A-41
        (d)   Corporate Action.........  A-42
ARTICLE VII  TERMINATION AND
  AMENDMENT............................  A-42
  7.1   Termination....................  A-42
  7.2   Effect of Termination..........  A-44
  7.3   Amendment......................  A-44
  7.4   Extension; Waiver..............  A-44
ARTICLE VIII  GENERAL PROVISIONS.......  A-45
</TABLE>
 
                                      A-ii
<PAGE>   101
 
<TABLE>
<CAPTION>
                                         PAGE
                                         ----
<S>     <C>   <C>                        <C>
  8.1   Nonsurvival of Representations
          and Warranties...............  A-45
  8.2   Notices........................  A-45
  8.3   Interpretation.................  A-46
  8.4   Counterparts...................  A-46
  8.5   Entire Agreement No Third Party
          Beneficiaries; Rights of
          Ownership....................  A-46
</TABLE>
 
<TABLE>
<CAPTION>
                                         PAGE
                                         ----
<S>     <C>   <C>                        <C>
  8.6   Governing Law; Consent to
          Jurisdiction.................  A-46
  8.7   Severability...................  A-47
  8.8   Assignment.....................  A-47
  8.9   Publicity......................  A-47
</TABLE>
 
                                      A-iii
<PAGE>   102
 
                             INDEX OF DEFINED TERMS
 
<TABLE>
<S>                                     <C>
Affiliate.............................   A-8
Agreement.............................   A-1
Agreement of Merger...................   A-1
Average BC Closing Price..............  A-43
Average BC Starting Price.............  A-43
Bank Merger...........................   A-1
Bank Regulators.......................  A-11
BC....................................   A-1
BC Class A Common Stock...............  A-23
BC Disclosure Schedule................  A-23
BC Permits............................  A-26
BC Preferred Stock....................  A-23
BC Ratio..............................  A-43
BC SEC Reports........................  A-25
BC Stock Plans........................  A-23
BC Common Stock.......................   A-3
Benefit Plans.........................  A-14
BHC Act...............................   A-6
BIF...................................   A-6
Burdensome Condition..................  A-41
Business Day..........................   A-2
BW....................................   A-1
Change in Control.....................  A-36
Closing...............................   A-2
Closing Date..........................   A-2
Code..................................   A-1
Confidentiality Agreement.............  A-34
Consents..............................  A-39
Constituent Corporations..............   A-2
CRA...................................  A-19
Derivatives Contract..................  A-22
Determination Date....................  A-44
DPC Shares............................   A-3
Effective Time........................   A-1
Environmental Laws....................  A-17
ERISA.................................  A-14
Exchange Act..........................   A-9
Exchange Agent........................   A-4
Exchange Ratio........................   A-3
FDIC..................................   A-6
Federal Reserve.......................   A-9
FFIEC.................................  A-20
FHB...................................  A-23
Final Index Price.....................  A-44
GAAP..................................  A-10
Governmental Entity...................   A-9
HSR Act...............................   A-9
Indemnified Liabilities...............  A-37
Indemnified Parties...................  A-37
Index.................................  A-44
Index Ratio...........................  A-43
Initial Index Price...................  A-44
Injunction............................  A-39
Litigation............................  A-11
material..............................   A-6
material adverse effect...............   A-6
Merger................................   A-1
NYSE..................................   A-4
OREO..................................  A-21
PBGC..................................  A-15
person................................   A-8
Preferred Lender......................  A-11
Proxy Statement.......................   A-9
Representatives.......................  A-34
Requisite Regulatory Approvals........  A-39
S-4...................................  A-25
SAS 71................................  A-41
SBA...................................  A-11
SBA Act...............................  A-11
SEC...................................   A-7
Securities Act........................  A-10
SFAS No. 5............................   A-6
SierraWest............................   A-1
SierraWest Benefit Plans..............  A-14
SierraWest Certificates...............   A-4
SierraWest Common Stock...............   A-1
SierraWest Consolidated Financial
  Statements..........................   A-9
</TABLE>
 
                                      A-iv
<PAGE>   103
<TABLE>
<S>                                     <C>
SierraWest Disclosure Schedule........   A-7
SierraWest Intellectual Property......  A-19
SierraWest Option.....................   A-5
SierraWest Permits....................  A-11
SierraWest Preferred Stock............   A-7
SierraWest Rights Agreement...........  A-19
SierraWest SEC Reports................  A-10
SierraWest Shareholder Approval.......   A-8
SierraWest Shareholders' Meeting......  A-10
SierraWest Stock Option Plans.........   A-5
SierraWest's Current Premium..........  A-37
Significant Subsidiary................   A-7
State Banking Approvals...............   A-9
State Takeover Approvals..............   A-9
Stock Option Agreement................   A-1
Subsidiary............................   A-6
Surviving Corporation.................   A-2
SWB...................................   A-1
SWB Common Stock......................   A-1
Takeover Proposal.....................  A-35
tax...................................  A-13
Tax return............................  A-13
taxable...............................  A-13
taxes.................................  A-13
trading day...........................   A-4
Transaction Agreements................   A-7
Trust Account Shares..................   A-3
Violation.............................   A-8
Year 2000 Ready.......................  A-20
</TABLE>
 
                                       A-v
<PAGE>   104
 
     AGREEMENT AND PLAN OF MERGER dated as of February 25, 1999 (this
"Agreement") among BANCWEST CORPORATION, a Delaware corporation ("BC"), BANK OF
THE WEST, a California state-chartered bank and a wholly-owned subsidiary of BC
("BW"), and SIERRAWEST BANCORP, a California corporation ("SierraWest").
 
     WHEREAS, SierraWest is the beneficial and record owner of 4,116,597 shares
of the issued and outstanding common stock, no par value per share (the "SWB
Common Stock"), of SierraWest Bank, a California state-chartered bank ("SWB"),
constituting all of the issued and outstanding shares of SWB Common Stock;
 
     WHEREAS, the Boards of Directors of BC and BW have approved this Agreement,
declared it advisable and deem it advisable and in the best interests of the
stockholders of BC and BW to consummate the transactions provided for herein in
which, inter alia, SierraWest would merge with and into BW (the "Merger");
 
     WHEREAS, the Board of Directors of SierraWest has approved this Agreement
and declared it advisable and deems it advisable and in the best interests of
the shareholders of SierraWest to consummate the Merger;
 
     WHEREAS, it is the intention of the parties that the Merger qualify as a
tax-free reorganization pursuant to Section 368(a) of the Internal Revenue Code
of 1986, as amended (the "Code"), and that the Merger shall be accounted for as
a "pooling of interests";
 
     WHEREAS, it is the intention of the parties that immediately after the
consummation of the Merger, SWB shall be merged with and into BW (the "Bank
Merger") and the resulting bank, which shall be BW, shall continue as a
wholly-owned Subsidiary (as defined herein) of BC;
 
     WHEREAS, the Boards of Directors of BC and SierraWest have each determined
that the Merger, the Bank Merger and the other transactions contemplated by this
Agreement are consistent with, and will contribute to the furtherance of, their
respective business strategies and goals; and
 
     WHEREAS, as a condition and inducement to BC's and BW's willingness to
enter into this Agreement, BC and SierraWest are entering into a Stock Option
Agreement dated as of the date hereof in the form of Exhibit A hereto (the
"Stock Option Agreement") pursuant to which SierraWest has granted to BC an
option to purchase shares of the Common Stock, no par value, of SierraWest (the
"SierraWest Common Stock") constituting 19.9% of the presently outstanding
shares of SierraWest Common Stock.
 
     NOW, THEREFORE, in consideration of the foregoing and the respective
representations, warranties, covenants and agreements set forth herein, the
parties hereto agree as follows:
 
                                   ARTICLE I
 
                                   THE MERGER
 
     1.1  Effective Time of the Merger.  Subject to the provisions of this
Agreement, the Merger shall become effective upon the occurrence of the filing
of an agreement of merger in substantially the form of Exhibit B hereto (the
"Agreement of Merger") with the Secretary of State of the State of California,
or at such time thereafter as is provided in the Agreement of Merger (the
"Effective Time").
 
                                       A-1
<PAGE>   105
 
     1.2  Closing.  The closing of the Merger (the "Closing") will take place at
10:00 a.m., California time, on the fifth Business Day after satisfaction or
waiver (subject to applicable law) of the conditions (excluding conditions that,
by their terms, cannot be satisfied until the Closing) set forth in Article VI
(the "Closing Date"), unless another time or date is agreed to in writing by the
parties hereto. The Closing shall be held at the offices of Pillsbury Madison &
Sutro LLP, 235 Montgomery Street, San Francisco, California 94104, or at such
other location as is agreed to in writing by the parties hereto. As used in this
Agreement, "Business Day" shall mean any day that is not a Saturday, Sunday or
other day on which banks are required or authorized by law to be closed in San
Francisco, California.
 
     1.3  Effects of the Merger.
 
     (a) At the Effective Time (i) SierraWest shall be merged with and into BW
and the separate corporate existence of SierraWest shall cease, (ii) the
Articles of Incorporation of BW as in effect immediately prior to the Effective
Time shall be the Articles of Incorporation of the Surviving Corporation, (iii)
the By-laws of BW as in effect immediately prior to the Effective Time shall be
the By-laws of the Surviving Corporation, (iv) the directors of BW at the
Effective Time shall be the directors of the Surviving Corporation (except that
BC and BW shall take all necessary action to appoint two representatives of
SierraWest, mutually acceptable to BC and SierraWest, to serve on the Surviving
Corporation's board of directors for a period of two years after the Effective
Time), such directors to serve until the earlier of their resignation or removal
or until their respective successors are duly elected and qualified, as the case
may be, and (v) the officers of BW immediately prior to the Effective Time shall
be the officers of the Surviving Corporation, until the earlier of their
resignation or removal or until their respective successors are duly elected and
qualified, as the case may be.
 
     (b) As used in this Agreement, "Constituent Corporations" shall mean each
of BC, BW and SierraWest, and "Surviving Corporation" shall mean BW, at and
after the Effective Time, as the surviving corporation in the Merger.
 
     (c) At and after the Effective Time, the Merger will have the effects set
forth in the California Corporations Code.
 
     1.4  Alternative Structure.  Notwithstanding anything contained in this
Agreement to the contrary, BC may specify, for any reasonable business, tax or
regulatory purpose, that, before the Merger, BC, BW, SierraWest and SWB shall
enter into transactions other than those described herein in order to effect the
purposes of this Agreement, and the parties hereto shall take all action
necessary and appropriate to effect, or cause to be effected, such transactions,
provided, however, that no such specification may (a) materially and adversely
affect the timing of the consummation of the transactions contemplated herein or
(b) adversely affect the economic benefits, the form of consideration or the tax
effect of the Merger to the holders of SierraWest Common Stock.
 
     1.5  Absence of Control.  Subject to any specific provisions of this
Agreement, it is the intent of the parties hereto that neither BC nor SierraWest
by reason of this Agreement shall be deemed (until consummation of the
transactions contemplated hereby) to control, directly or indirectly, the other
party and shall not exercise, or be deemed to exercise, directly or indirectly,
a controlling influence over the management or policies of such other party.
 
                                       A-2
<PAGE>   106
 
                                   ARTICLE II
 
          EFFECT OF THE MERGER ON THE CAPITAL STOCK OF THE CONSTITUENT
                     CORPORATIONS; EXCHANGE OF CERTIFICATES
 
     2.1  Effect on Capital Stock of the Constituent Corporations.  As of the
Effective Time, by virtue of the Merger and without any action on the part of
the holder of any shares of SierraWest or BW capital stock:
 
          (a) Cancellation of Treasury Stock.  All shares of SierraWest that are
     owned directly or indirectly by BC or SierraWest or any of their respective
     Subsidiaries (other than shares of SierraWest Common Stock held directly or
     indirectly in trust accounts, managed accounts and the like or otherwise
     held in a fiduciary or nominee capacity that are beneficially owned by
     third parties (any such shares, and shares of BC Common Stock (as
     hereinafter defined) which are similarly held, whether held directly or
     indirectly by BC or SierraWest or any of their respective Subsidiaries, as
     the case may be, being referred to herein as "Trust Account Shares") and
     other than any shares of SierraWest Common Stock held by BC or SierraWest
     or any of their respective Subsidiaries in respect of a debt previously
     contracted (any such shares of SierraWest Common Stock, and shares of BC
     Common Stock which are similarly held, whether held directly or indirectly
     by BC or SierraWest or any of their respective Subsidiaries, being referred
     to herein as "DPC Shares")) shall be cancelled and shall cease to exist and
     no stock of BC or other consideration shall be delivered in exchange
     therefor. All shares of BC Common Stock that are owned by SierraWest or any
     of its Subsidiaries (other than Trust Account Shares and DPC Shares) shall
     continue to be owned by a Subsidiary of BC.
 
          (b) Conversion of SierraWest Common Stock.  Subject to Sections 2.3,
     2.5 and 7.1(h), each of the shares of SierraWest Common Stock issued and
     outstanding immediately prior to the Effective Time (other than shares to
     be cancelled in accordance with Section 2.1(a)) shall be converted into the
     right to receive 0.82 shares (the "Exchange Ratio") of fully paid and
     nonassessable shares of Common Stock, $1.00 par value per share (the "BC
     Common Stock"), of BC. All such shares of SierraWest Common Stock shall no
     longer be outstanding and shall automatically be cancelled and retired and
     shall cease to exist, and each certificate previously representing any such
     shares shall thereafter represent the shares of BC Common Stock into which
     such SierraWest Common Stock has been converted and, if applicable, the
     right to receive cash in lieu of fractional shares as provided in Section
     2.3 hereof. Certificates previously representing shares of SierraWest
     Common Stock shall be exchanged for certificates representing whole shares
     of BC Common Stock issued in consideration therefor (and, if applicable,
     cash in lieu of fractional shares as provided in Section 2.3 hereof) upon
     the surrender of such certificates.
 
          (c) BW Common Stock.  Each of the issued and outstanding shares of the
     capital stock of BW immediately prior to the Effective Time shall remain
     issued and outstanding after the Merger as shares of the Surviving
     Corporation, which shall thereafter constitute all of the issued and
     outstanding shares of capital stock of the Surviving Corporation. No
     capital stock of BW will be issued or used in the Merger.
 
          (d) BC Capital Stock.  At and after the Effective Time, each share of
     BC Common Stock issued and outstanding immediately prior to the Effective
     Time shall remain an issued and outstanding share of capital stock of BC
     and shall not be affected by the Merger.
 
                                       A-3
<PAGE>   107
 
     2.2  No Further Ownership Rights in SierraWest Common Stock.  All shares of
BC Common Stock issued upon conversion of shares of SierraWest Common Stock in
accordance with the terms hereof shall be deemed to represent all rights
pertaining to such shares of SierraWest Common Stock, and, after the Effective
Time, there shall be no further registration of transfers on the stock transfer
books of SierraWest of the shares of SierraWest Common Stock which were
outstanding immediately prior to the Effective Time. If, after the Effective
Time, certificates formerly representing shares of SierraWest Common Stock are
presented to BC for any reason, they shall be cancelled and, if applicable,
exchanged as provided in this Article II.
 
     2.3  Fractional Shares.  Notwithstanding any other provision hereof, no
fractional shares of BC Common Stock shall be issued to holders of shares of
SierraWest Common Stock. In lieu thereof, each such holder entitled to a
fraction of a share of BC Common Stock (after taking into account all shares of
SierraWest Common Stock held at the Effective Time by such holder) shall
receive, at the time of surrender of the certificates representing such holder's
SierraWest Common Stock, an amount in cash equal to the closing price as
reported on the New York Stock Exchange ("NYSE") for BC Common Stock on the
trading day immediately preceding the Closing Date, multiplied by the fraction
of a share of BC Common Stock to which such holder would otherwise be entitled.
No such holder shall be entitled to dividends, voting rights, interest on the
value of, or any other rights in respect of a fractional share. The term
"trading day" shall mean a day on which trading generally takes place on the
NYSE and on which trading in BC Common Stock has not been halted or suspended.
 
     2.4  Surrender of Shares of SierraWest Common Stock.
 
     (a) Prior to the Effective Time, BC shall appoint American Stock Transfer &
Trust Company, New York, New York or its successor, or any other bank or trust
company (having capital of at least $50 million) mutually acceptable to
SierraWest and BC, as exchange agent (the "Exchange Agent") for the purpose of
exchanging certificates representing the BC Common Stock which are to be issued
pursuant to Section 2.1, and at and after the Effective Time, BC shall issue and
deliver to the Exchange Agent certificates representing the shares of BC Common
Stock, as shall be required to be delivered to holders of shares of SierraWest
Common Stock pursuant to Section 2.1 hereof. As soon as practicable after the
Effective Time, each holder of shares of SierraWest Common Stock converted
pursuant to Section 2.1, upon surrender to the Exchange Agent of one or more
SierraWest share certificates (the "SierraWest Certificates") for cancellation,
will be entitled to receive a certificate representing the number of shares of
BC Common Stock determined in accordance with Section 2.1 and a payment in cash
with respect to fractional shares, if any, determined in accordance with Section
2.3.
 
     (b) No dividends or other distributions of any kind which are declared
payable to shareholders of record of the shares of BC Common Stock after the
Effective Time will be paid to persons entitled to receive such certificates for
shares of BC Common Stock until such persons surrender their SierraWest
Certificates. Upon surrender of such SierraWest Certificate, the holder thereof
shall be paid, without interest, any dividends or other distributions with
respect to the shares of BC Common Stock as to which the record date and payment
date occurred on or after the Effective Time and on or before the date of
surrender.
 
     (c) If any certificate for shares of BC Common Stock is to be issued in a
name other than that in which the SierraWest Certificate surrendered in exchange
therefor is registered, it shall be a condition of such exchange that the person
requesting such
 
                                       A-4
<PAGE>   108
 
exchange shall pay to the Exchange Agent any transfer costs, taxes or other
expenses required by reason of the issuance of certificates for such shares of
BC Common Stock in a name other than the registered holder of the SierraWest
Certificate surrendered, or such persons shall establish to the satisfaction of
BC and the Exchange Agent that such costs, taxes or other expenses have been
paid or are not applicable.
 
     (d) All dividends or distributions, and any cash to be paid in lieu of
fractional shares pursuant to Section 2.3, if held by the Exchange Agent for
payment or delivery to the holders of unsurrendered SierraWest Certificates
representing shares of SierraWest Common Stock and unclaimed at the end of one
year from the Effective Time, shall (together with any interest earned thereon)
at such time be paid or redelivered by the Exchange Agent to BC, and after such
time any holder of a SierraWest Certificate who has not surrendered such
SierraWest Certificate to the Exchange Agent shall, subject to applicable law,
look as a general creditor only to BC for payment or delivery of such dividends
or distributions or cash, as the case may be.
 
     (e) Neither BC nor the Surviving Corporation shall be liable to any holder
of SierraWest Common Stock for such shares (or dividends or distributions
thereon) or cash payable in lieu of fractional shares pursuant to Section 2.3
delivered to a public official pursuant to any applicable abandoned property,
escheat or similar law.
 
     2.5  Adjustments.  If, between the date of this Agreement and the Effective
Time, the outstanding shares of BC Common Stock shall have been changed into a
different number of shares or a different class by reason of any
reclassification, recapitalization, split up, combination, exchange of shares or
readjustment, or a stock dividend thereon shall be declared with a record date
within such period, the Exchange Ratio shall be correspondingly adjusted and BC
shall give SierraWest prompt written notice of the declaration of the record
date with respect thereto.
 
     2.6  Options.  At the Effective Time, each option granted by SierraWest to
purchase shares of SierraWest Common Stock (each a "SierraWest Option") which is
outstanding and unexercised immediately prior thereto shall cease to represent a
right to acquire shares of SierraWest Common Stock and shall be converted
automatically into an option to purchase shares of BC Common Stock in an amount
and at an exercise price determined as provided below (and otherwise subject to
the terms of the SierraWest 1988 Non-Qualified Stock Option Agreement and 1996
Stock Option Plan, the California Community Bancshare Corporation 1993 Stock
Option Plan, as amended and restated March 19, 1996, and the Continental Pacific
Bank 1990 Amended Stock Option Plan, each as amended to date (collectively, the
"SierraWest Stock Option Plans") and the agreements evidencing grants
thereunder, including but not limited to, the accelerated vesting of such
options which shall occur in connection with and by virtue of the consummation
of the Merger as and to the extent required by such plans and agreements): (a)
the number of shares of BC Common Stock to be subject to the new option shall be
equal to the product of the number of shares of SierraWest Common Stock subject
to the original option and the Exchange Ratio, provided that any fractional
shares of BC Common Stock resulting from such multiplication shall be rounded
down to the nearest share; and (b) the exercise price per share of BC Common
Stock under the new option shall be equal to the exercise price per share of
SierraWest Common Stock under the original option divided by the Exchange Ratio,
provided that such exercise price shall be rounded up to the nearest cent. In
the case of any options which are "incentive stock options" (as defined in
section 422 of the Code), the exercise price, the number of shares purchasable
pursuant to such options and the terms and conditions of exercise of such
 
                                       A-5
<PAGE>   109
 
options shall be determined in order to comply with section 424(a) of the Code.
The duration and other terms of the new option shall be the same as the original
option except that all references to SierraWest shall be deemed to be references
to BC.
 
                                  ARTICLE III
 
                         REPRESENTATIONS AND WARRANTIES
 
     3.1  Representations and Warranties of SierraWest.  SierraWest represents
and warrants to BC and BW as follows:
 
          (a) Organization, Standing and Power.  SierraWest is a bank holding
     company registered under the Bank Holding Company Act of 1956, as amended
     (the "BHC Act"). SWB is a wholly-owned Subsidiary of SierraWest and is a
     California banking association chartered under the laws of the State of
     California. The deposit accounts of SWB are insured by the Bank Insurance
     Fund ("BIF") of the Federal Deposit Insurance Corporation ("FDIC") to the
     fullest extent permitted by law, and all premiums and assessments required
     in connection therewith have been paid when due. SierraWest and each of its
     Subsidiaries, including SWB, is a bank or corporation duly organized,
     validly existing and in good standing under the laws of its jurisdiction of
     incorporation or organization, has all requisite power and authority to
     own, lease and operate its properties and to carry on its business as now
     being conducted and is duly qualified and in good standing to do business
     in each jurisdiction in which the nature of its business or the ownership
     or leasing of its properties makes such qualification necessary, other than
     in such jurisdictions where the failure so to qualify would not, either
     individually or in the aggregate, have a material adverse effect on
     SierraWest. The Articles of Incorporation and By-laws of each of
     SierraWest, and each other Subsidiary of SierraWest, copies of which were
     previously made available to BC, are true, complete and correct. The minute
     books of SierraWest and its Subsidiaries which have been made available to
     BC contain a complete (except for certain portions thereof relating to the
     Merger and the transactions contemplated hereby) and accurate record of all
     meetings of the respective Boards of Directors (and committees thereof) and
     shareholders. As used in this Agreement, (i) the term "Subsidiary" when
     used with respect to any party means any corporation or other organization,
     whether incorporated or unincorporated, (x) of which such party or any
     other Subsidiary of such party is a general partner (excluding
     partnerships, the general partnership interests of which held by such party
     or any Subsidiary of such party do not have a majority of the voting
     interests in such partnership), or (y) at least a majority of the
     securities or other interests of which having by their terms ordinary
     voting power to elect a majority of the board of directors or others
     performing similar functions with respect to such corporation or other
     organization is directly or indirectly owned or controlled by such party or
     by any one or more of its Subsidiaries, or by such party and one or more of
     its Subsidiaries, (ii) any reference to any event, change or effect being
     "material" with respect to any entity means an event, change or effect
     which is material in relation to the condition (financial or otherwise),
     properties, assets, liabilities, businesses, results of operations or
     prospects of such entity and its Subsidiaries taken as a whole and (iii)
     the term "material adverse effect" means, with respect to any entity, a
     material adverse effect (whether or not required to be accrued or disclosed
     under Statement of Financial Accounting Standards No. 5 ("SFAS No. 5")) (A)
     on the condition (financial or otherwise), properties, assets, liabilities,
     businesses, results of operations or prospects of such entity and its
     Subsidiaries taken
 
                                       A-6
<PAGE>   110
 
     as a whole (but does not include any such effect resulting from or
     attributable to (1) any action or omission by SierraWest or BC or any
     Subsidiary of either of them taken with the prior written consent of the
     other parties hereto, in contemplation of the transactions contemplated
     hereby or (2) any expenses incurred by such party in connection with this
     Agreement or the transactions contemplated hereby, but only to the extent
     set forth in Section 3.1(a) of the SierraWest Disclosure Schedule (as
     defined herein), with respect to SierraWest), or (y) on the ability of such
     entity to perform its obligations under the Transaction Agreements (as
     defined below) on a timely basis. A "Significant Subsidiary" means any
     Subsidiary of a person that would constitute a Significant Subsidiary of
     such person within the meaning of Rule 1-02 of Regulation S-X of the
     Securities and Exchange Commission (the "SEC"). The term "Transaction
     Agreements" shall mean this Agreement, the Stock Option Agreement and the
     Agreement of Merger. The term "to best knowledge of" any person means the
     actual knowledge of such person after reasonable inquiry under all the
     circumstances.
 
          (b) Capital Structure; Ownership of BC Common Stock.
 
          (i) The authorized capital stock of SierraWest consists of 10,000,000
     shares of SierraWest Common Stock, 9,800,000 shares of preferred stock, no
     par value, and 200,000 shares of preferred stock series A, no par value
     (such classes of preferred stock, collectively, the "SierraWest Preferred
     Stock") of which (A) as of February 24, 1999, 5,324,074 shares of
     SierraWest Common Stock were outstanding (none having been issued
     thereafter except from the exercise of SierraWest Options) and (B) as of
     the date hereof, no shares of SierraWest Preferred Stock are outstanding.
     All outstanding shares of SierraWest Common Stock have been duly authorized
     and validly issued and are fully paid and non-assessable and not subject to
     preemptive rights.
 
          (ii) The authorized capital stock of SWB consists of 10,000,000 shares
     of SWB Common Stock of which 4,116,597 shares are outstanding. All
     outstanding shares of SWB Common Stock have been duly authorized and
     validly issued and are fully paid and non-assessable and not subject to
     preemptive rights.
 
          (iii) Except for this Agreement and the Stock Option Agreement and
     except as set forth in Section 3.1(b)(iii) of the disclosure schedule of
     SierraWest delivered to BC on the date hereof (the "SierraWest Disclosure
     Schedule"), (A) there are no options, warrants, calls, rights, commitments
     or agreements of any character to which SierraWest or any of its
     Subsidiaries or Affiliates (as defined herein) is a party or by which any
     of the foregoing are bound obligating SierraWest or any of its Subsidiaries
     or Affiliates to issue, deliver or sell, or cause to be issued, delivered
     or sold, additional shares of capital stock of SierraWest or any of its
     Subsidiaries or obligating SierraWest or any of its Subsidiaries or
     Affiliates to grant, extend or enter into any such option, warrant, call,
     right, commitment or agreement, (B) there are no outstanding contractual
     obligations of SierraWest or any of its Subsidiaries or Affiliates to
     repurchase, redeem or otherwise acquire any shares of capital stock of
     SierraWest or any of its Subsidiaries and (C) there are no outstanding
     securities of any kind convertible into or exchangeable for the capital
     stock of SierraWest or any of its Subsidiaries (or any interest therein).
     Except as set forth in Section 3.1(b)(iii) of the SierraWest Disclosure
     Schedule, there is no agreement of any kind that gives any person any right
     to participate in the equity, value or income of, or to vote (x) in the
     election of directors or officers of or (y) otherwise with respect to the
     affairs of,
 
                                       A-7
<PAGE>   111
 
     SierraWest or any of its Subsidiaries. As used in this Agreement, the term
     "Affiliate" means, as to any person, a person which controls, is controlled
     by or is under common control with such person. The term "person" shall
     mean an individual, corporation, partnership, limited liability company,
     joint venture, association, trust, unincorporated organization or other
     entity.
 
          (iv) Neither SierraWest nor any of its Subsidiaries or, to
     SierraWest's best knowledge (without inquiry), its Affiliates, beneficially
     owns, directly or indirectly, any shares of capital stock of BC, securities
     of BC convertible into, or exchangeable for, such shares, or options,
     warrants or other rights to acquire such shares (regardless of whether such
     securities, options, warrants or other rights are then exercisable or
     convertible), nor is SierraWest or any of such Subsidiaries or Affiliates a
     party to any agreement, arrangement or understanding for the purpose of
     acquiring, holding, voting or disposing of shares of capital stock of BC or
     any such other securities, options, warrants or other rights.
 
          (c) Authority; No Violation.
 
          (i) SierraWest has all requisite corporate power and authority to
     enter into this Agreement and the other Transaction Agreements and to
     consummate the transactions contemplated hereby and thereby. The execution
     and delivery of this Agreement and the other Transaction Agreements and the
     consummation of the transactions contemplated hereby and thereby have been
     duly authorized by all necessary corporate action on the part of
     SierraWest, other than the approval of this Agreement by the holders of a
     majority of the outstanding shares of SierraWest Common Stock (the
     "SierraWest Shareholder Approval"). The SierraWest Shareholder Approval is
     the only vote of any class or series of SierraWest capital stock necessary
     to approve this Agreement and the other Transaction Agreements and the
     consummation of the transactions contemplated hereby and thereby. This
     Agreement and the other Transaction Agreements have been duly executed and
     delivered by SierraWest and (assuming due authorization, execution and
     delivery by BC and BW) constitute the valid and binding obligations of
     SierraWest, enforceable against it in accordance with their terms. SWB has
     full corporate power and authority to consummate the Bank Merger.
 
          (ii) Except as set forth in Section 3.1(c)(ii) of the SierraWest
     Disclosure Schedule, the execution and delivery by SierraWest of this
     Agreement and the other Transaction Agreements does not, and the
     consummation of the transactions contemplated hereby and thereby will not,
     conflict with, or result in any violation of, or constitute a default (with
     or without notice or lapse of time, or both) under, or give rise to a right
     of termination, cancellation or acceleration of any obligation or the loss
     of a material benefit under, or the creation of a lien, pledge, security
     interest, charge or other encumbrance on any assets (any such conflict,
     violation, default, right of termination, cancellation or acceleration,
     loss or creation, a "Violation") pursuant to, (x) any provision of the
     articles of incorporation or by-laws or comparable organizational documents
     of SierraWest or any Subsidiary of SierraWest, or (y) subject to obtaining
     or making the consents, approvals, orders, authorizations, registrations,
     declarations and filings referred to in paragraph (iii) below, any loan or
     credit agreement, note, mortgage, indenture, lease, SierraWest Benefit Plan
     (as defined in Section 3.1(j)) or other agreement, obligation, instrument,
     permit, concession, franchise, license, judgment, order, decree, statute,
     law, ordinance, rule or regulation applicable to SierraWest or any
     Subsidiary of SierraWest or its properties
 
                                       A-8
<PAGE>   112
 
     or assets, which Violation, in the case of clause (y), individually or in
     the aggregate, would have a material adverse effect on SierraWest.
 
          (iii) No consent, approval, order or authorization of, or
     registration, declaration or filing with, any court, administrative agency
     or commission or other governmental authority or instrumentality, domestic
     or foreign (a "Governmental Entity"), is required by or with respect to
     SierraWest or any of its Subsidiaries in connection with the execution and
     delivery of this Agreement or the other Transaction Agreements or the
     consummation by SierraWest of the transactions contemplated hereby or
     thereby, the failure to make or obtain which would have a material adverse
     effect on SierraWest or on the ability of SierraWest to perform its
     obligations hereunder or thereunder on a timely basis, or on BC's ability
     to own, possess or exercise the rights of an owner with respect to the
     business and assets of SierraWest and its Subsidiaries, except for (A) the
     filing of applications and notices with the FDIC under the Bank Merger Act
     and, if required, the Board of Governors of the Federal Reserve System (the
     "Federal Reserve") under the BHC Act and approval of same, (B) the filing
     by SierraWest with the SEC of a proxy statement in definitive form relating
     to the meeting of SierraWest's shareholders to be held to approve and adopt
     this Agreement and the transactions contemplated hereby (the "Proxy
     Statement"), (C) the filing of applications with the California State
     Department of Financial Institutions and such other applications, filings,
     authorizations, orders and approvals as may be required under the banking
     laws of other states or jurisdictions, and approval thereof (collectively,
     the "State Banking Approvals") and pursuant to any applicable state
     takeover laws ("State Takeover Approvals"), (D) notices under the
     Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (the "HSR
     Act"), if required, (E) the filing with the Secretary of State of the State
     of California of the Agreement of Merger, and (F) the filing of a notice
     with the Department of Financial Institutions of the State of Nevada.
 
          (d) Financial Statements.  SierraWest has previously delivered to BC
     copies of (a) the consolidated statements of financial condition of
     SierraWest and its Subsidiaries, as of December 31, for the fiscal years
     1997 and 1998, and the related consolidated statements of income,
     shareholders' equity and cash flows for the fiscal years 1996 through 1998,
     inclusive, as reported (except in respect of such consolidated financial
     statements as at the end of and for the fiscal year 1998) in SierraWest's
     Annual Reports on Form 10-K for the relevant fiscal years filed with the
     SEC under the Securities Exchange Act of 1934, as amended (the "Exchange
     Act"), in each case accompanied by the report of Deloitte & Touche LLP,
     independent auditors with respect to SierraWest (except for the
     consolidated financial statements as at and for the year ended December 31,
     1998 which, as of the date hereof, are not accompanied by such report) (the
     consolidated financial statements of SierraWest and its Subsidiaries
     referred to in this sentence being hereinafter sometimes referred to as the
     "SierraWest Consolidated Financial Statements"). Each of the financial
     statements referred to in this Section 3.1(d) (including the related notes,
     where applicable) fairly present, and the financial statements referred to
     in Section 5.14 hereof will fairly present (subject, in the cases of the
     unaudited statements, to normal recurring adjustments, none of which are
     expected to be material in nature or amount), the results of the
     consolidated operations and changes in stockholders' equity and
     consolidated financial condition of SierraWest and its Subsidiaries for the
     respective fiscal periods or as of the respective dates therein set forth.
     Each of such statements (including the related notes, where applicable)
     complies, and the financial statements
 
                                       A-9
<PAGE>   113
 
     referred to in Section 5.14 hereof will comply, in all material respects
     with applicable accounting requirements and with the published rules and
     regulations of the SEC with respect thereto and each of such statements
     (including the related notes, where applicable) has been, and the financial
     statements referred to in Section 5.14 will be, prepared in accordance with
     United States generally accepted accounting principles ("GAAP")
     consistently applied during the periods involved, except in each case as
     indicated in such statements or in the notes thereto or, in the case of
     unaudited statements, as permitted by Form 10-Q. The books and records of
     SierraWest and its Subsidiaries have been, and are being, maintained in all
     material respects in accordance with GAAP and any other applicable legal
     and accounting requirements and, where such books and records purport to
     reflect any transactions, the transactions so reflected are actual
     transactions.
 
          (e) SierraWest SEC Documents.  SierraWest has made available to BC a
     true and complete copy of each report, schedule, registration statement and
     definitive proxy statement filed by SierraWest with the SEC pursuant to the
     Securities Act of 1933 (the "Securities Act") or the Exchange Act (other
     than reports filed pursuant to Section 13(g) of the Exchange Act), since
     December 31, 1997 (as such documents have since the time of their filing
     been amended, the "SierraWest SEC Reports"), which are all the documents
     (other than preliminary material and reports required pursuant to Section
     13(g) of the Exchange Act) that SierraWest was required to file with the
     SEC since such date. As of their respective dates of filing with the SEC,
     the SierraWest SEC Reports complied in all material respects with the
     requirements of the Securities Act or the Exchange Act, as the case may be,
     and the rules and regulations of the SEC thereunder applicable to such
     SierraWest SEC Reports, and did not contain any untrue statement of a
     material fact or omit to state a material fact required to be stated
     therein or necessary to make the statements therein, in light of the
     circumstances under which they were made, not misleading. The financial
     statements of SierraWest included in the SierraWest SEC Reports complied as
     to form, as of their respective dates of filing with the SEC, in all
     material respects with applicable accounting requirements and with the
     published rules and regulations of the SEC with respect thereto, have been
     prepared in accordance with GAAP (as defined hereinafter) applied on a
     consistent basis during the periods involved (except as may be indicated in
     the notes thereto or, in the case of the unaudited statements, as permitted
     by Form 10-Q of the SEC) and fairly present in all material respects the
     consolidated financial position of SierraWest and its consolidated
     Subsidiaries as at the dates thereof and the consolidated results of
     operations, changes in stockholders' equity and cash flows of such
     companies for the periods then ended.
 
          (f) SierraWest Information Supplied.  None of the information supplied
     or to be supplied by SierraWest for inclusion or incorporation by reference
     in the Proxy Statement relating to the meeting of the shareholders (the
     "SierraWest Shareholders' Meeting") of SierraWest at which the SierraWest
     Shareholder Approval will be sought or for inclusion in the S-4 (as defined
     herein) will, at the date of mailing to shareholders of SierraWest and at
     the time of the SierraWest Shareholders' Meeting, (i) contain any untrue
     statement of a material fact or omit to state any material fact required to
     be stated therein or necessary in order to make the statements therein, in
     light of the circumstances under which they were made, not misleading or
     (ii) at the time and in the light of the circumstances under which it is
     made, be false or misleading with respect to any material fact, or omit to
     state any material fact necessary in order to make the statements therein
     not false or misleading or necessary
 
                                      A-10
<PAGE>   114
 
     to correct any statement in any earlier communication with respect to the
     solicitation of a proxy for the SierraWest Shareholders' Meeting which has
     become false or misleading. The Proxy Statement (except for such portions
     thereof furnished in writing to SierraWest by BC or any Subsidiary of BC as
     to which no warranty is made) will comply as to form in all material
     respects with the requirements of the Exchange Act and the rules and
     regulations of the SEC thereunder.
 
          (g) Compliance with Applicable Laws.  SierraWest and its Subsidiaries
     hold, and at all relevant times have held, all permits, licenses,
     variances, exemptions, orders and approvals of all Governmental Entities
     which are material to the operation of the businesses of SierraWest and its
     Subsidiaries, taken as a whole (the "SierraWest Permits"). SierraWest and
     its Subsidiaries are in compliance and have complied with the terms of the
     SierraWest Permits, except where the failure so to comply, individually or
     in the aggregate, would not have a material adverse effect on SierraWest.
     The businesses of SierraWest and its Subsidiaries are not being conducted
     in violation of any law, ordinance or regulation of any Governmental
     Entity, except for possible violations which, individually or in the
     aggregate, do not, and, insofar as reasonably can be foreseen, in the
     future will not, have a material adverse effect on SierraWest. Except for
     routine examinations by Federal or state Governmental Entities charged with
     the supervision or regulation of banks or bank holding companies or engaged
     in the insurance of bank deposits ("Bank Regulators") or by the United
     States Small Business Administration (the "SBA"), no investigation by any
     Governmental Entity with respect to SierraWest or any of its Subsidiaries
     is pending or threatened, and no proceedings by any Bank Regulator are
     pending or threatened which seek to revoke or materially limit any of the
     SierraWest Permits. Without limiting the generality of the foregoing
     provisions of this Section 3.1(g), SierraWest and its Subsidiaries are in
     compliance with and have been in compliance with any applicable federal or
     state insurance or securities law, or other rule, regulation, guidelines or
     policy statements of any Governmental Entity, as to all insurance and/or
     securities products offered or sold by SierraWest or its Subsidiaries for
     its own account or the account of others, including but not limited to
     annuity products. SWB is approved to conduct operations as a "Preferred
     Lender" by the SBA under the Small Business Administration Act of 1953 (the
     "SBA Act") and the SBA's rules and regulations at each of its offices
     except for those offices listed in Section 3.1(g) of the SierraWest
     Disclosure Schedule. All loans originated by SWB under the SBA's loan
     guarantee program have been originated in full compliance with the rules
     and regulations of the SBA and the SBA Act except to the extent that any
     noncompliance would not be reasonably likely to result in a material
     adverse effect with respect to SierraWest. Neither SierraWest nor any of
     its Subsidiaries has received any notice from the SBA which calls into
     question SierraWest and its Subsidiaries' compliance with the rules and
     regulations of the SBA in any respect. Except as described in Section
     3.1(g) of the SierraWest Disclosure Schedule, the SBA has not given to
     SierraWest or any of its Subsidiaries notice that it will not honor its
     guarantee with respect to any loan originated by any of them which loan is
     on the books of SierraWest or its Subsidiaries as of the date hereof or is
     the subject of any securitization heretofore consummated.
 
          (h) Litigation.  Except as set forth in Section 3.1(h) of the
     SierraWest Disclosure Schedule, there is no suit, action, proceeding,
     arbitration or investigation ("Litigation") pending to which SierraWest or
     any Subsidiary of SierraWest is a party or by which any of such persons or
     their respective assets may be bound or, to
 
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     the best knowledge of SierraWest, threatened against or affecting
     SierraWest or any Subsidiary of SierraWest, or challenging the validity or
     propriety of the transactions contemplated hereby which, if adversely
     determined, would, individually or in the aggregate, have or reasonably be
     expected to have a material adverse effect on SierraWest or on the ability
     of SierraWest to perform its obligations under this Agreement or the Stock
     Option Agreement in a timely manner, nor is there any judgment, decree,
     injunction, rule or order of any Governmental Entity or arbitrator
     outstanding against SierraWest or any Subsidiary of SierraWest.
 
          (i) Taxes.  SierraWest and each of its Subsidiaries have timely filed
     all tax returns required to be filed by any of them and all such tax
     returns are correct and complete in all material respects. SierraWest and
     each of its Subsidiaries have timely paid (or SierraWest has paid on their
     behalf), or have set up an adequate reserve for the payment of, all taxes
     required to be paid (whether or not shown as due on such returns), and the
     most recent financial statements that have been delivered to BC reflect an
     adequate reserve (other than reserves for deferred taxes established to
     reflect differences between tax and book basis of assets and liabilities)
     for all taxes accrued but not yet due and owing, by SierraWest and its
     Subsidiaries accrued through the date of such financial statements.
     SierraWest and its Subsidiaries file tax returns in all jurisdictions where
     required to file tax returns. No material deficiencies for any taxes have
     been proposed, asserted or assessed against SierraWest or any of its
     Subsidiaries that are not adequately reserved for (other than reserves for
     deferred taxes established to reflect differences between tax and book
     basis of assets and liabilities). Except as set forth in Section 3.1(i) of
     the SierraWest Disclosure Schedule: (i) there are no liens with respect to
     taxes upon any of the assets or properties of SierraWest and its
     Subsidiaries, other than with respect to taxes not yet due and payable,
     (ii) no material issue relating to taxes of SierraWest and its Subsidiaries
     has been raised in writing by any taxing authority in any audit or
     examination which can result in a proposed adjustment or assessment by a
     governmental authority in a taxable period (or portion thereof) ending on
     or before the Closing Date nor to the best knowledge of SierraWest does any
     basis exist for the raising of any such issue, (iii) SierraWest and its
     Subsidiaries have duly and timely withheld from all payments (including
     employee salaries, wages and other compensation) and paid over to the
     appropriate taxing authorities all amounts required to be so withheld and
     paid over for all periods for which the statute of limitations has not
     expired under all applicable laws and regulations, (iv) as of the Closing
     Date, none of SierraWest nor any of its Subsidiaries shall be a party to,
     be bound by or have any obligation under, any tax sharing agreement or
     similar contract or arrangement or any agreement that obligates any of them
     to make any payment computed by reference to the taxes, taxable income or
     taxable losses of any other person, (v) there is no contract or agreement,
     plan or arrangement by SierraWest or any of its Subsidiaries covering any
     person that, individually or collectively, could give rise to the payment
     of any amount that would not be deductible by SierraWest or any of its
     Subsidiaries by reason of Section 280G of the Code, (vi) SierraWest and its
     Subsidiaries have collected all material sales and use taxes required to be
     collected, and have remitted, or will remit on a timely basis, such amounts
     to the appropriate governmental authorities, or have been furnished
     properly completed exemption certificates and have maintained all such
     records and supporting documents in the manner required by all applicable
     sales and use tax statutes and regulations for all periods for which the
     statute of limitations has not expired, (vii) neither SierraWest nor any of
     its Subsidiaries has been a United States real property holding corporation
     within the
 
                                      A-12
<PAGE>   116
 
     meaning of Section 897(c)(2) of the Code during the applicable period
     specified in Section 897(c)(1)(A)(ii) of the Code, and (viii) none of
     SierraWest nor any of its Subsidiaries (A) has been a member of an
     affiliated group (other than the group to which they are currently members)
     filing a consolidated federal income tax return or (B) has any liability
     for the taxes of any person (other than the members of such current group)
     under Treasury Regulation Section 1.1502-6(a) (or any similar provision of
     state, local, or foreign law), as a transferee or successor, by contract,
     or otherwise. For the purpose of this Agreement, the term "tax" (including,
     with correlative meaning, the terms "taxes" and "taxable") shall include,
     except where the context otherwise requires, all Federal, state, local and
     foreign income, profits, franchise, gross receipts, payroll, sales,
     employment, use, property, withholding, excise, occupancy, custom, duty,
     capital stock, ad valorem, value added, estimated, stamp, alternative,
     environmental, any taxes imposed under Subchapter H of Chapter I of
     Subtitle A of the Code, and other taxes, duties or assessments of any
     nature whatsoever, together with all interest, penalties and additions
     imposed with respect to such amounts. As used in this Agreement, the term
     "Tax return" shall mean any return, declaration, report, claim for refund
     or information return or statement relating to taxes, including any
     schedule or attachment thereto, and including any amendment thereof.
     Neither SierraWest nor any of its Subsidiaries has filed a consent to the
     application of Section 341(f) of the Code.
 
     (j) Certain Agreements.  Section 3.1(j) of the SierraWest Disclosure
     Schedule sets forth a listing of all of the following contracts and other
     agreements, oral or written (which are currently in force or which may in
     the future be operative in any respect) to which SierraWest or any of its
     Subsidiaries is a party or by or to which SierraWest or any of its
     Subsidiaries or any of their respective assets or properties are bound or
     subject: (i) consulting agreements not terminable on six months or less
     notice involving the payment of more than $25,000 per annum, or union,
     guild or collective bargaining agreements covering any employees in the
     United States, (ii) agreements with any officer or other key employee of
     SierraWest or any of its Subsidiaries (x) providing any term of employment
     or (y) the benefits of which are contingent, or the terms of which are
     materially altered, upon the occurrence of a transaction involving
     SierraWest of the nature contemplated by this Agreement, (iii) any
     agreement or plan, any of the benefits of which will be increased, or the
     vesting of the benefits of which will be accelerated, by the occurrence of
     any of the transactions contemplated by this Agreement or the value of any
     of the benefits of which will be calculated on the basis of any of the
     transactions contemplated by this Agreement, (iv) contracts and other
     agreements for the sale or lease (other than where SierraWest or any of its
     Subsidiaries is a lessor) of any assets or properties (other than in the
     ordinary course of business) or for the grant to any person (other than to
     SierraWest or any of its Subsidiaries) of any preferential rights to
     purchase any assets or properties, (v) contracts and other agreements
     relating to the acquisition by SierraWest or any of its Subsidiaries of any
     operating business or entity or any interest therein, (vi) contracts or
     other agreements under which SierraWest or any of its Subsidiaries agrees
     to indemnify any party, other than in the ordinary course of business,
     consistent with past practice, or to share a tax liability of any party,
     (vii) contracts and other agreements containing covenants restricting
     SierraWest or any of its Subsidiaries from competing in any line of
     business or with any person in any geographical area or requiring
     SierraWest or any of its Subsidiaries to engage in any line of business,
     (viii) contracts or other agreements (other than contracts in the ordinary
     course of their banking business) relating to the borrowing of money by
 
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<PAGE>   117
 
     SierraWest or any of its Subsidiaries, or the direct or indirect guaranty
     by SierraWest or any of its Subsidiaries of any obligation for, or an
     agreement by SierraWest or any of its Subsidiaries to service, the
     repayment of borrowed money, or any other contingent obligations of
     SierraWest or any of its Subsidiaries in respect of indebtedness of any
     other person, and (ix) any other material contract or other agreement
     whether or not made in the ordinary course of business, including any
     contract required to be filed by SierraWest pursuant to Item 601(b)(10) of
     Regulation S-K of the SEC. There have been delivered or made available to
     BC true and complete copies of all of the contracts and other agreements
     set forth in Section 3.1(j) of the SierraWest Disclosure Schedule or in any
     other Section of the SierraWest Disclosure Schedule. Except as set forth in
     Section 3.1(j) of the SierraWest Disclosure Schedule, each such contract
     and other agreement is in full force and effect and constitutes a legal,
     valid, and binding obligation of SierraWest or its Subsidiaries, as the
     case may be, and to the best knowledge of SierraWest, each other party
     thereto, enforceable in accordance with its terms. Neither SierraWest nor
     any Subsidiary of SierraWest has received any notice, whether written or
     oral, of termination or intention to terminate from any other party to such
     contract or agreement. None of SierraWest or any of its Subsidiaries or (to
     the best knowledge of Sierra West) any other party to any such contract or
     agreement is in violation or breach of or default under any such contract
     or agreement (or with or without notice or lapse of time or both, would be
     in violation or breach of or default under any such contract or agreement),
     which violation, breach, or default has had or would have, individually or
     in the aggregate, a material adverse effect on SierraWest.
 
     (k) Benefit Plans.
 
          (i) Section 3.1(k) of the SierraWest Disclosure Schedule contains a
     true and complete list of each "employee benefit plan" (within the meaning
     of section 3(3) of the Employee Retirement Income Security Act of 1974, as
     amended ("ERISA")), including, without limitation, multiemployer plans
     (within the meaning of ERISA section 3(37)), and all stock purchase, stock
     option, severance, employment, change-in-control, fringe benefit,
     collective bargaining, bonus, incentive, deferred compensation, employee
     stock ownership, retirement, profit sharing and all other employee benefit
     plans, agreements, programs, policies or other arrangements, whether or not
     subject to ERISA, and whether formal or informal, oral or written (all the
     foregoing being herein called "Benefit Plans"), that are sponsored or are
     being maintained or contributed to, or required to be contributed to, by
     SierraWest or any of its Subsidiaries (the "SierraWest Benefit Plans"). No
     SierraWest Benefit Plan is a multiemployer plan or is subject to a
     collective bargaining agreement.
 
          (ii) With respect to each SierraWest Benefit Plan, SierraWest has
     delivered to BC a current, accurate and complete copy (or, to the extent no
     such copy exists, an accurate description) thereof and, to the extent
     applicable, (A) any related trust agreement or other funding instrument;
     (B) the most recent determination letter; (C) any summary plan description
     and other written communications (or a description of any oral
     communications) by SierraWest or any of its Subsidiaries to any of their
     respective employees concerning the extent of the benefits provided under
     any SierraWest Benefit Plan; and (D) except as described in Section 3.1(k)
     of the SierraWest Disclosure Schedule, for the two most recent years (I)
     the Form 5500 and attached schedules; (II) audited financial statements;
     and (III) actuarial valuation reports.
 
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<PAGE>   118
 
          (iii) (A) Each SierraWest Benefit Plan has been established and
     administered in accordance with its terms, and in compliance with the
     applicable provisions of ERISA, the Code and other applicable laws, rules
     and regulations; (B) each SierraWest Benefit Plan which is intended to be
     qualified within the meaning of Code section 401(a) is so qualified and has
     received a favorable determination letter as to its qualification and
     nothing has occurred, whether by action or failure to act, which would
     cause the loss of such qualification; (C) with respect to any SierraWest
     Benefit Plan, no audits, actions, suits or claims (other than routine
     claims for benefits in the ordinary course) are pending or threatened, and
     no facts or circumstances exist which could give rise to any such audits,
     actions, suits or claims; (D) neither SierraWest nor any other party has
     engaged in a prohibited transaction which could subject SierraWest or any
     of its Subsidiaries, or the Surviving Corporation to any taxes, penalties
     or other liabilities under Code section 4975 or ERISA sections 409 or
     502(i); (E) no event has occurred and no condition exists that could
     subject SierraWest or any of its Subsidiaries, or the Surviving
     Corporation, either directly or by reason of any such entity's affiliation
     with any member of any such entity's Controlled Group (defined as any
     organization which is a member of a controlled group of organizations
     within the meaning of Code sections 414(b), (c), (m) or (o)), to any tax,
     fine, liability or penalty imposed by ERISA, the Code or other applicable
     laws, rules and regulations; (F) all insurance and Pension Benefit Guaranty
     Corporation ("PBGC") premiums required to be paid with respect to
     SierraWest Benefit Plans through the Closing Date have been or will be paid
     prior thereto and adequate reserves will have been provided for on
     SierraWest's consolidated statement of financial condition as of the month
     end immediately prior to the Closing Date for any premiums (or portions
     thereof) attributable to service on or prior to the Closing Date; (G) all
     contributions required to be made prior to the Closing Date under the terms
     of each SierraWest Benefit Plan, the Code, ERISA or other applicable laws,
     rules and regulations have been or will be timely made and adequate
     reserves will have been provided for on SierraWest's consolidated statement
     of financial condition as of the month end immediately prior to the Closing
     Date for all benefits attributable to service on or prior to the Closing
     Date; (H) no SierraWest Benefit Plan has incurred any "accumulated funding
     deficiency" as such term is defined in ERISA section 302 and (including,
     but not limited to the voting of any securities held pursuant to a
     SierraWest Benefit Plan) Code section 412 (whether or not waived); and (I)
     the consummation of this Agreement will not result in a nonexempt
     prohibited transaction or a breach of fiduciary duty under ERISA.
 
          (iv) Except as set forth in Section 3.1(k)(iv) of the SierraWest
     Disclosure Schedule, with respect to each of the SierraWest Benefit Plans
     which is subject to Title IV of ERISA, as of the Closing Date, the assets
     of each such Plan shall be at least equal in value to the present value of
     the accrued benefits (vested and unvested) of the participants in such Plan
     on a termination and projected basis, based on the actuarial methods and
     assumptions indicated in the most recent actuarial valuation reports.
 
          (v) Except as set forth on Section 3.1(k)(v) of the SierraWest
     Disclosure Schedule, no SierraWest Benefit Plan exists which provides for
     an increase in benefits on or after the Closing Date or could result in the
     payment to any employee of SierraWest or any of its Subsidiaries of any
     money or other property or rights or accelerate or provide any other rights
     or benefits to any such employee as a result of the transactions
     contemplated by this Agreement. The aggregate amount of payments
 
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<PAGE>   119
 
     due from SierraWest under all such contracts and the amount due under each
     such contract, at the Effective Time, are as set forth in the schedule
     included in Section 3.1(k)(v) of the SierraWest Disclosure Schedule. None
     of such payments will constitute an "excess parachute" payment within the
     meaning of Code section 280G.
 
          (l) Subsidiaries.  Section 3.1(l) of the SierraWest Disclosure
     Schedule lists all the Subsidiaries of SierraWest. SierraWest owns,
     directly or indirectly, beneficially and of record 100% of the issued and
     outstanding voting securities of each such Subsidiary. All of the shares of
     capital stock of each of the Subsidiaries held by SierraWest or by another
     of its Subsidiaries are fully paid and nonassessable and are owned by
     SierraWest or one of its Subsidiaries free and clear of any lien, claim or
     other encumbrance. Neither SierraWest nor any of its Subsidiaries owns any
     shares of capital stock or other equity securities of any person (other
     than, in the case of SierraWest, the capital stock of its Subsidiaries and,
     in the case of such Subsidiaries, shares or equity securities acquired in
     satisfaction of debts previously contracted in good faith in the ordinary
     course of their banking business).
 
          (m) Agreements with Bank or Other Regulators.  Except as set forth in
     Section 3.1(m) of the SierraWest Disclosure Schedule, neither SierraWest
     nor any Subsidiary of SierraWest is a party to any written agreement or
     memorandum of understanding with, or a party to any commitment letter or
     similar undertaking to, or is subject to any order or directive by, or is a
     recipient of any extraordinary supervisory letter from, or has adopted any
     board resolutions at the request of, any Bank Regulator or the SBA or the
     U.S. Department of Agriculture which restricts materially the conduct by
     SierraWest and its Subsidiaries of their businesses, or in any manner
     relates to their capital adequacy, credit policies, loan underwriting or
     documentation or management, nor has SierraWest or any such Subsidiary been
     advised by any Bank Regulator or the SBA or the U.S. Department of
     Agriculture that it is contemplating issuing or requesting (or is
     considering the appropriateness of issuing or requesting) any such order,
     decree, agreement, memorandum of understanding, extraordinary supervisory
     letter, commitment letter or similar submission, or any such board
     resolutions.
 
          (n) Absence of Certain Changes or Events.  Since September 30, 1998,
     there has not been any change, or any event involving a prospective change,
     in the business, financial condition or results of operations or prospects
     of SierraWest or any of its Subsidiaries which has had, or would be
     reasonably likely to have, a material adverse effect on SierraWest. Except
     as set forth in Section 3.1(n) of the SierraWest Disclosure Schedule, since
     September 30, 1998, SierraWest and each of its Subsidiaries have conducted
     their respective businesses in the ordinary course consistent with their
     past practices and neither SierraWest nor any of its Subsidiaries has taken
     any action or entered into any transaction, and no event has occurred, that
     would have required BC's consent pursuant to Section 4.1 of this Agreement
     if such action had been taken, transaction entered into or event had
     occurred, in each case, after the date of this Agreement, nor has
     SierraWest or any of its Subsidiaries entered into any agreement, plan or
     arrangement to do any of the foregoing.
 
          (o) Undisclosed Liabilities.  Except (i) for those liabilities or
     obligations that are fully reflected or reserved against in the
     consolidated statement of financial condition at December 31, 1998 of
     SierraWest referred to in Section 3.1(d) or (ii) for liabilities or
     obligations incurred in the ordinary course of business consistent
 
                                      A-16
<PAGE>   120
 
     with past practice since December 31, 1998 and which are not material to
     SierraWest and its Subsidiaries taken as a whole, none of SierraWest or any
     of its Subsidiaries has incurred any liability or obligation of any nature
     whatsoever (whether absolute, accrued or contingent or otherwise and
     whether due or to become due) that, either alone or when combined with all
     similar liabilities or obligations, has had, or would have, a material
     adverse effect on SierraWest. Without limiting the generality of the
     foregoing, neither SierraWest nor any Subsidiary has any liabilities
     (whether or not required to be accrued or disclosed under SFAS No. 5)
     relating to or arising from its securitizations of SBA guaranteed loans
     except as described in Section 3.1(o) of the SierraWest Disclosure
     Schedule. No agreement pursuant to which any loans or other assets have
     been or will be sold by SierraWest or any Subsidiary entitle the buyer of
     such loans or other assets, unless there is material breach of a
     representation or covenant by SierraWest or its Subsidiaries not relating
     to the payment or other performance by an obligor of such loan or other
     asset of its obligations thereunder, to cause SierraWest or its
     Subsidiaries to repurchase such loan or other asset or the buyer to pursue
     any other form of recourse against SierraWest or its Subsidiaries.
 
          (p) Governmental Reports.  SierraWest and each of its Subsidiaries
     have timely filed all material reports, registrations and statements,
     together with any amendments required to be made with respect thereto, that
     they were required to file since January 1, 1995 with any Governmental
     Entity and have paid all fees and assessments due and payable in connection
     therewith. Except as set forth in Section 3.1(p) of the SierraWest
     Disclosure Schedule and except for normal examinations conducted by a
     Governmental Entity in the regular course of business of SierraWest and its
     Subsidiaries, no Governmental Entity has initiated any proceeding or, to
     the best knowledge of SierraWest, investigation into the business or
     operations of SierraWest or any of its Subsidiaries since January 1, 1995.
     Except as set forth in Section 3.1(p) of the SierraWest Disclosure
     Schedule, there is no material unresolved violation, criticism or exception
     by any Governmental Entity with respect to any report or statement relating
     to any examinations of SierraWest or any of its Subsidiaries.
 
          (q) Environmental Liability.  Except as set forth in Section 3.1(q) of
     the SierraWest Disclosure Schedule, there are no legal, administrative,
     arbitral or other proceedings, claims, actions, causes of action, private
     environmental investigations or remediation activities or governmental
     investigations of any nature seeking to impose, or that is reasonably
     likely to result in the imposition, on SierraWest or any of its
     Subsidiaries of any liability or obligation arising under common law
     standards relating to environmental protections, human health or safety, or
     under any local, state or federal environmental statute, regulation or
     ordinance, including, without limitation, the Comprehensive Environmental
     Response, Compensation and Liability Act of 1980, as amended (collectively,
     the "Environmental Laws"), pending or, to the best knowledge of SierraWest,
     threatened, against SierraWest or any of its Subsidiaries, which liability
     or obligation, individually or in the aggregate, would have or would be
     reasonably likely to have a material adverse effect on SierraWest. To the
     best knowledge of SierraWest, there is no reasonable basis for any such
     proceeding, claim, action or governmental investigation that would impose
     any liability or obligation that would have or would be reasonably likely
     to have a material adverse effect on SierraWest. Except as set forth in
     Section 3.1(q) of the SierraWest Disclosure Schedule, to the best knowledge
     of SierraWest, during or prior to the period of (i) the ownership by
     SierraWest or any of its Subsidiaries of any of their respective current
     properties, (ii) the participation by SierraWest or any of its Subsidiaries
     in the
 
                                      A-17
<PAGE>   121
 
     management of any property, or (iii) the holding by SierraWest or any of
     its Subsidiaries of a security interest or other interest in any property,
     there were no releases or threatened release of hazardous, toxic,
     radioactive or dangerous materials or other materials regulated under
     Environmental Laws in, on, under or affecting any such property which,
     individually or in the aggregate, would be reasonably likely to have a
     material adverse effect on SierraWest. Neither SierraWest nor any
     Subsidiary of SierraWest or SWB is subject to any agreement, order,
     judgment, decree, letter or memorandum by or with any Governmental Entity
     or third party imposing any material liability or obligation pursuant to or
     under any Environmental Law that would be reasonably likely to have a
     material adverse effect on SierraWest.
 
          (r) Properties.  Except as set forth in Section 3.1(r) of the
     SierraWest Disclosure Schedule, SierraWest or one of its Subsidiaries (i)
     has good and marketable title to all the properties and assets reflected in
     the SierraWest Consolidated Financial Statements as being owned by
     SierraWest or one of its Subsidiaries or acquired after the date thereof
     which are material to the business of SierraWest on a consolidated basis
     (except properties sold or otherwise disposed of since the date thereof in
     the ordinary course of business), free and clear of all claims, liens,
     charges, security interests or encumbrances of any nature whatsoever except
     (A) statutory liens securing payments not yet due, (B) liens on assets of
     SWB securing deposits incurred in the ordinary course of its banking
     business and (C) such imperfections or irregularities of title, claims,
     liens, charges, security interests or encumbrances as do not materially
     affect the use of the properties or assets subject thereto or affected
     thereby or otherwise materially impair business operations at such
     properties and (ii) is the lessee of all leasehold estates reflected in the
     SierraWest Consolidated Financial Statements or acquired after the date
     thereof which are material to its business on a consolidated basis (except
     for leases that have expired by their terms since the date thereof) and is
     in possession of the properties purported to be leased thereunder, and each
     such lease is valid without default thereunder by the lessee or, to the
     best knowledge of SierraWest, the lessor. Except as set forth in Section
     3.1(r) of the SierraWest Disclosure Schedule, all real properties owned by
     SierraWest or any of its Subsidiaries are owned in accordance with all
     requirements of applicable rules, regulations and policies of the Bank
     Regulators.
 
          (s) Transactions with Affiliates.  Except as set forth on Section
     3.1(s) of the SierraWest Disclosure Schedule and except for those
     arrangements, contracts, agreements or transactions which either (A)
     involve per annum payments by SierraWest and its Subsidiaries of less than
     $25,000 individually or $100,000 in the aggregate or (B) are terminable by
     SierraWest or such Subsidiary on 30 days or less notice with no financial
     penalty, (i) since December 31, 1996, none of SierraWest or any of its
     Subsidiaries has engaged in any business arrangement or relationship with
     any of its Affiliates and (ii) there are no, and since December 31, 1996,
     there have not been, any, liabilities, contracts or other agreements or
     other transactions between SierraWest or any of its Subsidiaries, on the
     one hand, and any of its Affiliates or any officer, director or employee of
     any such Affiliate, on the other hand.
 
          (t) Brokers or Finders.  No agent, broker, investment banker,
     financial advisor or other firm or person is or will be entitled to any
     broker's or finder's fee or any other similar commission or fee in
     connection with any of the transactions contemplated by this Agreement,
     except for NationsBanc Montgomery Securities LLC, whose fees and expenses
     will be paid by SierraWest in accordance with SierraWest's agreement with
 
                                      A-18
<PAGE>   122
 
     such firm (a copy of which agreement has been delivered to BC prior to the
     date of this Agreement).
 
          (u) Intellectual Property.  Except as set forth in Section 3.1(u) of
     the SierraWest Disclosure Schedule, SierraWest and its Subsidiaries own or
     have a valid license to use all trademarks, service marks and trade names
     (including any registrations or applications for registration of any of the
     foregoing) (collectively, the "SierraWest Intellectual Property") necessary
     to carry on their business substantially as currently conducted, except for
     such SierraWest Intellectual Property the failure of which to own or
     validly license individually or in the aggregate would not reasonably be
     expected to have a material adverse effect on SierraWest. Neither
     SierraWest nor any such Subsidiary has received any notice of infringement
     of or conflict with, and, to SierraWest's knowledge, there are no
     infringements of or conflicts with, the rights of others with respect to
     the use of any SierraWest Intellectual Property that individually or in the
     aggregate, in either such case, would reasonably be expected to have a
     material adverse effect on SierraWest.
 
          (v) Pooling of Interests.  As of the date of this Agreement,
     SierraWest has no reason (in respect to matters pertaining to SierraWest
     existing as of the date hereof or expected to exist as of the Closing Date)
     to believe that SierraWest will not qualify for a pooling of interests
     treatment for accounting purposes under GAAP as presently in effect.
 
          (w) Opinion of Financial Advisor.  SierraWest has received the written
     opinion of NationsBanc Montgomery Securities LLC, dated February 25, 1999,
     to the effect that, as of such date, subject to the limitations and
     conditions contained therein, the consideration to be received by the
     holders of SierraWest Common Stock pursuant to the Merger is fair to such
     holders from a financial point of view.
 
          (x) Rights Agreement; Anti-takeover Provisions.  SierraWest has taken
     all action so that the entering into of this Agreement and the other
     Transaction Agreements and the consummation of the transactions
     contemplated hereby and thereby do not and will not result in the grant of
     any rights to any person under the Rights Agreement between SierraWest and
     American Stock Transfer & Trust Co. dated January 16, 1996, as amended
     January 29, 1998 and February 25, 1999 (the "SierraWest Rights Agreement")
     or enable or require the rights thereunder to be exercised, distributed or
     triggered. The Board of Directors of SierraWest has taken all necessary
     action so that the provisions of Section 1203 of the California
     Corporations Code (and any applicable provisions of the takeover laws of
     any other state) and any comparable provisions of SierraWest's Articles of
     Incorporation do not and will not apply to this Agreement and the other
     Transaction Agreements, or the transactions contemplated hereby or thereby.
 
          (y) Community Reinvestment Act Compliance.  SWB is in substantial
     compliance with the applicable provisions of the Community Reinvestment Act
     of 1977 and the regulations promulgated thereunder (collectively, the
     "CRA") and has received a CRA rating of "satisfactory" from the FDIC in its
     most recent examination, and SierraWest has no knowledge of the existence
     of any fact or circumstance or set of facts or circumstances which could be
     reasonably expected to result in SWB failing to be in substantial
     compliance with such provisions or having its current rating lowered.
 
          (z) Year 2000 Readiness.  SierraWest has a plan and organization in
     place to minimize any material adverse effect caused by the failure of any
     system, equipment
 
                                      A-19
<PAGE>   123
 
     or product which is material to its operations or financial condition to be
     Year 2000 Ready (as defined below). Such plan addresses, at a minimum, the
     issues set forth in the statement of the Federal Financial Institutions
     Examination Council ("FFIEC"), dated May 5, 1997, entitled "Year 2000
     Project Management Awareness," and December 1997, entitled "Safety and
     Soundness Guidelines Concerning the Year 2000 Business Risk," as well as
     any other statements of the FFIEC related to the Year 2000, as such issues
     might affect SierraWest and its Subsidiaries. SierraWest has provided to BC
     a complete and accurate copy of the plan, which includes SierraWest's Year
     2000 organization and an estimate of anticipated associated costs.
     SierraWest is using its best efforts to implement such plan, which includes
     seeking assurances from its vendors and suppliers that such vendors' and
     suppliers' products and services which are material to its operations are
     Year 2000 Ready, replacing any material products and services supplied by
     such vendors or suppliers which are not Year 2000 Ready with new products
     and services which are Year 2000 Ready, and/or working with such vendors
     and suppliers to achieve Year 2000 Readiness with respect to such material
     products and services. Such plan shall also establish procedures to
     evaluate, manage and mitigate Year 2000-related risks to SierraWest posed
     by SierraWest's material customers who may not themselves be Year 2000
     Ready. Neither SierraWest nor its Subsidiaries have received, or expect to
     receive, a "Year 2000 Deficiency Notification Letter" (as such term is
     employed in the Federal Reserve's Supervision and Regulation Letter No. SR
     98-3 (SUP), dated March 4, 1998). As used herein, the term "Year 2000
     Ready" shall mean that the functionality and the performance of any system
     or piece of equipment will not be materially adversely affected as a result
     of the date change for any date on or after January 1, 2000, including leap
     year calculations, and that, to the extent applicable to normal operating
     specifications, the system or equipment will in all material respects
     accurately accept, store, retrieve, calculate, compare and otherwise
     process dates of January 1, 2000 and later.
 
          (aa) Dissenters' Rights.  The SierraWest Common Stock is included on
     the list of OTC margin stocks maintained by the Federal Reserve and
     therefore the holders of such shares shall not be entitled to have their
     shares treated as "dissenting shares" under Section 1300 of the California
     Corporations Code by reason of the Merger unless demands for payment shall
     be filed with respect to 5% or more of the outstanding shares of such
     class.
 
          (ab) Insurance.  SierraWest has previously delivered to BC a list
     identifying all insurance policies maintained on behalf of SierraWest and
     its Subsidiaries (other than mortgage, title and other similar policies for
     the benefit of SierraWest or its Subsidiaries as mortgagees under
     residential mortgage loans). All of the material insurance policies and
     bonds maintained by or for the benefit of SierraWest and its Subsidiaries
     are in full force and effect, SierraWest and its Subsidiaries are not in
     default thereunder and all material claims thereunder have been filed in
     due and timely fashion and neither SierraWest nor any of its Subsidiaries
     have received notice that any of such material claims have been or will be
     denied. The insurance policies and bonds maintained by SierraWest and its
     Subsidiaries are written by reputable insurers and are in such amounts,
     cover such risks and have such other terms as is customary for banks and
     bank holding companies comparable in size and operations to SierraWest and
     its Subsidiaries. Since December 31, 1997, there has not been any damage
     to, destruction of, or loss of any assets of SierraWest and its
     Subsidiaries (whether or not covered by insurance) that could have a
     material adverse effect on
 
                                      A-20
<PAGE>   124
 
     SierraWest. Neither SierraWest nor any SierraWest Subsidiaries has received
     any notice of a premium increase or cancellation with respect to any of its
     insurance policies or bonds, and within the last three years, neither
     SierraWest nor any of its Subsidiaries has been refused any insurance
     coverage sought or applied for, and SierraWest has no reason to believe
     that existing insurance coverage cannot be renewed as and when the same
     shall expire, upon terms and conditions as favorable as those presently in
     effect, other than possible increases in premiums or unavailability in
     coverage that have not resulted from an extraordinary loss experience of
     SierraWest or any SierraWest Subsidiary.
 
          (ac) Loans and Other Assets.
 
          (i) SierraWest has disclosed to BC prior to the date hereof the
     amounts of all loans, leases, other extensions of credit, commitments or
     other interest-bearing assets presently owned by SierraWest or any of its
     Subsidiaries that have been classified by any Bank Regulator, SierraWest's
     independent auditors, or the management of SierraWest or any Subsidiary of
     SierraWest as "Other Loans Especially Mentioned," "Substandard,"
     "Doubtful," or "Loss" or classified using categories with similar import
     and will have disclosed to BC prior to the Closing Date all such items
     which will be so classified hereafter and prior to the Closing Date. All
     such assets or portions thereof classified "Loss" or which are subsequently
     so classified, have been (or will be) charged off on a timely basis in
     full, collected or otherwise placed in a bankable condition. SierraWest
     regularly reviews and appropriately classifies its and its Subsidiaries'
     loans and other assets in accordance with all applicable legal and
     regulatory requirements and GAAP. SierraWest has disclosed to BC the
     amounts and identities of all other real estate owned ("OREO") that have
     been classified as such as of the date hereof by SierraWest's independent
     auditors, management or any Bank Regulator and will have disclosed to BC
     prior to the Closing Date all such terms which will be so classified
     hereafter and prior to the Closing Date. As of the date hereof and the
     Closing Date, the recorded values of all OREO on the books of SierraWest
     and its Subsidiaries accurately reflect and will reflect the net realizable
     values of each OREO parcel thereof in compliance with GAAP. SierraWest and
     its Subsidiaries have recorded on a timely basis all expenses associated
     with or incidental to its OREO including but not limited to taxes,
     maintenance and repairs as required by GAAP.
 
          (ii) All loans, leases, other extensions of credit, commitments or
     other interest-bearing assets and investments of SierraWest and its
     Subsidiaries are legal, valid and binding obligations enforceable in
     accordance with their respective terms and are not subject to any setoffs,
     counterclaims or disputes known to SierraWest (subject to applicable
     bankruptcy, insolvency and similar laws affecting creditors' rights
     generally and subject, as to enforceability, to equitable principles of
     general applicability), except as previously disclosed to BC in Section
     3.1(ac)(ii) of the SierraWest Disclosure Schedule or reserved for in the
     consolidated statement of financial condition of SierraWest as of December
     31, 1998 referred to in Section 3.1(d) in accordance with GAAP, and were
     duly authorized under and made in compliance with applicable federal and
     state laws and regulations. SierraWest and its Subsidiaries do not have any
     extensions or letters of credit, investments, guarantees, indemnification
     agreements or commitments for the same (including without limitation
     commitments to issue letters of credit, to create acceptances, or to
     repurchase securities, federal funds or other assets) other than those
     documented on the books and records of SierraWest and its Subsidiaries.
 
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<PAGE>   125
 
          (ad) Restrictions on Investments.  Except for pledges to secure public
     and trust deposits and repurchase agreements in the ordinary course of
     business, none of the investments reflected in the consolidated statement
     of financial condition of SierraWest as of December 31, 1998 referred to in
     Section 3.1(d), and none of the investments made by SierraWest and its
     Subsidiaries since December 31, 1998, is subject to any restriction,
     whether contractual or statutory, which materially impairs the ability of
     SierraWest or its Subsidiaries freely to dispose of such investment at any
     time.
 
          (ae) No Brokered Deposits.  Except as described in Section 3.1(ae) of
     the SierraWest Disclosure Schedule, as of the date hereof, neither
     SierraWest nor any of its Subsidiaries now has any "brokered deposits" as
     such deposits are defined by applicable regulations of the FDIC as of the
     date hereof.
 
          (af) Derivatives Contracts; Structured Notes; Etc.  Except as set
     forth in Section 3.1(af) of the SierraWest Disclosure Schedule, neither
     SierraWest nor any Subsidiary is a party to or has agreed to enter into an
     exchange traded or over-the-counter equity, interest rate, foreign exchange
     or other swap, forward, future, option, cap, floor or collar or any other
     contract that is not included on the balance sheet and is a derivatives
     contract (including various combinations thereof) (each, a "Derivatives
     Contract") or owns securities that (1) are referred to generically as
     "structured notes," "high risk mortgage derivatives," "capped floating rate
     notes" or "capped floating rate mortgage derivatives" or (2) are likely to
     have changes in value as a result of interest or exchange rate changes that
     significantly exceed normal changes in value attributable to interest or
     exchange rate changes, except for those Derivatives Contracts and other
     instruments legally purchased or entered into in the ordinary course of
     their banking business, consistent with safe and sound banking practices
     and regulatory guidance, and with counterparties reasonably believed by
     SierraWest to be financially responsible. All of such Derivatives Contracts
     or other instruments are legal, valid and binding obligations of SierraWest
     or one of its Subsidiaries and to the best knowledge of SierraWest, each of
     the other counterparties thereto, enforceable in accordance with their
     terms (except as enforcement may be limited by general principles of equity
     whether applied in a court of law or a court of equity and by bankruptcy,
     insolvency and similar laws affecting creditors' rights and remedies
     generally), and are in full force and effect. SierraWest and each of its
     Subsidiaries and to the best knowledge of SierraWest, each of the other
     counterparties thereto, have duly performed in all material respects all of
     their material obligations thereunder to the extent that such obligations
     to perform have accrued; and there are no breaches, violations or defaults
     or allegations or assertions of such by any party thereunder which would
     have or would reasonably be expected to have a material adverse effect on
     SierraWest.
 
          (ag) Labor Matters.  Neither SierraWest nor any of its Subsidiaries is
     a party to, or is bound by, any collective bargaining agreement, contract
     or other agreement or understanding with a labor union or labor
     organization, nor is it or any of its Subsidiaries the subject to a
     proceeding asserting that it or any such Subsidiary has committed an unfair
     labor practice (within the meaning of the National Labor Relations Act) or
     seeking to compel it or such Subsidiary to bargain with any labor
     organization as to wages and conditions of employment, nor is there any
     strike or other labor dispute involving it or any of its Subsidiaries
     pending or, to the best of its knowledge, threatened, nor is it aware of
     any activity involving it or any of its
 
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<PAGE>   126
 
     Subsidiaries' employees seeking to certify a collective bargaining unit or
     engaging in any other organization activity.
 
     3.2  Representations and Warranties of BC.  BC represents and warrants to
SierraWest as follows:
 
          (a) Organization, Standing and Power.  BC is a bank holding company
     registered under the BHC Act and organized under the laws of the State of
     Delaware. Each of BW and First Hawaiian Bank ("FHB") is a wholly-owned
     Subsidiary of BC and each is a banking corporation organized under the laws
     of the State of California in the case of BW and the State of Hawaii in the
     case of FHB. The deposit accounts of BC's bank Subsidiaries are insured by
     the BIF or the Savings Association Insurance Fund of the FDIC to the
     fullest extent permitted by law, and all premiums and assessments required
     to be paid in connection therewith have been paid when due. Each of BC and
     its Significant Subsidiaries is a bank or corporation duly organized,
     validly existing and in good standing under the laws of its jurisdiction of
     incorporation or organization, has all requisite power and authority to
     own, lease and operate its properties and to carry on its business as now
     being conducted, and is duly qualified and in good standing to do business
     in each jurisdiction in which the nature of its business or the ownership
     or leasing of its properties makes such qualification necessary, other than
     in such jurisdictions where the failure so to qualify would not, either
     individually or in the aggregate, have a material adverse effect on BC. The
     Certificate of Incorporation and By-laws of BC, copies of which were
     previously made available to SierraWest, are true, complete and correct
     copies of such documents as in effect on the date of this Agreement.
 
          (b) Capital Structure.
 
          (i) As of the date hereof, the authorized capital stock of BC consists
     of 200,000,000 shares of BC Common Stock, 75,000,000 shares of Class A
     Common Stock, par value $1.00 per share (the "BC Class A Common Stock") and
     50,000,000 shares of preferred stock, par value $1.00 per share ("BC
     Preferred Stock"). As of February 19, 1999 (A) 31,572,627 shares of BC
     Common Stock were outstanding, 1,203,129 shares of BC Common Stock were
     reserved for issuance upon the exercise of outstanding stock options or
     awards under incentive plans (such plans or programs, collectively, the "BC
     Stock Plans"), and 1,617,747 shares of BC Common Stock were held by BC in
     its treasury or by its Subsidiaries (other than shares held in trust,
     managed, custodial or nominee accounts and the like, or held by mutual
     funds for which a Subsidiary of BC acts as investment advisor, that in any
     such case are beneficially owned by third parties); (B) 25,814,768 shares
     of BC Class A Common Stock were outstanding and (C) no shares of BC
     Preferred Stock were outstanding. All outstanding shares of BC Common Stock
     and BC Class A Common Stock have been duly authorized and validly issued
     and are fully paid and non-assessable and not subject to preemptive rights.
     At the Effective Time, the BC Common Stock to be issued hereunder will be,
     when issued in accordance with the terms hereof, duly authorized, validly
     issued, fully paid and non-assessable and not subject to preemptive rights.
 
          (ii) Except as set forth in Section 3.2(b) of the disclosure schedule
     of BC delivered to SierraWest on the date hereof (the "BC Disclosure
     Schedule") or as disclosed in the BC SEC Reports (as defined herein)
     heretofore filed with the SEC prior to the date of this Agreement, as of
     the date of this Agreement, except for this Agreement, (A) there are no
     options, warrants, calls, rights, commitments or
 
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<PAGE>   127
 
     agreements of any character to which BC or any of its Subsidiaries is a
     party or by which any of them are bound obligating BC or any of its
     Subsidiaries to issue, deliver or sell, or cause to be issued, delivered or
     sold, additional shares of capital stock of BC or of its Subsidiaries or
     obligating BC or any of its Subsidiaries to grant, extend or enter into any
     such option, warrant, call, right, commitment or agreement, (B) there are
     no outstanding contractual obligations of BC or of its Subsidiaries to
     repurchase, redeem or otherwise acquire any shares of capital stock of BC
     or any of its Subsidiaries and (C) there are no outstanding securities of
     any kind convertible into or exchangeable for the capital stock of BC or
     any of its Subsidiaries (or any interest therein). Except as set forth in
     Section 3.2(b) of the BC Disclosure Schedule or as disclosed in the BC SEC
     Reports (as defined herein) filed with the SEC prior to the date of this
     Agreement, there is no agreement of any kind that gives any person any
     right to participate in the equity, value or income of, or to vote (i) in
     the election of directors or officers of or (ii) otherwise with respect to
     the affairs of, BC of any of its Subsidiaries.
 
          (c) Authority; No Violation.
 
          (i) BC and BW have all requisite corporate power and authority to
     enter into this Agreement and the other Transaction Agreements and to
     consummate the transactions contemplated hereby and thereby. The execution
     and delivery of this Agreement and the other Transaction Agreements and the
     consummation of the transactions contemplated hereby and thereby have been
     duly authorized by all necessary corporate action on the part of BC and BW.
     This Agreement and the other Transaction Agreements have been duly executed
     and delivered by BC and BW and (assuming due authorization, execution and
     delivery by SierraWest) constitute the valid and binding obligations of BC
     and BW, enforceable against BC and BW in accordance with their terms.
 
          (ii) The execution and delivery by BC and BW of this Agreement and the
     other Transaction Agreements does not, and the consummation of the
     transactions contemplated hereby and thereby will not result in any
     Violation pursuant to (x) any provision of the Certificate of Incorporation
     or By-laws or comparable organizational documents of BC, BW or any other
     Significant Subsidiary of BC, or (y) subject to obtaining or making the
     consents, approvals, orders, authorizations, registrations, declarations
     and filings referred to in paragraph (iii) below, any loan or credit
     agreement, note, mortgage, indenture, lease, benefit plan or other
     agreement, obligation, instrument, permit, concession, franchise, license,
     judgment, order, decree, statute, law, ordinance, rule or regulation
     applicable to BC, BW or any other Significant Subsidiary of BC or their
     respective properties or assets which Violation (in the case of clause
     (y)), individually or in the aggregate, would have a material adverse
     effect on BC.
 
          (iii) No consent, approval, order or authorization of, or
     registration, declaration or filing with, any Governmental Entity, is
     required by or with respect to BC, BW or any other Subsidiary of BC in
     connection with the execution and delivery by BC and BW of this Agreement
     and the other Transaction Agreements or the consummation by BC and BW of
     the transactions contemplated hereby and thereby, the failure to make or
     obtain which would have a material adverse effect on BC, or on BC's ability
     to own, possess or exercise the rights of an owner with respect to its
     Subsidiaries, except for (A) the filing of applications and notices with
     the FDIC under the Bank Merger Act and, if required, the Board of Governors
     of the Federal Reserve under the
 
                                      A-24
<PAGE>   128
 
     BHC Act and approval of same, (B) the filing by BC with the SEC of a
     registration statement on Form S-4 (the "S-4") with respect to the BC
     Common Stock issuable pursuant hereto, (C) the State Banking Approvals and
     any applicable State Takeover Approvals, (D) approval for listing upon
     official notice of issuance on the NYSE of the BC Common Stock issuable
     pursuant hereto, (E) notices under the HSR Act, if required, (F) compliance
     with applicable state blue sky laws, (G) the filing with the Secretary of
     State of the State of California of the Agreement of Merger, and (H) the
     filing of a notice with the Department of Financial Institutions of the
     State of Nevada.
 
          (d) Financial Statements.  BC has previously delivered to SierraWest
     copies of (a) the consolidated balance sheets of BC and its Subsidiaries,
     as of December 31, for the fiscal years 1997 and 1998, and the related
     consolidated statements of income changes in stockholders' equity and cash
     flows for the fiscal years 1996 through 1998, inclusive, as reported
     (except in respect of such financial statements as at the end of and for
     the fiscal year 1998) in BC's Annual Reports on Form 10-K for the relevant
     fiscal years filed with the SEC under the Exchange Act, in each case
     accompanied by the report of PricewaterhouseCoopers L.L.P. or its
     predecessors, independent auditors with respect to BC. Each of the
     financial statements referred to in this Section 3.2(d) (including the
     related notes, where applicable) fairly present, and the consolidated
     financial statements referred to in Section 5.14 hereof will fairly present
     (subject, in the cases of the unaudited statements, to normal recurring
     adjustments, none of which are expected to be material in nature or
     amount), the results of the consolidated operations and changes in
     stockholders' equity and consolidated financial condition of BC and its
     Subsidiaries for the respective fiscal periods or as of the respective
     dates therein set forth. Each of such statements (including the related
     notes, where applicable) complies, and the financial statements referred to
     in Section 5.14 hereof will comply, in all material respects with
     applicable accounting requirements and with the published rules and
     regulations of the SEC with respect thereto and each of such statements
     (including the related notes, where applicable) has been, and the financial
     statements referred to in Section 5.14 will be, prepared in accordance with
     GAAP consistently applied during the periods involved, except in each case
     as indicated in such statements or in the notes thereto or, in the case of
     unaudited statements, as permitted by Form 10-Q. The books and records of
     BC and its Subsidiaries have been, and are being, maintained in all
     material respects in accordance with GAAP and any other applicable legal
     and accounting requirements and, where such books and records purport to
     reflect any transactions, the transactions so reflected are actual
     transactions.
 
          (e) BC SEC Documents.  BC has made available to SierraWest a true and
     complete copy of each report, schedule, registration statement and
     definitive proxy statement filed by BC with the SEC (other than reports
     filed pursuant to Section 13(g) of the Exchange Act), since December 31,
     1997 (as such documents have since the time of their filing been amended,
     the "BC SEC Reports"), which are all the documents (other than preliminary
     material and reports required pursuant to Section 13(g) of the Exchange
     Act) that BC was required to file with the SEC since such date. As of their
     respective dates of filing with the SEC, the BC SEC Reports complied in all
     material respects with the requirements of the Securities Act or the
     Exchange Act, as the case may be, and the rules and regulations of the SEC
     thereunder applicable to such BC SEC Reports, and did not contain any
     untrue statement of a material fact or omit to state a material fact
     required to be stated
 
                                      A-25
<PAGE>   129
 
     therein or necessary to make the statements therein, in light of the
     circumstances under which they were made, not misleading. The financial
     statements of BC included in the BC SEC Reports complied as to form, as of
     their respective dates of filing with the SEC, in all material respects
     with applicable accounting requirements and with the published rules and
     regulations of the SEC with respect thereto, have been prepared in
     accordance with GAAP applied on a consistent basis during the periods
     involved (except as may be indicated in the notes thereto or, in the case
     of the unaudited statements, as permitted by Form 10-Q of the SEC) and
     fairly present in all material respects the consolidated financial position
     of BC and its consolidated Subsidiaries as at the dates thereof and the
     consolidated results of operations, changes in stockholders' equity and
     cash flows of such companies for the periods then ended.
 
          (f) BC Information Supplied.  None of the information supplied or to
     be supplied by BC for inclusion or incorporation by reference in the Proxy
     Statement will, at the date of mailing to SierraWest's shareholders and at
     the time of the SierraWest Shareholders' Meeting, (i) contain any untrue
     statement of a material fact or omit to state any material fact required to
     be stated therein or necessary in order to make the statements therein, in
     light of the circumstances under which they were made, not misleading or
     (ii) at the time and in the light of the circumstances under which it is
     made, be false or misleading with respect to any material fact, or omit to
     state any material fact necessary in order to make the statements therein
     not false or misleading or necessary to correct any statement in any
     earlier communication with respect to the solicitation of a proxy for the
     SierraWest Shareholders' Meeting which has become false or misleading. The
     S-4 (except for such portions thereof furnished in writing to BC by
     SierraWest or any Subsidiary of SierraWest as to which no warranty is made)
     will comply as to form in all material respects with the requirements of
     the Exchange Act and the rules and regulations of the SEC thereunder.
 
          (g) Compliance with Applicable Laws.  BC and its Subsidiaries hold,
     and at all times have held, all permits, licenses, variances, exemptions,
     orders and approvals of all Governmental Entities which are material to the
     operation of the businesses of BC and its Subsidiaries, taken as a whole
     (the "BC Permits"). BC and its Subsidiaries are in compliance and have
     complied with the terms of the BC Permits and all applicable laws and
     regulations, except where the failure so to comply, individually or in the
     aggregate, would not have a material adverse effect on BC. Except as
     disclosed in the BC SEC Reports filed prior to the date of this Agreement,
     the businesses of BC and its Subsidiaries are not being conducted in
     violation of any law, ordinance or regulation of any Governmental Entity,
     except for possible violations which, individually or in the aggregate, do
     not, and, insofar as reasonably can be foreseen, in the future will not,
     have a material adverse effect on BC. Except as described in Section 3.2(g)
     of the BC Disclosure Schedule and except for routine examinations by Bank
     Regulators, to the best knowledge of BC, no investigation by any
     Governmental Entity with respect to BC or any of its Subsidiaries is
     pending or threatened, other than, in each case, those the outcome of
     which, individually or in the aggregate, as far as reasonably can be
     foreseen, would not have a material adverse effect on BC, and no
     proceedings by any Bank Regulator are pending or threatened which seek to
     revoke or materially limit any of the BC Permits.
 
          (h) Litigation.  Except as disclosed in the BC SEC Reports filed prior
     to the date of this Agreement or as listed in Section 3.2(h) of the BC
     Disclosure Schedule, there is no Litigation pending or, to the best
     knowledge of BC, threatened, against or
 
                                      A-26
<PAGE>   130
 
     affecting BC or any Subsidiary of BC which could reasonably be expected,
     individually or in the aggregate, to have a material adverse effect on BC,
     nor is there any judgment, decree, injunction, rule or order of any
     Governmental Entity or arbitrator outstanding against BC or any Subsidiary
     of BC having, or which, insofar as reasonably can be foreseen, in the
     future could have, individually or in the aggregate, any such effect.
 
          (i) Subsidiaries.  Exhibit 21 to BC's Annual Report on Form 10-K for
     the fiscal year ended December 31, 1997 includes all the Significant
     Subsidiaries of BC as of the date of this Agreement except for BW and its
     Subsidiaries which were acquired by BC on November 1, 1998. The
     Subsidiaries of BW as of the date hereof are as listed in Section 3.2(i) of
     the BC Disclosure Schedule. BC owns, directly or indirectly, beneficially
     and of record 100% of the issued and outstanding voting securities of each
     such Significant Subsidiary (other than directors' qualifying shares, if
     any). Except as provided in any provision of applicable state law in the
     case of Significant Subsidiaries of BC that are state chartered banks, all
     of the shares of capital stock of each of the Significant Subsidiaries held
     by BC or by another Subsidiary of BC are fully paid and nonassessable and
     are owned by BC or a Subsidiary of BC free and clear of any claim, lien or
     encumbrance.
 
          (j) Agreements with Bank Regulators.  Except as set forth in Section
     3.2(j) of the BC Disclosure Schedule, neither BC nor any of its
     Subsidiaries is a party to any written agreement or memorandum of
     understanding with, or a party to any commitment letter or similar
     undertaking to, or is subject to any order or directive by, or is a
     recipient of any extraordinary supervisory letter from, or has adopted any
     board resolutions at the request of, any Bank Regulator which restricts
     materially the conduct by BC and its Subsidiaries of their businesses, or
     in any manner relates to their capital adequacy, credit policies or
     management, nor has BC or any such Subsidiary been advised by any Bank
     Regulator that it is contemplating issuing or requesting (or is considering
     the appropriateness of issuing or requesting) any such order, decree,
     agreement, memorandum of understanding, extraordinary supervisory letter,
     commitment letter or similar submission, or any such board resolutions.
 
          (k) Absence of Certain Changes or Events.  Except as disclosed in the
     BC SEC Reports filed prior to the date of this Agreement and except as set
     forth in Section 3.2(k) of the BC Disclosure Schedule, since September 30,
     1998, (i) there has not been any change, or any event involving a
     prospective change, in the business, financial condition, results of
     operations or prospects of BC or any of its Subsidiaries which has had, or
     would be reasonably likely to have, a material adverse effect on BC, (ii)
     BC and its Subsidiaries have conducted their respective businesses in the
     ordinary course consistent with their past practices, and (iii) neither BC
     nor any of its Subsidiaries has taken any action or entered into any
     transaction, and no event has occurred, that would have required
     SierraWest's consent pursuant to Section 4.2 of this Agreement if such
     action had been taken, transaction entered into or event had occurred, in
     each case, after the date of this Agreement, nor has BC or any of its
     Subsidiaries entered into any agreement, plan or arrangement to do any of
     the foregoing.
 
          (l) Undisclosed Liabilities.  Except (i) for those liabilities or
     obligations that are fully reflected or reserved against in the
     consolidated balance sheet at December 31, 1998 of BC referred to in
     Section 3.2(d) or (ii) for liabilities or obligations incurred in the
     ordinary course of business consistent with past practice since December
     31,
 
                                      A-27
<PAGE>   131
 
     1998 and which are not material to BC and its Subsidiaries taken as a
     whole, none of BC or any of its Subsidiaries has incurred any liability or
     obligation of any nature whatsoever (whether absolute, accrued or
     contingent or otherwise and whether due or to become due) that, either
     alone or when combined with all similar liabilities or obligations, has
     had, or would have, a material adverse effect on BC.
 
          (m) Governmental Reports.  BC and each of its Subsidiaries have timely
     filed all material reports, registrations and statements, together with any
     amendments required to be made with respect thereto, that they were
     required to file since January 1, 1995 with any Governmental Entity and
     have paid all fees and assessments due and payable in connection therewith.
     Except as set forth in Section 3.2(m) of the BC Disclosure Schedule and
     except for normal examinations conducted by a Governmental Entity in the
     regular course of business of BC and its Subsidiaries, no Governmental
     Entity has initiated any proceeding or, to the best knowledge of BC,
     investigation into the business or operations of BC or any of its
     Subsidiaries since January 1, 1995 which would be reasonably likely to have
     a material adverse effect on BC and its Subsidiaries taken as a whole.
     Except as set forth in Section 3.2(m) of the BC Disclosure Schedule, there
     is no material unresolved violation, criticism or exception by any
     Governmental Entity with respect to any report or statement relating to any
     examinations of BC or any of its Subsidiaries.
 
          (n) Pooling of Interests.  As of the date of this Agreement, BC has no
     reason (in respect of matters pertaining to BC existing as of the date
     hereof or expected to exist as of the Closing Date) to believe that BC will
     not qualify for pooling of interests treatment for accounting purposes
     under GAAP as presently in effect.
 
          (o) Vote Required.  No vote of the holders of any securities of BC is
     required with respect to the adoption of this Agreement.
 
          (p) Brokers or Finders.  No agent, broker, investment banker,
     financial advisor or other firm or person is or will be entitled to any
     broker's or finder's fee or any other similar commission or fee by BC in
     connection with any of the transactions contemplated by this Agreement.
 
          (q) Community Reinvestment Act Compliance.  BW and FHB are in
     substantial compliance with the applicable provisions of the CRA and have
     received a CRA rating of "satisfactory" and "outstanding," respectively,
     from the FDIC in their most recent examinations, and BC has no knowledge of
     the existence of any fact or circumstance or set of facts or circumstances
     which could be reasonably expected to result in BW or FHB failing to be in
     substantial compliance with such provisions or having their current ratings
     lowered.
 
          (r) Year 2000 Readiness.  Each of FHB and BW has a plan and
     organization in place to minimize any material adverse effect caused by the
     failure of any system, equipment or product which is material to its
     respective operations or financial condition to be Year 2000 Ready (as
     defined in Section 3.1(z) hereto). Such plans address, at a minimum, the
     issues set forth in the statement of the FFIEC, dated May 5, 1997, entitled
     "Year 2000 Project Management Awareness," and December 1997, entitled
     "Safety and Soundness Guidelines Concerning the Year 2000 Business Risk,"
     as well as any other statements of the FFIEC related to the Year 2000, as
     such issues might affect FHB and BW. FHB and BW have provided to SierraWest
     an accurate summary of such plans, which includes FHB's and BW's Year 2000
     organizations and estimates of anticipated associated costs. Each
 
                                      A-28
<PAGE>   132
 
     of FHB and BW is using its best efforts to implement such plan, which
     includes seeking assurances from its vendors and suppliers that such
     vendors' and suppliers' products and services which are material to its
     operations are Year 2000 Ready, replacing any material products and
     services supplied by such vendors or suppliers which are not Year 2000
     Ready with new products and services which are Year 2000 Ready, and/or
     working with such vendors and suppliers to achieve Year 2000 Readiness with
     respect to such material products and services. Such plan shall also
     establish procedures to evaluate, manage and mitigate Year 2000-related
     risks to FHB and BW posed by their respective material customers who may
     not themselves be Year 2000 Ready. None of FHB, BW or their Subsidiaries
     have received, or expect to receive, a "Year 2000 Deficiency Notification
     Letter" (as such term is employed in the Federal Reserve's Supervision and
     Regulation Letter No. SR 98-3 (SUP), dated March 4, 1998).
 
                                   ARTICLE IV
 
                   COVENANTS RELATING TO CONDUCT OF BUSINESS
 
     4.1  Covenants of SierraWest.  During the period from the date of this
Agreement and continuing until the Effective Time (except as expressly
contemplated or permitted by this Agreement or to the extent that BC shall
otherwise consent in writing) SierraWest agrees that it will and will cause each
of its Subsidiaries to carry on the business of SierraWest and each of its
Subsidiaries in the usual, regular and ordinary course in substantially the same
manner as heretofore conducted and use all reasonable efforts to preserve intact
the present business organizations of SierraWest and each of its Subsidiaries,
maintain the rights and franchises of, and preserve the relationships with
customers, suppliers and others having business dealings with, SierraWest and
each of its Subsidiaries to the end that their goodwill and ongoing businesses
shall not be impaired in any material respect at the Effective Time. Without
limiting the generality of the foregoing, during the period from the date of
this Agreement to the Effective Time, SierraWest shall not, and shall not permit
any of its Subsidiaries to, without the prior consent of BC in writing:
 
          (a) (i) declare or pay any dividends on or make other distributions in
     respect of any of its capital stock, except (A) for dividends by a
     wholly-owned Subsidiary of SierraWest to SierraWest, and (B) SierraWest may
     pay on March 31, 1999, a cash dividend in the amount of up to $0.26 per
     share on the SierraWest Common Stock and, if the Closing occurs after the
     record date set by BC's Board of Directors for the cash dividend payable by
     BC on its outstanding Common Stock in respect to the third fiscal quarter
     of 1999 a further cash dividend of up to $0.26 per share on the SierraWest
     Common Stock, (ii) set any record or payment dates for the payment of any
     dividends or distribution on its capital stock except in the ordinary
     course of business consistent with past practice, (iii) split, combine or
     reclassify any of its capital stock or issue or authorize or propose the
     issuance of any other securities in respect of, in lieu of or in
     substitution for, shares of its capital stock or (iv) repurchase, redeem or
     otherwise acquire, or permit any Subsidiary to purchase or otherwise
     acquire, any shares of its capital stock or the capital stock of any other
     Subsidiary of SierraWest or any securities convertible into or exercisable
     for any shares of such capital stock;
 
          (b) issue, deliver or sell, or authorize or propose the issuance,
     delivery or sale of, any shares of its capital stock of any class, any
     securities convertible into or
 
                                      A-29
<PAGE>   133
 
     exercisable for, or any rights, warrants or options to acquire, any such
     shares, or enter into any agreement with respect to any of the foregoing,
     other than issuances of SierraWest Common Stock pursuant to the exercise of
     SierraWest Options;
 
          (c) amend or propose to amend its Articles of Incorporation or its
     By-laws or other organizational documents or that of any Subsidiary;
 
          (d) (i) enter into any new material line of business, (ii) change its
     lending, investment, liability management and other material banking
     policies in any respect which is material to SierraWest, except as required
     by law or by policies imposed by a Bank Regulator or the SBA, or (iii)
     except as set forth in Section 4.1(d) of the SierraWest Disclosure
     Schedule, incur or commit to any capital expenditures or any obligations or
     liabilities in connection therewith other than capital expenditures and
     obligations or liabilities incurred or committed to in the ordinary course
     of business consistent with past practice but in no event for more than
     $50,000 as to any one such item or $200,000 as to all such items in the
     aggregate;
 
          (e) acquire or agree to acquire by merging or consolidating with, or
     by purchasing a substantial equity interest in or a substantial portion of
     the assets of, or by any other means, any business or any corporation,
     partnership, association or other business organization or division
     thereof; provided, however, that the foregoing shall not prohibit
     foreclosures and other debt-previously-contracted acquisitions in the
     ordinary course of business consistent with past practice.
 
          (f) sell, lease, encumber or otherwise dispose of, or agree to sell,
     lease, encumber or otherwise dispose of, any of its assets (including
     capital stock of Subsidiaries of SierraWest), which are material,
     individually or in the aggregate, to SierraWest, other than in the ordinary
     course of business consistent with past practice;
 
          (g) incur any long-term indebtedness for borrowed money or guarantee
     any such long-term indebtedness or issue or sell any long-term debt
     securities or warrants or rights to acquire any long-term debt securities
     of SierraWest or any of its Subsidiaries or guarantee any long-term debt
     securities of others other than (i) indebtedness of any Subsidiary of
     SierraWest to SierraWest or to another Subsidiary of SierraWest, (ii)
     deposits taken in the ordinary course of business consistent with past
     practice, or (iii) renewals or extensions of existing long-term
     indebtedness without any change in the material terms thereof;
 
          (h) intentionally take or fail to take any action that would, or
     reasonably might be expected to, result in any of the representations and
     warranties set forth in this Agreement being or becoming untrue in any
     material respect, or in any of the conditions to the Closing set forth in
     Article VI (including without limitation the conditions set forth in
     Sections 6.1(f) and 6.3(c)) not being satisfied, or (unless such action is
     required by applicable law or sound banking practice) which would adversely
     affect the ability of BC or SierraWest to obtain any of the Requisite
     Regulatory Approvals without imposition of a condition or restriction of
     the type referred to in Section 6.2 (f);
 
          (i) change the methods of accounting of SierraWest or any of its
     Subsidiaries, except as required by changes in GAAP as concurred in by such
     party's independent auditors;
 
          (j) (i) except for the retention bonus arrangements and the severance
     arrangements specified in Section 4.1(j) of the SierraWest Disclosure
     Schedule, enter
 
                                      A-30
<PAGE>   134
 
     into, adopt, amend (except for technical amendments and such amendments as
     may be required by law) or terminate any SierraWest Benefit Plan or any
     other Benefit Plan or any agreement, arrangement, plan or policy between
     SierraWest or any of its Subsidiaries and one or more of its directors or
     officers, increase in any manner the compensation or fringe benefits of any
     director, officer or employee of SierraWest or any of its Subsidiaries
     (except for general salary increases for non-officer employees and the
     scheduled annual merit increases to be effective as of March 1, 1999, as
     previously disclosed to BC, in respect to certain officers of SierraWest or
     its Subsidiaries, other than the chief executive officer and the chief
     financial officer of SierraWest, in each case, in the ordinary course of
     business consistent with established salary guidelines and policies as
     heretofore furnished to BC by SierraWest) or pay or grant any benefit not
     required by any plan and arrangement as in effect as of the date hereof
     (including, without limitation, the granting of stock options, stock
     appreciation rights, restricted stock, restricted stock units or
     performance units or shares or any similar awards) or enter into any
     contract, agreement, commitment or arrangement to do any of the foregoing,
     (ii) except for the retention bonus arrangements specified in Section
     4.1(j) of the SierraWest Disclosure Schedule, enter into or renew any
     contract, agreement, commitment or arrangement providing for the payment to
     any director, officer or employee of SierraWest or any of its Subsidiaries
     of compensation or benefits contingent, or the terms of which are
     materially altered, upon the occurrence of any of the transactions
     contemplated by this Agreement, or (iii) with respect to any SierraWest
     Benefit Plan which is a defined benefit or defined contribution pension
     plan, permit or cause (A) a consolidation or merger of any such Plan, (B) a
     spin-off involving any such Plan, (C) a transfer of assets and/or
     liabilities from or to any such Plan, or (D) any similar transaction
     involving any such Plan;
 
          (k) enter into any contract that would be required to be disclosed on
     Section 3.1(j) of the SierraWest Disclosure Schedule or renew or terminate
     any contract listed in Section 3.1(j) of the SierraWest Disclosure
     Schedule, other than renewals of contracts or leases for a term of one year
     or less without material adverse changes to the terms thereof;
 
          (l) make or acquire any loan or issue a commitment for any loan except
     for loans and commitments that are made in the ordinary course of business
     consistent with past practice or issue or agree to issue any letters of
     credit or otherwise guarantee the obligations of any other persons except
     in the ordinary course of business consistent with past practice;
 
          (m) engage or participate in any material transaction or incur or
     sustain any material obligation not in the ordinary course of business
     consistent with past practice;
 
          (n) settle any claim, action or proceeding involving money damages
     involving a payment in excess of $50,000 as to any such matter, or settle
     any other matter not involving money damages which is material to
     SierraWest;
 
          (o) change or make any tax elections, change any method of accounting
     with respect to taxes, file any amended tax return, or settle or compromise
     any federal, state, local or foreign material tax liability;
 
          (p) except as set forth in Section 4.1(p) of the SierraWest Disclosure
     Schedule, make an application for the opening, relocation or closing of, or
     open, relocate or close any branch or loan production office;
 
                                      A-31
<PAGE>   135
 
          (q) except as provided in Section 3.1(x) hereof, modify, amend or
     waive any provision of, or terminate the SierraWest Rights Plan, or redeem
     any of the rights outstanding thereunder;
 
          (r) except as described in Section 4.1(r) of the SierraWest Disclosure
     Schedule, enter into any securitization or similar transactions with
     respect to any loans, leases or other assets of SierraWest or any of its
     Subsidiaries;
 
          (s) agree to, or make any commitment to, take any of the actions
     prohibited by this Section 4.1.
 
     4.2  Covenants of BC.  During the period from the date of this Agreement
and continuing until the Effective Time, BC agrees as to itself and its
Subsidiaries that (except as expressly contemplated or permitted by this
Agreement or to the extent that SierraWest shall otherwise consent in writing),
BC will and will cause each of its Subsidiaries to carry on its business in the
usual, regular and ordinary course in substantially the same manner as
heretofore conducted and use all reasonable efforts to preserve intact its
present business organizations, maintain its rights and franchises and preserve
its relationships with customers, suppliers and others having business dealings
with them to the end that their goodwill and ongoing businesses shall not be
impaired in any material respect at the Effective Time. Without limiting the
generality of the foregoing, during the period from the date of this Agreement
to the Effective Time, BC shall not, and shall not permit any of its
Subsidiaries to, without the prior consent of SierraWest in writing:
 
          (a) amend or propose to amend its Certificate of Incorporation or its
     By-laws in a manner that would materially and adversely affect its ability
     to perform its obligations under this Agreement or consummate the
     transactions contemplated hereunder, or otherwise materially and adversely
     affect the rights, powers and privileges of the shares of BC Common Stock
     to be issued in the Merger;
 
          (b) intentionally take or fail to take any action that would, or
     reasonably might be expected to, result in any of its representations and
     warranties set forth in this Agreement being or becoming untrue, subject to
     such exceptions as do not have, and would not reasonably be expected to
     have, individually or in the aggregate, a material adverse effect on BC
     following the Effective Time, or in any of the conditions to the Closing
     set forth in Article VI (including without limitation the conditions set
     forth in Sections 6.1(f) and 6.2(d)) not being satisfied, or (unless such
     action is required by applicable law or sound banking practice) which would
     adversely affect the ability of BC or SierraWest to obtain any of the
     Requisite Regulatory Approvals without imposition of a condition or
     restriction of the type referred to in Section 6.2(f);
 
          (c) agree to, or make any commitment to, take any of the actions
     prohibited by this Section 4.2.
 
                                      A-32
<PAGE>   136
 
                                   ARTICLE V
 
                             ADDITIONAL AGREEMENTS
 
     5.1  Regulatory Matters.
 
     (a) SierraWest shall promptly prepare and file with the SEC a Proxy
Statement and BC shall promptly prepare and file with the SEC the S-4, in which
the Proxy Statement will be included as a prospectus. Each of BC and SierraWest
shall use all reasonable efforts to have the S-4 declared effective under the
Securities Act as promptly as practicable after such filing, and SierraWest
shall thereafter promptly mail the Proxy Statement to its shareholders.
 
     (b) The parties hereto shall cooperate with each other and use reasonable
best efforts to promptly prepare and file all necessary documentation, to effect
all applications, notices, petitions and filings, to obtain as promptly as
practicable all permits, consents, approvals and authorizations of all third
parties and Governmental Entities which are necessary or advisable to consummate
the transactions contemplated by this Agreement and the other Transactions
Agreements (including without limitation the Merger and the Bank Merger) and any
branch consolidation, relocation or closure applications or notices with respect
to the branches of SWB which BC shall request SWB to file and to comply with the
terms and conditions of all such permits, consents, approvals and authorizations
of all such Governmental Entities. BC and SierraWest shall have the right to
review in advance and to the extent practicable each will consult the other on,
in each case subject to applicable laws relating to the exchange of information,
all the information relating to SierraWest or BC, as the case may be, and any of
their respective Subsidiaries which appears in any filing made with, or written
materials submitted to, any third party or any Governmental Entity in connection
with the transactions contemplated by this Agreement. In exercising the
foregoing right, each of the parties hereto shall act reasonably and as promptly
as practicable. The parties hereto agree that they will consult with each other
with respect to the obtaining of all permits, consents, approvals and
authorizations of all third parties and Governmental Entities necessary or
advisable to consummate the transactions contemplated by this Agreement
(including without limitation the Merger and the SierraWest Merger) and each
party will keep the other apprised of the status of matters relating to
completion of the transactions contemplated herein.
 
     (c) BC and SierraWest shall, upon request, furnish each other with all
information concerning themselves, their Subsidiaries, directors, officers and
shareholders and such other matters as may be reasonably necessary or advisable
in connection with the Proxy Statement, the S-4 or any other statement, filing,
notice or application made by or on behalf of BC, SierraWest or any of their
respective Subsidiaries to any Governmental Entity in connection with the Merger
and the other transactions contemplated by this Agreement.
 
     (d) BC and SierraWest shall promptly advise each other upon receiving any
communication from any Governmental Entity whose consent or approval is required
for consummation of the transactions contemplated by this Agreement which causes
such party to believe that there is a reasonable likelihood that any Requisite
Regulatory Approval (as defined below) will not be obtained or that the receipt
of any such approval will be materially delayed.
 
                                      A-33
<PAGE>   137
 
     5.2  Access to Information.
 
     (a) Upon reasonable notice, SierraWest and BC shall (and shall cause each
of their respective Subsidiaries to) afford to the other and their
representatives and advisors access, during normal business hours during the
period prior to the Closing Date, to all the properties, books, contracts,
commitments and records of SierraWest (in the case of SierraWest) and of BC (in
the case of BC) and, during such period, each of SierraWest and BC shall (and
shall cause each of their respective Subsidiaries to) make available to the
other and their representatives and advisors (a) a copy of each report,
schedule, registration statement and other document filed or received by
SierraWest or BC, as the case may be, during such period pursuant to the
requirements of Federal securities laws or Federal or state banking laws (other
than reports or documents which such party is not permitted to disclose under
applicable law or reports or documents which are subject to an attorney-client
privilege or which constitute attorney work product) and (b) all other
information concerning the business, properties and personnel of SierraWest or
of BC, as the case may be, as such other party may reasonably request. BC will
hold any such information with respect to SierraWest and its Subsidiaries which
is nonpublic in confidence to the extent required by, and in accordance with,
the provisions of the letter dated October 27, 1998, between SierraWest and BC
(the "Confidentiality Agreement"). SierraWest will hold all such information
with respect to BC and its Subsidiaries which is nonpublic in confidence and
will otherwise deal with such information to the extent required by, and in
accordance with, the provisions of the Confidentiality Agreement, deeming for
purpose of this sentence, such information to be subject to the provision of the
Confidentiality Agreement as if such provisions applied by their terms to such
information of BC and its Subsidiaries, as well as to such information of
SierraWest and its Subsidiaries. No investigation by either BC, on the one hand,
or SierraWest, on the other hand, shall affect the representations and
warranties of the other.
 
     5.3  SierraWest Shareholders' Meeting.  SierraWest shall call a meeting of
its shareholders to be held as promptly as practicable for the purpose of voting
upon the adoption of this Agreement. SierraWest will, through its Board of
Directors, recommend to its shareholders adoption of this Agreement unless the
Board of Directors of SierraWest determines in good faith, based upon the
written advice of outside counsel, that making such recommendation, or failing
to withdraw, modify or amend any previously made recommendation, would
constitute a breach of fiduciary duty by SierraWest's Board of Directors under
applicable law. In addition, nothing in this Section 5.3 or elsewhere in this
Agreement shall prohibit accurate disclosure by SierraWest of information that
is required to be disclosed in the Proxy Statement or any other document
required to be filed with the SEC (including without limitation a
Solicitation/Recommendation Statement on Schedule 14D-9) or otherwise required
to be disclosed by applicable law or regulation or the rules of any securities
exchange or automated quotation system on which the securities of SierraWest may
then be traded.
 
     5.4  No Solicitations.  From the date hereof until the earlier of the
Effective Time or the termination of this Agreement, SierraWest agrees that
neither it, nor any of its Subsidiaries, Affiliates or agents shall, nor shall
it authorize or permit any of its officers, directors or employees or any
investment banker, financial advisor, attorney, accountant or other
representative or agent (collectively, "Representatives") retained by it or any
of its Subsidiaries, Affiliates or agents to, solicit, initiate or knowingly
encourage the submission of, or enter into discussions or negotiations with or
provide information to any person or group of persons (other than the respective
parties to this Agreement) concerning, any Takeover Proposal (as defined below)
or enter into any agreement with a third party
 
                                      A-34
<PAGE>   138
 
relating to a Takeover Proposal or assist, participate in, facilitate or
encourage any effort or attempt by any other person to do or seek to do any of
the foregoing. As used in this Agreement, "Takeover Proposal" shall mean any
proposal for the acquisition of a 15% or greater equity interest in, or a
merger, consolidation, liquidation, dissolution or other disposition of 15% or
more of the assets of, SierraWest or any Significant Subsidiary of SierraWest,
or any tender offer or exchange offer that if consummated would result in any
person beneficially owning 15% or more of any class of equity securities of
SierraWest or any Significant Subsidiary of SierraWest (other than pursuant to
the transactions contemplated by this Agreement and the Stock Option Agreement).
 
     5.5  Legal Conditions.
 
     (a) Each of SierraWest and BC shall, and shall cause its respective
Subsidiaries to, use all reasonable efforts (i) to take, or cause to be taken,
all actions necessary to comply promptly with all legal requirements which may
be imposed on such party or its Subsidiaries with respect to the transactions
contemplated by this Agreement and as promptly as practicable, (ii) to obtain
(and to cooperate with the other party to obtain) any consent, authorization,
order or approval of, or any exemption by, any Governmental Entity and or any
other public or private third party which is required to be obtained or made by
such party or any of its Subsidiaries in connection with the Merger and the
other transactions contemplated by this Agreement. Each of SierraWest and BC
will promptly cooperate with and furnish information to the other in connection
with any such burden suffered by, or requirement imposed upon, any of them or
any of their Subsidiaries in connection with the foregoing.
 
     (b) Each of SierraWest and BC agrees to use all reasonable best efforts to
take, or cause to be taken, all actions, and to do, or cause to be done, all
things necessary and proper or advisable to consummate, as soon as practicable
after the date of this Agreement, the transactions contemplated hereby,
including, without limitation, using all reasonable best efforts to (i) lift or
rescind any injunction or restraining order or other order adversely affecting
the ability of the parties to consummate the transactions contemplated hereby,
(ii) defend any Litigation seeking to enjoin, prevent or delay the consummation
of the transactions contemplated hereby or seeking material damages, (iii)
provide to counsel to the other party hereto representations and certifications
as to such matters as such counsel may reasonably request in order to render the
opinions referred to in Sections 6.2(d) and 6.3(c), and (iv) to obtain the
letters of the independent accountants referred to in Section 6.1(f).
 
     5.6  Employee Benefit Plans.
 
     (a) For purposes of all employee benefit plans of BC or its Subsidiaries in
which the employees of SierraWest who shall remain in the employment of BC or
its Subsidiaries after the Closing Date shall participate from and after such
date (including all policies and employee fringe benefit programs, including
vacation policies), and under which an employee's benefit depends, in whole or
in part, on length of service, credit will be given to SierraWest Employees for
vesting and eligibility purposes only for service previously credited with
SierraWest or its Subsidiaries prior to the Effective Time to the extent that
such crediting of service does not result in duplication of benefits, provided,
however, that BC shall determine each employee's length of service in a manner
consistent with the customary practice with respect to the employees of the BC
Subsidiary by which they shall be employed. BC shall also cause each employee
benefit plan in which SierraWest Employees participate from and after the
Effective Time to waive (i) any preexisting condition restriction which was
waived under the terms of any analogous Plan immediately
 
                                      A-35
<PAGE>   139
 
prior to the Effective Time or (ii) any waiting period limitation which would
otherwise be applicable to a SierraWest Employee on or after the Effective Time
to the extent such SierraWest Employee had satisfied any similar waiting period
limitation under an analogous Plan prior to the Effective Time.
 
     (b) Notwithstanding the foregoing, except as otherwise expressly provided
in this Agreement, BC shall, and shall cause its Subsidiaries to, honor in
accordance with their terms all Plans, each as amended to the date hereof and as
otherwise amended prior to the Closing Date in accordance with the terms of
paragraph (e) hereof, and other contracts, arrangements, commitments or
understandings described in the SierraWest Disclosure Schedule; provided,
however, that this paragraph (b) shall be subject to the provisions of paragraph
(d) hereof. BC and SierraWest hereby acknowledge that consummation of the Merger
will constitute a "Change in Control" for purposes of all Plans, contracts,
arrangements and commitments of SierraWest identified pursuant to Section 3.1(j)
that contain change in control provisions.
 
     (c) Except as provided in Section 5.6(g), SierraWest and its Subsidiaries
shall take all action necessary to ensure that no further mortgage loans will be
made to employees and to amend any retiree medical plans so that no additional
retirees shall become entitled to continuing medical insurance benefits
thereunder.
 
     (d) Except as otherwise provided herein, nothing in this Section 5.6 shall
be interpreted as preventing BC or its Subsidiaries after the Effective Time
from amending, modifying or terminating any of the Plans, or other contracts,
arrangements, commitments or understandings, in accordance with their terms and
applicable law.
 
     (e) Prior to the Closing Date, SierraWest and its Subsidiaries shall
implement the modifications to its present incentive compensation and bonus
plans which are described more fully in Section 5.6(e) of the SierraWest
Disclosure Schedule which will provide for the settlement promptly after the
Effective Time of the accrued entitlement of the participants in such plans as
of such date. BC and BW shall cause such entitlements to be paid to the
respective participants promptly after the Effective Time subject to required
withholdings. For the period from the Closing Date to and including December 31,
1999, BC and BW shall either cause the existing incentive compensation and bonus
plans of SierraWest and its Subsidiaries to be continued by the Surviving
Corporation or shall make comparable plans available to the participants in such
plans.
 
     (f) After the Closing Date, BC and BW shall cause the Surviving Corporation
to honor the obligations of SierraWest under Section 2.6(b) of the Plan of
Acquisition and Merger dated November 13, 1997, among SierraWest, SWB,
California Community Bancshares and Continental Pacific Bank.
 
     (g) After the Closing Date, BC shall and BC and BW shall cause the
Surviving Corporation to honor the arrangements regarding health and life
insurance coverage, continuation of directors' fees and deferral of directors'
fees for certain individuals which are described in Section 5.6(g) of the
SierraWest Disclosure Schedule and SierraWest shall have performed all necessary
actions so that the modification or termination of the various plans or
arrangements which are described in Section 5.6(g) of the SierraWest Disclosure
Schedule shall be effective at the Effective Time.
 
     5.7  Indemnification; Directors' and Officers' Insurance.
 
     (a) From and after the Effective Time, the Surviving Corporation shall
indemnify, defend and hold harmless each person who is now, or has been at any
time prior to the
 
                                      A-36
<PAGE>   140
 
date hereof or who becomes prior to the Effective Time, an officer or director
of SierraWest or any of its Subsidiaries (the "Indemnified Parties") against (i)
all losses, claims, damages, costs, expenses, liabilities or judgments or
amounts of any nature whatsoever, governmental or non-governmental (including
but not limited to reasonable expenses of counsel and investigation) that are
paid in settlement of or in connection with any claim, action, suit, proceeding
or investigation based in whole or in part on or arising in whole or in part out
of the fact that such person is or was a director or officer of SierraWest or
any Subsidiary of SierraWest, whether pertaining to any matter existing or
occurring at or prior to the Effective Time and whether asserted or claimed
prior to, or at or after, the Effective Time ("Indemnified Liabilities") and
(ii) all Indemnified Liabilities based in whole or in part on, or arising in
whole or in part out of, or pertaining to this Agreement or the transactions
contemplated hereby, in each case to the full extent that SierraWest would have
been permitted under applicable law and its Articles of Incorporation, and the
Surviving Corporation is permitted under California law, to indemnify such
person (and the Surviving Corporation shall pay expenses in advance of the final
disposition of any such action or proceeding to each Indemnified Party to the
full extent permitted by law with no bond or security to be required upon
receipt of any undertaking required by Section 317(f) of the California
Corporations Code). Without limiting the foregoing, in the event any such claim,
action, suit, proceeding or investigation is brought against any Indemnified
Parties (whether arising before or after the Effective Time), (i) any counsel
retained by the Indemnified Parties for any period after the Effective Time
shall be reasonably satisfactory to the Surviving Corporation; (ii) after the
Effective Time, the Surviving Corporation shall pay all reasonable fees and
expenses of such counsel for the Indemnified Parties promptly as statements
therefor are received; and (iii) after the Effective Time, the Surviving
Corporation will use all reasonable efforts to assist in the vigorous defense of
any such matter, provided that the Surviving Corporation shall not be liable for
any settlement of any claim effected without its written consent, which consent,
however, shall not be unreasonably withheld or delayed. Any Indemnified Party
wishing to claim indemnification under this Section 5.7, upon learning of any
such claim, action, suit, proceeding or investigation, shall notify the
Surviving Corporation (but the failure so to notify the Surviving Corporation
shall not relieve it from any liability which it may have under this Section 5.7
except to the extent such failure materially prejudices the Surviving
Corporation), and shall deliver to the Surviving Corporation the undertaking, if
any, required by Section 317(f) of the California Corporations Code. The
Surviving Corporation shall be liable for the fees and expenses hereunder with
respect to only one law firm, in addition to local counsel in each applicable
jurisdiction, to represent the Indemnified Parties as a group with respect to
each such matter unless there is, under applicable standards of professional
conduct, a conflict between the positions of any two or more Indemnified Parties
that would preclude or render inadvisable joint or multiple representation of
such parties.
 
     (b) For a period of four years after the Effective Time, BC shall cause to
be maintained in effect the current policies of directors' and officers'
liability insurance maintained by SierraWest and its Subsidiaries (provided that
BC may substitute therefor policies of at least the same coverage and amounts
containing terms and conditions which are not materially less advantageous in
the aggregate) with respect to claims arising from facts or events which
occurred before the Effective Time; provided, however, that BC shall not be
obligated to make annual premium payments for such insurance to the extent such
premiums exceed 150% of the premiums paid by SierraWest and SWB in respect of
1998 for such insurance, as previously disclosed to BC ("SierraWest's Current
Premium"), and if such premiums for such insurance would at any time exceed 150%
of SierraWest's
 
                                      A-37
<PAGE>   141
 
Current Premium, then BC shall cause to be maintained policies of insurance
which, in BC's good faith determination, provide the maximum coverage available
at an annual premium equal to 150% of SierraWest's Current Premium.
 
     (c) The provisions of this Section 5.7 are intended to be for the benefit
of, and shall be enforceable by, each Indemnified Party, his or her heirs and
his or her representatives and are in addition to, and not in substitution for,
any other rights to indemnification or contribution that any such person may
have by contract or otherwise.
 
     (d) BW acknowledges and agrees to honor the obligations contained in
Section 3.3(d) of the Plan of Acquisition and Merger dated as of November 13,
1997, among SierraWest, SWB, California Community Bancshares Corporation and
Continental Pacific Bank.
 
     5.8  Additional Agreements.  In case at any time after the Effective Time
any further action is necessary or desirable to carry out the purposes of this
Agreement, the proper officers and directors of each party to this Agreement
shall take all such necessary action.
 
     5.9  Fees and Expenses.  Except as otherwise expressly provided herein,
whether or not the transactions contemplated hereby are consummated, all costs
and expenses incurred in connection with this Agreement and the transactions
contemplated hereby shall be paid by the party incurring such expense.
 
     5.10  Cooperation.  During the period from the date of this Agreement to
the Effective Time, each of SierraWest and BC shall, (i) confer on a regular and
frequent basis with the other, report on operational matters, policies and
banking practices and promptly advise the other orally and in writing of any
change or event having, or which, insofar as can reasonably be foreseen, could
have, a material adverse effect on SierraWest or BC, as the case may be, or
which would cause or constitute a material breach of any of the representations,
warranties or covenants of such party contained herein and (ii) cause each
Subsidiary of SierraWest and BC that is a bank to file all call reports with the
appropriate Bank Regulators and all other reports, applications and other
documents required to be filed with the applicable Governmental Entities between
the date hereof and the Effective Time.
 
     5.11  Affiliates.  Each of BC and SierraWest shall use its commercially
reasonable efforts to cause each director, executive officer and other person
who is an "affiliate" (for purposes of Rule 145 under the Securities Act, in the
case of affiliates of SierraWest, and for purposes of qualifying the Merger for
pooling of interests accounting treatment, in the case of affiliates of either
BC or SierraWest) of such party to execute and deliver, as soon as practicable
after the date of this Agreement, and in any event prior to the date of the
SierraWest Shareholders meeting, a written agreement, in substantially the form
heretofore agreed to by the parties hereto.
 
     5.12  Stock Exchange Listing.  BC shall use its commercially reasonable
efforts to cause the shares of BC Common Stock to be issued in the Merger to be
approved for listing on the NYSE, subject to official notice of issuance, prior
to the Effective Time.
 
     5.13  Advice of Changes.  BC and SierraWest shall promptly advise the other
party of any change or event which, individually or in the aggregate with other
such changes or events, has a material adverse effect on it or which it believes
would or would be reasonably likely to cause or constitute a material breach of
any of its representations, warranties or covenants contained herein.
 
                                      A-38
<PAGE>   142
 
     5.14  Subsequent Interim and Annual Financial Statements; Certain
Reports.  As soon as reasonably available, but in no event more than 45 days
after the end of each fiscal quarter (other than the fourth quarter of a fiscal
year) or 90 days after December 31, 1998 or the end of each fiscal year ending
after the date of this Agreement, each party will deliver to the other party its
Quarterly Report on Form 10-Q or its Annual Report on Form 10-K, as the case may
be, as filed with the SEC under the Exchange Act and SierraWest will furnish to
BC copies of its management's monthly interim reports (which do not comply with
the published rules and regulations of the SEC or GAAP) to SierraWest's Board of
Directors within two days after such reports are so furnished to the Board. No
later than March 31, 1999, SierraWest will cause there to be delivered to BC
copies of the consolidated statements of financial condition of SierraWest and
its Subsidiaries, as of December 31, 1998, and the related consolidated
statements of income, shareholders' equity and cash flows for the fiscal year
ended December 31, 1998, accompanied by the report of Deloitte & Touche LLP,
independent auditors with respect to SierraWest.
 
     5.15  Dissenters' Rights.  SierraWest shall include in the notice of the
SierraWest Shareholders' Meeting the summary of certain provisions of the
California Corporations Code as required by California Corporation's Code, ss.
1300(b)(1), so that the shares of SierraWest Common Stock shall not constitute
"dissenting shares."
 
                                   ARTICLE VI
 
                              CONDITIONS PRECEDENT
 
     6.1  Conditions to Each Party's Obligation.  The respective obligations of
each party to consummate the transactions contemplated by this Agreement shall
be subject to the satisfaction on or prior to the Closing Date of the following
conditions:
 
          (a) Shareholder Approval.  The SierraWest Shareholder Approval shall
     have been obtained.
 
          (b) Other Approvals.  All authorizations, consents, orders or
     approvals of, or declarations or filings with, and all expirations of
     waiting periods imposed by, any Governmental Entity (all the foregoing,
     "Consents") which are necessary pursuant to the Merger, other than
     immaterial Consents the failure to obtain which would have no material
     adverse effect on the consummation of the transactions contemplated by this
     Agreement and the Agreement of Merger or on either BC or the Surviving
     Corporation, shall have been filed, have occurred or been obtained (all
     such permits, approvals, filings and consents and the lapse of all such
     waiting periods being referred to as the "Requisite Regulatory Approvals")
     and all such Requisite Regulatory Approvals shall be in full force and
     effect.
 
          (c) No Injunctions or Restraints.  No temporary restraining order,
     preliminary or permanent injunction or other order issued by any court of
     competent jurisdiction or other legal restraint or prohibition (an
     "Injunction") preventing the consummation of the transactions contemplated
     by this Agreement or the Transaction Agreements shall be in effect. There
     shall not be any action taken, or any statute, rule, regulation or order
     enacted, entered, enforced or deemed applicable to the transactions
     contemplated by this Agreement or the Transaction Agreements, by any
     Federal, state or foreign Governmental Entity of competent jurisdiction
     which makes the consummation of the transactions contemplated by this
     Agreement or the Transaction Agreements illegal.
 
                                      A-39
<PAGE>   143
 
          (d) NYSE Listing.  The shares of BC Common Stock which shall be issued
     to the shareholders of SierraWest upon consummation of the Merger shall
     have been approved for listing upon official notice of issuance on the
     NYSE.
 
          (e) S-4.  The S-4 shall become effective under the Securities Act, no
     stop orders suspending the effectiveness of the S-4 shall have been issued
     and no proceedings for that purpose shall have been initiated or threatened
     by the SEC.
 
          (f) Pooling.  Each of BC and SierraWest shall have received a letter
     from its independent public accountants, dated the Closing Date, in form
     and substance reasonably satisfactory to BC and SierraWest, respectively,
     to the effect that the Merger will qualify for "pooling of interests"
     accounting treatment, provided that if either party shall have knowingly
     taken or omitted to take any action which shall have prevented such party's
     independent public accountants from rendering such letter, then this
     condition shall not be applicable to such party.
 
     6.2  Conditions to Obligations of BC.  The obligation of BC to consummate
the transactions contemplated by this Agreement is subject to the satisfaction
of the following conditions unless waived by BC:
 
          (a) Representations and Warranties.  The representations and
     warranties of SierraWest set forth in this Agreement shall be true and
     correct in all material respects as of the date of this Agreement and
     (except to the extent such representations and warranties speak as of an
     earlier date) as of the Closing Date as though made on and as of the
     Closing Date, and BC shall have received a certificate signed on behalf of
     SierraWest by its President and Chief Executive Officer and its Chief
     Financial Officer to such effect.
 
          (b) Performance of Obligations.  SierraWest shall have performed in
     all material respects all obligations required to be performed by it under
     this Agreement at or prior to the Closing Date, and BC shall have received
     a certificate signed on behalf of SierraWest by its President and Chief
     Executive Officer and its Chief Financial Officer to such effect.
 
          (c) Corporate Action.  BC shall have received a copy of the resolution
     or resolutions duly adopted by the Board of Directors (or a duly authorized
     committee thereof) of SierraWest and of the holders of the SierraWest
     Common Stock authorizing the execution, delivery and performance by
     SierraWest of this Agreement, certified by the Secretary or an Assistant
     Secretary of SierraWest.
 
          (d) Tax Opinion.  BC shall have received the opinion of Pillsbury
     Madison & Sutro LLP, counsel to BC, dated the Closing Date, to the effect
     that (i) the Merger will be treated for Federal income tax purposes as a
     reorganization within the meaning of Section 368(a) of the Code and (ii) BC
     and SierraWest will each be a party to that reorganization within the
     meaning of Section 368(b) of the Code. In rendering such opinion, such
     counsel may require and rely upon representations and covenants contained
     in certificates of officers of BC, SierraWest and others.
 
          (e) SierraWest Rights Agreement.  The rights issued pursuant to the
     SierraWest Rights Agreement shall not have become nonredeemable,
     exercisable, distributed or triggered pursuant to the terms of such
     agreement.
 
          (f) Burdensome Condition.  There shall not be any action taken, or any
     statute, rule, regulation, order or decree enacted, entered, enforced or
     deemed applicable to the Merger or the other Transaction Agreements by any
     Federal, state or foreign
 
                                      A-40
<PAGE>   144
 
     Governmental Entity which, in connection with the grant of a Requisite
     Regulatory Approval or otherwise, imposes any condition or restriction (a
     "Burdensome Condition") upon BC or its Subsidiaries or any Affiliate hereof
     which would reasonably be expected to (i) have a material adverse effect
     after the Effective Time on the present or prospective consolidated
     financial condition, business, operating results or prospects of BC or the
     Surviving Corporation (including, without limitation, any requirement to
     dispose of any material assets or businesses or restrict in any significant
     way any material operations or activities), (ii) prevent BC or its
     Subsidiaries from realizing all or a substantial portion of the economic
     benefits of the transactions contemplated by this Agreement, or (iii)
     materially impair BC's ability to exercise and enforce its rights under the
     Transaction Agreements.
 
          (g) SAS 71 Review Letters.  SierraWest shall have provided to BC a
     review report prepared in accordance with the provisions of Statement of
     Accounting Standards No. 71 ("SAS 71"), Interim Financial Information, by
     SierraWest's independent accountants covering SierraWest's quarterly
     financial report for the most recent quarter ending at least 45 days prior
     to the Closing Date.
 
          (h) Bank Merger.  All conditions precedent (including required
     regulatory approvals) to the consummation of the Bank Merger shall have
     been satisfied and the Bank Merger shall be able to be consummated
     immediately following the Effective Time of the Merger.
 
          (i) No Dissenters' Rights.  BC shall have determined to its
     satisfaction that no holders of SierraWest Common Stock are entitled to
     have their shares treated as "dissenting shares" under Section 1300 of the
     California Corporations Code by reason of the Merger because such shares
     are listed on the list of OTC margin stocks maintained by the Federal
     Reserve and demands for payment shall not have been filed with respect to
     5% or more of the shares of the SierraWest Common Stock then outstanding.
 
     6.3  Conditions to Obligations of SierraWest.  The obligation of SierraWest
to consummate the transactions contemplated by this Agreement is subject to the
satisfaction of the following conditions unless waived by SierraWest:
 
          (a) Representations and Warranties.  The representations and
     warranties of BC set forth in this Agreement shall be true and correct in
     all material respects as of the date of this Agreement and (except to the
     extent such representations speak as of an earlier date) as of the Closing
     Date as though made on and as of the Closing Date, and SierraWest shall
     have received a certificate signed on behalf of BC by its Chairman and
     Chief Executive Officer or its President or Chief Financial Officer to such
     effect.
 
          (b) Performance of Obligations.  BC shall have performed in all
     material respects all obligations required to be performed by it under this
     Agreement at or prior to the Closing Date, and SierraWest shall have
     received a certificate signed on behalf of BC by its President or its Chief
     Financial Officer to such effect.
 
          (c) Tax Opinion.  SierraWest shall have received the opinion of
     McCutchen, Doyle, Brown & Enersen LLP, counsel to SierraWest, dated the
     Closing Date, to the effect that (i) the Merger will be treated for Federal
     income tax purposes as a reorganization within the meaning of Section
     368(a) of the Code and (ii) BC and SierraWest will each be a party to that
     reorganization within the meaning of Section 368(b) of the Code. In
     rendering such opinion, such counsel may require and
 
                                      A-41
<PAGE>   145
 
     rely upon representations and covenants contained in certificates of
     officers of BC, SierraWest and others.
 
          (d) Corporate Action.  SierraWest shall have received a copy of the
     resolution or resolutions duly adopted by the Board of Directors of BC
     authorizing the execution, delivery or performance by BC of this Agreement
     and the other Transaction Agreements, certified by the Secretary or an
     Assistant Secretary of BC.
 
                                  ARTICLE VII
 
                           TERMINATION AND AMENDMENT
 
     7.1  Termination.  This Agreement may be terminated at any time prior to
the Effective Time, by action taken or authorized by the Board of Directors of
the terminating party or parties, whether before or after adoption of the
Agreement by the shareholders of SierraWest:
 
          (a) by mutual consent of BC and SierraWest in a written instrument;
 
          (b) by (i) either BC or SierraWest upon written notice to the other
     party if any Bank Regulator shall have issued an order denying approval of
     the Merger and the other material aspects of the transactions contemplated
     by this Agreement or if any Governmental Entity of competent jurisdiction
     shall have issued a final permanent order enjoining or otherwise
     prohibiting the consummation of the transactions contemplated by this
     Agreement or (ii) by BC, if any Governmental Entity of competent
     jurisdiction shall have issued an order in connection with the transactions
     contemplated hereby imposing a Burdensome Condition on BC or the Surviving
     Corporation, and in any such case the time for appeal or petition for
     reconsideration of any such order referred to in clauses (i) or (ii) shall
     have expired without such appeal or petition being granted;
 
          (c) by either BC or SierraWest if the Merger shall not have been
     consummated on or before September 30, 1999; provided that if the Merger
     shall not been consummated on or before such date due to the act or
     omission of BC or SierraWest, then that party may not terminate this
     Agreement pursuant to this paragraph (c);
 
          (d) by BC in the event of a breach by SierraWest of any
     representation, warranty or covenant contained in this Agreement, which
     breach (i) either is not cured within 45 days after the giving of written
     notice to SierraWest, or is of a nature which cannot be cured prior to the
     Closing and (ii) would entitle the non-breaching party to elect not to
     consummate the transactions contemplated hereby pursuant to Article VI;
     provided, however, that BC may immediately terminate this agreement upon
     notice to SierraWest in the event that SierraWest shall breach the covenant
     provided for in Section 5.4 hereof;
 
          (e) by SierraWest in the event of a breach by BC of any
     representation, warranty or covenant contained in this Agreement, which
     breach (i) either is not cured within 45 days after the giving of written
     notice to BC or is of a nature which cannot be cured prior to the Closing
     and (ii) would entitle the non-breaching party to elect not to consummate
     the transactions contemplated hereby pursuant to Article VI;
 
          (f) by BC if, in accordance with Section 5.3, the Board of Directors
     of SierraWest fails to recommend adoption of this Agreement by the
     shareholders of SierraWest, or amends or modifies such recommendation in a
     manner materially adverse to BC or withdraws such recommendation to the
     shareholders of SierraWest;
 
                                      A-42
<PAGE>   146
 
          (g) by BC or SierraWest, if the SierraWest Shareholder Approval shall
     not have been obtained at a duly held meeting of shareholders of SierraWest
     held for such purpose or at any adjournment, postponement or continuation
     thereof; or
 
          (h) by the Board of Directors of SierraWest, if the Board of Directors
     so determines by a vote of a majority of the members of its entire Board,
     at any time during the two-Business Day period commencing on the first
     Business Day after the Determination Date (as defined herein), if (i) the
     Average BC Closing Price shall be less than the product of 0.85 and the
     Average BC Starting Price, and (ii) the number obtained by dividing the
     Average BC Closing Price by the Average BC Starting Price (the "BC Ratio")
     shall be less than the number obtained by dividing the Final Index Price by
     the Initial Index Price and by multiplying such quotient by 0.85 (the
     "Index Ratio"); subject, however, to the following provisions of this
     paragraph (h). If SierraWest elects to exercise its termination right
     pursuant to the immediately preceding sentence, it shall give prompt
     written notice to BC during such two-Business Day period by means of
     facsimile transmission (as provided in Section 8.2 hereof); provided that
     such notice of election to terminate may be withdrawn at any time within
     the aforementioned two-Business Day period. During the five Business-Day
     period commencing on the day after receipt of such notice of election to
     terminate, BC shall have the option of adjusting the Exchange Ratio to
     equal the lesser of (A) a number equal to a quotient (rounded to the
     nearest one ten-thousandth), the numerator of which is the product of 0.85,
     the Average BC Starting Price and the Exchange Ratio (as then in effect)
     and the denominator of which is the Average BC Closing Price and (B) a
     number equal to a quotient (rounded to the nearest one ten-thousandth), the
     numerator of which is the Index Ratio multiplied by the Exchange Ratio (as
     then in effect) and the denominator of which is the BC Ratio. If BC makes
     the election contemplated by the preceding sentence, within such five
     Business-Day period, it shall give prompt written notice to SierraWest of
     such election and the revised Exchange Ratio, whereupon no termination
     shall have occurred pursuant to this paragraph (h) and this Agreement shall
     remain in effect in accordance with its terms (except as the Exchange Ratio
     shall have been so modified), and any references in this Agreement to
     "Exchange Ratio" shall thereafter be deemed to refer to the Exchange Ratio
     as adjusted pursuant to this paragraph (h).
 
          For purposes hereof, the following terms have the following meanings:
 
             (i) "Average BC Closing Price" shall mean the average of the
        closing prices of BC Common Stock on the NYSE for the 20 consecutive
        trading days ending on the Determination Date, rounded to four decimal
        places, whether or not trades occurred on those days (subject to
        adjustment as provided below and provided that if no trades of BC Common
        Stock shall occur on a given trading day the closing price thereof on
        the next preceding day when a trade shall have occurred shall be deemed
        to be the closing price on such day for the purposes hereof). In the
        event BC pays, declares or otherwise effects a stock split, reverse
        stock split, reclassification or stock dividend or stock distribution
        with respect to the BC Common Stock between the date of this Agreement
        and the Effective Time, appropriate adjustments will be made to the
        Average BC Closing Price of BC Common Stock.
 
             (ii) "Average BC Starting Price" shall mean the average of the
        closing prices of BC Common Stock on the NYSE for the five consecutive
        trading days
 
                                      A-43
<PAGE>   147
 
        immediately preceding the day on which a press release regarding this
        Agreement shall be issued.
 
             (iii) "Determination Date" shall mean the date on which approval of
        the FDIC required for consummation of the Merger shall be received.
 
             (iv) "Index" shall mean the Standard & Poor's Mid-Cap Regional Bank
        Index (MBKRG) as published by Bloomberg Financial Markets.
 
             (v) "Initial Index Price" shall mean the average of the Index on
        the five consecutive trading days immediately preceding the public
        announcement of this Agreement.
 
             (vi) "Final Index Price" shall mean the average of the Index for
        the 20 trading day period referred to in the definition of Average BC
        Closing Price.
 
             (vii) "Trading day" shall have the meaning ascribed thereto in
        Section 2.3 hereof.
 
          (i) by BC if SierraWest shall have failed to deliver the financial
     statements and report of Deloitte & Touche LLP accompanying such financial
     statements pursuant to the last sentence of Section 5.14 hereto or if the
     financial statements so delivered shall reflect any material adverse change
     in the consolidated financial condition, income, shareholders' equity or
     cash flows of SierraWest and its Subsidiaries compared to the financial
     condition, income, shareholders' equity or cash flows reflected in the
     unaudited consolidated financial statements of SierraWest and its
     Subsidiaries, or if the report of Deloitte & Touche LLP accompanying such
     financial statements contains any qualifications which are not satisfactory
     to BC.
 
     7.2  Effect of Termination.  In the event of termination of this Agreement
by either SierraWest or BC as provided in Section 7.1, this Agreement shall
forthwith become void and there shall be no liability or obligation on the part
of BC or SierraWest or their respective officers or directors except (i) with
respect to Sections 3.1(t) and the penultimate sentence of Section 5.2, and (ii)
with respect to any liabilities or damages incurred or suffered by a party as a
result of the willful breach by the other party or parties of any of its
representations, warranties, covenants or agreements set forth in this
Agreement.
 
     7.3  Amendment.  This Agreement may be amended by the parties hereto at any
time before or after adoption of this Agreement by the shareholders of
SierraWest, but after any such approval, no amendment shall be made which by law
requires further approval by such shareholders without such further approval.
This Agreement may not be amended except by an instrument in writing signed on
behalf of each of the parties hereto.
 
     7.4  Extension; Waiver.  At any time prior to the Closing Date, the parties
hereto, by action taken or authorized by their respective Board of Directors,
may, to the extent legally allowed, (i) extend the time for the performance of
any of the obligations or other acts of the other parties hereto, (ii) waive any
inaccuracies in the representations and warranties contained herein or in any
document delivered pursuant hereto and (iii) waive compliance with any of the
agreements or conditions contained herein. Any agreement on the part of a party
hereto to any such extension or waiver shall be valid only if set forth in a
written instrument signed on behalf of such party.
 
                                      A-44
<PAGE>   148
 
                                  ARTICLE VIII
 
                               GENERAL PROVISIONS
 
     8.1  Nonsurvival of Representations and Warranties.  None of the
representations or warranties in this Agreement shall survive the Effective
Time.
 
     8.2  Notices.  All notices and other communications hereunder shall be in
writing and shall be deemed duly given (a) on the date of delivery if delivered
personally, or by telecopy or telefacsimile, upon confirmation of receipt, (b)
on the first Business Day following the date of dispatch if delivered by a
recognized next-day courier service, or (c) on the third Business Day following
the date of mailing if delivered by registered or certified mail, return receipt
requested, postage prepaid. All notices hereunder shall be delivered as set
forth below, or pursuant to such other instructions as may be designated in
writing by the party to receive such notice.
 
     (a) if to SierraWest, to
 
         SierraWest Bancorp
         10181 Truckee-Tahoe Airport Road
         P.O. Box 61000
         Truckee, CA 96160-9010
         Fax: (530) 582-2953
         Attention:  Mr. William T. Fike, President and
                     Chief Executive Officer
 
          with a copy to
 
          James M. Rockett, Esq.
          McCutchen, Doyle, Brown & Enersen, LLP
          Three Embarcadero Center, Suite 2700
          San Francisco, CA 94111-4067
          Fax: (415) 393-2286
 
          and
 
     (b) if to BC and BW, to
 
         BancWest Corporation
         999 Bishop Street, 29th Floor
         Honolulu, HI 96813
         Fax: (808) 533-7844
         Attention:  Mr. Walter A. Dods, Jr., Chairman and
                     Chief Executive Officer
 
          with a copy to
 
          Pillsbury Madison & Sutro LLP
          235 Montgomery Street
          San Francisco, California 94104
          Attention:  Rodney R. Peck, Esq.
          Fax: (415) 983-1200
 
          and
 
          Simpson Thacher & Bartlett
          425 Lexington Avenue
          New York, NY 10017
          Attention:  Lee Meyerson, Esq.
          Fax: (212) 455-2502
                                      A-45
<PAGE>   149
 
     8.3  Interpretation.  When a reference is made in this Agreement to
Sections, Exhibits or Schedules, such reference shall be to a Section of or
Exhibit or Schedule to this Agreement unless otherwise indicated. The table of
contents and headings contained in this Agreement are for reference purposes
only and shall not affect in any way the meaning or interpretation of this
Agreement. Whenever the words "include", "includes" or "including" are used in
this Agreement, they shall be deemed to be followed by the words "without
limitation". The phrase "made available" in this Agreement shall mean that the
information referred to has been made available if requested by the party to
whom such information is to be made available. The phrases "the date of this
Agreement", "the date hereof" and terms of similar import, unless the context
otherwise requires, shall be deemed to refer to February 25, 1999.
 
     8.4  Counterparts.  This Agreement may be executed in two or more
counterparts, all of which shall be considered one and the same agreement and
shall become effective when two or more counterparts have been signed by each of
the parties and delivered to the other party, it being understood that both
parties need not sign the same counterpart.
 
     8.5  Entire Agreement No Third Party Beneficiaries; Rights of
Ownership.  This Agreement (including the documents and the instruments referred
to herein) (a) constitutes the entire agreement and supersedes all prior
agreements and understandings, both written and oral, among the parties with
respect to the subject matter hereof, other than the Confidentiality Agreements,
which shall survive the execution and delivery of this Agreement and (b) is not
intended to confer upon any person other than the parties hereto any rights or
remedies hereunder except as otherwise expressly provided in Section 5.7. The
parties hereby acknowledge that, except as hereinafter agreed to in writing, no
party shall have the right to acquire or shall be deemed to have acquired shares
of common stock of the other party pursuant to the Merger until consummation
thereof. No current or former employee of SierraWest, BC, or any of their
respective Subsidiaries, shall be construed as a third party beneficiary under
this Agreement, and no provision in this Agreement shall create any right in any
such employee (or his or her beneficiary or dependent) for any reason,
including, without limitation, in respect of employment, continued employment,
or resumed employment with the Surviving Corporation, SierraWest or BC (or any
of their respective Affiliates) or in respect of any benefits that may be
provided, directly or indirectly, under any Benefit Plan maintained by the
Surviving Corporation, SierraWest or BC (or any of their respective Affiliates).
 
     8.6  Governing Law; Consent to Jurisdiction.  This Agreement shall be
governed by and construed in accordance with the laws of the State of California
without giving effect to the principles of conflicts of law. Each of the parties
hereto hereby irrevocably and unconditionally consents to submit to the
non-exclusive jurisdiction of the courts of the State of California and of the
United States of America, in each case located in the City and County of San
Francisco, for any Litigation in any court or before any governmental authority
arising out of or relating to this Agreement and the transactions contemplated
hereby. Each of the parties hereto hereby irrevocably and unconditionally
waives, and agrees not to assert, by way of motion, as a defense, counterclaim
or otherwise, in any such Litigation, any claim that it is not personally
subject to the jurisdiction of the aforesaid courts for any reason other than
the failure to serve process in accordance with this Section 8.6, that it or its
property is exempt or immune from jurisdiction of any such court or from any
legal process commenced in such courts (whether through service of notice,
attachment prior to judgment, attachment in aid of execution of judgment,
execution of judgment or otherwise), and to the fullest extent permitted by
applicable law, that the Litigation in any such court is brought in an
inconvenient forum, that the venue
 
                                      A-46
<PAGE>   150
 
of such Litigation is improper, or that this Agreement, or the subject matter
hereof, may not be enforced in or by such courts and further irrevocably waives,
to the fullest extent permitted by applicable law, the benefit of any defense
that would hinder, fetter or delay the levy, execution or collection of any
amount to which the party is entitled pursuant to the final judgment of any
court having jurisdiction. Each of the parties irrevocably and unconditionally
waives, to the fullest extent permitted by applicable law, any and all rights to
trial by jury in connection with any Litigation arising out of or relating to
this Agreement or the transactions contemplated hereby.
 
     8.7  Severability.  Any term or provision of this Agreement which is
invalid or unenforceable in any jurisdiction shall, as to that jurisdiction, be
ineffective to the extent of such invalidity or unenforceability and, unless the
effect of such invalidity or unenforceability would prevent the parties from
realizing the major portion of the economic benefits of the Merger that they
currently anticipate obtaining therefrom, shall not render invalid or
unenforceable the remaining terms and provisions of this Agreement or affect the
validity or enforceability of any of the terms or provisions of this Agreement
in any other jurisdiction. If any provision of this Agreement is so broad as to
be unenforceable, the provision shall be interpreted to be only so broad as is
enforceable.
 
     8.8  Assignment.  Neither this Agreement nor any of the rights, interests
or obligations hereunder shall be assigned by either of the parties hereto
(whether by operation of law or otherwise) without the prior written consent of
the other party, and any attempt to make any such assignment without such
consent shall be null and void. Subject to the preceding sentence, this
Agreement will be binding upon, inure to the benefit of and be enforceable by
the parties and their respective successors and assigns.
 
     8.9  Publicity.  BC and SierraWest shall consult with each other before
issuing any press release with respect to the Merger or this Agreement and shall
not issue any such press release or make any such public statement without the
prior consent of the other party, which shall not be unreasonably withheld;
provided, however, that a party may, without the prior consent of the other
party (but after prior consultation, to the extent practicable in the
circumstances) issue such press release or make such public statement as may
upon the advice of outside counsel be required by law or the rules and
regulations of the NYSE (in the case of BC) or the National Association of
Securities Dealers (in the case of SierraWest). Without limiting the reach of
the preceding sentence, BC and SierraWest shall cooperate to develop all public
announcement materials and make appropriate management available at
presentations related to the transactions contemplated by this Agreement as
reasonably requested by the other party. In addition, SierraWest and its
Subsidiaries shall (a) consult with BC regarding communications with customers,
shareholders, prospective investors and employees related to the transactions
contemplated hereby, (b) provide BC with shareholders lists of SierraWest and
(c) allow and facilitate BC contact with shareholders of SierraWest and other
prospective investors.
 
                                      A-47
<PAGE>   151
 
     IN WITNESS WHEREOF, BC, BW and SierraWest have caused this Agreement to be
executed by their respective officers thereunto duly authorized, all as of date
first above written.
 
                                          BANCWEST CORPORATION
 
                                          By:   /s/ WALTER A. DODS, JR.
                                          --------------------------------------
                                          Name: Walter A. Dods, Jr.
                                          Title: Chairman and Chief Executive
                                          Officer
 
                                          BANK OF THE WEST
 
                                          By:     /s/ DON J. MCGRATH
                                          --------------------------------------
                                          Name: Don J. McGrath
                                          Title: President and Chief Executive
                                          Officer
 
                                          SIERRAWEST BANCORP
 
                                          By:     /s/ WILLIAM T. FIKE
                                          --------------------------------------
                                          Name: William T. Fike
                                          Title: President and Chief Executive
                                          Officer
 
                                      A-48
<PAGE>   152
 
                                                                      APPENDIX B
 
                             STOCK OPTION AGREEMENT
 
     STOCK OPTION AGREEMENT, dated as of February 25, 1999 (the "Agreement"), by
and between SIERRAWEST BANCORP, a California corporation ("Issuer"), and
BANCWEST CORPORATION, a Delaware corporation ("Grantee").
 
                                    RECITALS
 
     A.  The Plan.  Grantee, Issuer and Issuer's wholly-owned subsidiary, Bank
of the West, a California state-chartered bank ("BW"), are concurrently herewith
entering into an Agreement and Plan of Merger, dated as of the date hereof (the
"Plan"), providing for, among other things, the merger of Issuer with and into
BW with BW being the surviving corporation.
 
     B.  Condition to Plan.  As a condition and inducement to Grantee's
execution of the Plan, Grantee has required that Issuer agree, and Issuer has
agreed, to grant Grantee the Option (as hereinafter defined).
 
     NOW, THEREFORE, in consideration of the foregoing and the respective
representations, warranties, covenants and agreements set forth herein and in
the Plan, and intending to be legally bound hereby, Issuer and Grantee agree as
follows:
 
          1.  Defined Terms.  Capitalized terms which are used but not defined
     herein shall have the meanings ascribed to such terms in the Plan.
 
          2.  Grant of Option.  Subject to the terms and conditions set forth
     herein, Issuer hereby grants to Grantee an irrevocable option (the
     "Option") to purchase up to 1,059,490 shares of common stock, no par value
     ("Issuer Common Stock"), of Issuer (as adjusted as set forth herein, the
     "Option Shares," which shall include the Option Shares before and after any
     transfer of such Option Shares, but in no event shall the number of Option
     Shares for which this Option is exercisable exceed 19.9% of the issued and
     outstanding shares of Issuer Common Stock) at a purchase price per Option
     Share (as adjusted as set forth herein, the "Purchase Price") equal to
     $28.875. Each Option Share issued upon exercise of the Option shall be
     accompanied by Rights (the "SierraWest Rights") as provided in the
     SierraWest Rights Agreement.
 
          3.  Exercise of Option.
 
          (a) The Holder (as hereinafter defined) may exercise the Option, in
     whole or in part, at any time and from time to time following the
     occurrence of a Purchase Event (as hereinafter defined); provided that the
     option shall terminate and be of no further force or effect upon the
     earliest to occur of (A) the Effective Time, (B) termination of the Plan in
     accordance with the terms thereof prior to the occurrence of a Purchase
     Event or a Preliminary Purchase Event (as hereinafter defined) or (C) 12
     months after termination of the Plan following the occurrence of a Purchase
     Event or a Preliminary Purchase Event; provided, however, that any purchase
     of shares upon exercise of the Option shall be subject to compliance with
     applicable law. Notwithstanding the termination of the Option, Grantee or
     Holder as the case may be, shall be entitled to purchase those Option
     Shares with respect to which it has exercised the Option in accordance
     herewith prior to the termination of the Option.
 
                                       B-1
<PAGE>   153
 
     The term "Holder" shall mean the holder or holders of the Option from time
     to time, and which initially is Grantee. The termination of the Option
     shall not affect any rights hereunder which by their terms extend beyond
     the date of such termination.
 
          (b) As used herein, a "Purchase Event" means any of the following
     events:
 
             (i) Without Grantee's prior written consent, Issuer or any of its
        Significant Subsidiaries shall have recommended, publicly proposed or
        publicly announced an intention to authorize, recommend or propose, or
        entered into an agreement with any person (other than Grantee or any
        Subsidiary of Grantee) to effect (A) a merger, consolidation or similar
        transaction involving Issuer or any of its Significant Subsidiaries
        (other than transactions solely between Issuer's subsidiaries that are
        not violative of the Plan), (B) the disposition, by sale, lease,
        exchange or otherwise, of assets or deposits of Issuer or any of its
        Significant Subsidiaries representing in either case 15% or more of the
        consolidated assets or deposits of Issuer and its subsidiaries or (C)
        the issuance, sale or other disposition by Issuer (including by way of
        merger, consolidation, share exchange or any similar transaction) of
        securities representing 15% or more of the voting power of Issuer or any
        of its Significant Subsidiaries, other than, in each case of (A), (B),
        or (C), any merger, consolidation, share exchange or similar transaction
        involving Issuer or any of its Significant Subsidiaries in which the
        voting securities of Issuer outstanding immediately prior thereto
        continue to represent (by either remaining outstanding or being
        converted into the voting securities of the surviving entity of any such
        transaction) at least 80% of the combined voting power of the voting
        securities of the Issuer or the surviving entity outstanding immediately
        after the completion of such merger, consolidation, or similar
        transaction (provided any such transaction is not violative of the Plan)
        (each of (A), (B), or (C), an "Acquisition Transaction"); or
 
             (ii) any person (other than Grantee or any Subsidiary of Grantee)
        shall have acquired beneficial ownership (as such term is defined in
        Rule 13d-3 promulgated under the Exchange Act) of or the right to
        acquire beneficial ownership of, or any "group" (as such term is defined
        in Section 13(d)(3) of the Exchange Act), other than a group of which
        Grantee or any Subsidiary of Grantee is a member, shall have been formed
        which beneficially owns or has the right to acquire beneficial ownership
        of 15% or more of the voting power of Issuer or any of its Significant
        Subsidiaries; or
 
             (iii) any person (other than Grantee or any Subsidiary of Grantee)
        shall have commenced (as such term is defined in Rule 14d-2 under the
        Exchange Act) or shall have filed a registration statement under the
        Securities Act with respect to, a tender offer or exchange offer to
        purchase any shares of Issuer Common Stock such that, upon consummation
        of such offer, such person would own or control 15% or more of the then
        outstanding shares of Issuer Common Stock (such an offer being referred
        to herein as a "Tender Offer" or an "Exchange Offer," respectively); or
 
             (iv) the shareholders shall not have approved the Plan by the
        requisite vote at the SierraWest Shareholders Meeting, the SierraWest
        Shareholders Meeting shall not have been held or shall have been
        canceled prior to termination of the Plan, or Issuer's Board of
        Directors shall have failed to make, withdrawn or modified in a manner
        adverse to Grantee the recommendation of Issuer's Board of Directors
        with respect to the Plan, in each case after it shall have been
 
                                       B-2
<PAGE>   154
 
        publicly announced or disclosed that any person (other than Grantee or
        any Subsidiary of Grantee) shall have (A) made, or disclosed an
        intention to make, a bona fide proposal to engage in an Acquisition
        Transaction, (B) commenced a Tender Offer or filed a registration
        statement under the Securities Act with respect to an Exchange Offer or
        (C) filed an application (or given a notice), whether in draft or final
        form, under the BHC Act, the Bank Merger Act, as amended (the "BMA") or
        the Change in Bank Control Act of 1978, as amended (the "CBCA"), for
        approval to engage in an Acquisition Transaction.
 
          (c) As used herein, a "Preliminary Purchase Event" means any of the
     following events:
 
             (i) any person (other than Grantee or any Subsidiary of Grantee)
        shall have made a bona fide proposal to Issuer or its shareholders by
        public announcement, or written communication that is or becomes the
        subject of public disclosure, to engage in an Acquisition Transaction;
        or
 
             (ii) after a proposal is made by a third party to Issuer or its
        shareholders to engage in an Acquisition Transaction, or such third
        party states its intention to the Issuer to make such a proposal if the
        Plan terminates, Issuer shall have breached any representation,
        warranty, covenant or agreement contained in the Plan, which breach
        would entitle Issuer to terminate the Plan pursuant to Section 7.1(d)
        thereof; or
 
             (iii) any person (other than Grantee or any Subsidiary of Grantee)
        other than in connection with a transaction to which Grantee has given
        its prior written consent, shall have filed an application or notice
        with any Governmental Entity for approval to engage in an Acquisition
        Transaction; or
 
             (iv) any event entitling Grantee to terminate the Plan pursuant to
        Section 7.1(f) of the Plan.
 
     As used in this Agreement, "person" shall have the meaning specified in
     Sections 3(a)(9) and 13(d)(3) of the Exchange Act.
 
          (d) Issuer shall notify Grantee promptly in writing of the occurrence
     of any Preliminary Purchase Event or Purchase Event, it being understood
     that the giving of such notice by Issuer shall not be a condition to the
     right of Holder to exercise the Option.
 
          (e) In the event Holder wishes to exercise the Option, it shall send
     to Issuer a written notice (the date of which being herein referred to as
     the "Notice Date") specifying (i) the total number of Option Shares it
     intends to purchase pursuant to such exercise and (ii) a place and date not
     earlier than three business days nor later than 20 business days from the
     Notice Date for the closing (the "Closing") of such purchase (the "Closing
     Date"); provided that if the Closing cannot be consummated by reason of any
     applicable judgment, decree, order, law or regulation, the period of time
     that otherwise would run pursuant to this sentence shall run instead from
     the date on which such restriction on consummation has expired or been
     terminated; and provided, further, without limiting the foregoing, that if
     prior notification to or approval of any Governmental Entity is required in
     connection with such purchase, Issuer shall cooperate with the Holder in
     the filing of the required notice of application for approval and the
     obtaining of such approval and the Closing shall occur immediately
     following such regulatory approvals (and any mandatory waiting
 
                                       B-3
<PAGE>   155
 
     periods). Any exercise of the Option shall be deemed to occur on the Notice
     Date relating thereto.
 
          (f) Notwithstanding Section 3(e), in no event shall any Closing Date
     be more than 18 months after the related Notice Date, and if the Closing
     Date shall not have occurred within 18 months after the related Notice Date
     due to the failure to obtain any such required approval, the exercise of
     the Option effected on the Notice Date shall be deemed to have expired. In
     the event (i) Holder receives official notice that an approval of any other
     Governmental Entity required for the purchase of Option Shares will not be
     issued or granted or (ii) a Closing Date shall not have occurred within 18
     months after the related Notice Date due to the failure to obtain any such
     required approval, Grantee shall be entitled to exercise its right as set
     forth in Section 8 to exercise the option in connection with the resale of
     Issuer Common Stock or other securities pursuant to a registration
     statement as provided in Section 9. The provisions of this Section 3 and
     Section 4 shall apply with appropriate adjustments to any such exercise.
 
     4.  Payment and Delivery of Certificates.
 
     (a) On each Closing Date, Holder shall (i) pay to Issuer, in immediately
available funds by wire transfer to a bank account designated by Issuer, an
amount equal to the Purchase Price multiplied by the number of Option Shares to
be purchased on such Closing Date, and (ii) present and surrender this Agreement
to the Issuer at the address of the Issuer specified in Section 12(f).
 
     (b) At each Closing, simultaneously with the delivery of immediately
available funds and surrender of this Agreement as provided in Section 4(a), (i)
Issuer shall deliver to Holder (A) a certificate or certificates representing
the Option Shares to be purchased at such Closing, which Option Shares shall be
free and clear of any liens, claims or encumbrances and subject to no preemptive
rights, and (B) if the Option is exercised in part only, an executed new
agreement with the same terms as this Agreement evidencing the right to purchase
the balance of the shares of Issuer Common Stock purchasable hereunder, and (ii)
Holder shall deliver to Issuer a letter agreeing that Holder shall not offer to
sell or otherwise dispose of such Option Shares in violation of applicable
federal and state law or of the provisions of this Agreement.
 
     (c) In addition to any other legend that is required by applicable law,
certificates for the Option Shares delivered at each Closing shall be endorsed
with a restrictive legend which shall read substantially as follows:
 
          THE TRANSFER OF THE STOCK REPRESENTED BY THIS CERTIFICATE IS SUBJECT
     TO RESTRICTIONS ARISING UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND
     PURSUANT TO THE TERMS OF A STOCK OPTION AGREEMENT DATED AS OF FEBRUARY 25,
     1999. A COPY OF SUCH AGREEMENT WILL BE PROVIDED TO THE HOLDER HEREOF
     WITHOUT CHARGE UPON RECEIPT BY THE ISSUER OF A WRITTEN REQUEST THEREFOR.
 
     It is understood and agreed that (i) the portion of the above legend
relating to the Securities Act shall be removed by delivery of substitute
certificates without such legend if Holder shall have delivered to Issuer a copy
of a letter from the staff of the SEC, or an opinion of counsel in form and
substance reasonably satisfactory to Issuer and its counsel, to the effect that
such legend is not required for purposes of the Securities Act and (ii) the
reference to restrictions pursuant to this Agreement in the above legend shall
be
 
                                       B-4
<PAGE>   156
 
removed by delivery of substitute certificate(s) without such reference if the
Option Shares evidenced by certificate(s) containing such reference have been
sold or transferred in compliance with the provisions of this Agreement under
circumstances that do not require the retention of such reference.
 
     (d) Upon the giving by Holder to Issuer of the written notice of exercise
of the Option provided for under Section 3(e), the tender of the applicable
Purchase Price in immediately available funds and the tender of this Agreement
to Issuer, Holder shall be deemed to be the holder of record of the shares of
Issuer Common Stock issuable upon such exercise, notwithstanding that the stock
transfer books of Issuer shall then be closed or that certificates representing
such shares of Issuer Common Stock shall not then be actually delivered to
Holder. Issuer shall pay all expenses, and any and all United States federal,
state, and local taxes and other charges that may be payable in connection with
the preparation, issuance and delivery of stock certificates under this Section
4(d) in the name of Holder or its assignee, transferee, or designee.
 
     (e) Issuer agrees (i) that it shall at all times maintain, free from
preemptive rights, sufficient authorized but unissued or treasury shares of
Issuer Common Stock so that the Option may be exercised without additional
authorization of Issuer Common Stock after giving effect to all other options,
warrants, convertible securities and other rights to purchase Issuer Common
Stock, (ii) that it will not, by charter amendment or through reorganization,
consolidation, merger, dissolution or sale of assets, or by any other voluntary
act, avoid or seek to avoid the observance or performance of any of the
covenants, stipulations or conditions to be observed or performed hereunder by
Issuer, (iii) promptly to take all action as may from time to time be required
(including (A) complying with all premerger notification, reporting and waiting
period requirements and (B) in the event prior approval of or notice to any
Governmental Entity is necessary before the Option may be exercised (the
"Governmental Approvals"), cooperating fully with Holder in preparing such
applications or notices and providing such information to such Governmental
Entity as it may require) in order to permit Holder to exercise the Option and
Issuer duly and effectively to issue shares of the Issuer Common Stock pursuant
hereto, and (iv) promptly to take all action provided herein to protect the
rights of Holder against dilution.
 
     5.  Representations and Warranties of Issuer.  Issuer hereby represents and
warrants to Grantee (and Holder, if different than Grantee) as follows:
 
          (a) Corporate Authority. Issuer has full corporate power and authority
     to execute and deliver this Agreement and to consummate the transactions
     contemplated hereby; the execution and delivery of this Agreement and,
     subject to receiving any necessary Governmental Approvals, the consummation
     of the transactions contemplated hereby have been duly and validly
     authorized by the Board of Directors of Issuer, and no other corporate
     proceedings on the part of Issuer are necessary to authorize this Agreement
     or to consummate the transactions so contemplated; this Agreement has been
     duly and validly executed and delivered by Issuer.
 
          (b) Beneficial Ownership. To the best knowledge of Issuer, as of the
     date of this Agreement, no person or group has beneficial ownership of more
     than 10% of the issued and outstanding shares of Issuer Common Stock.
 
          (c) Shares Reserved for Issuance; Capital Stock. Issuer has taken all
     necessary corporate action to authorize and reserve and permit it to issue,
     and at all times from the date hereof through the termination of this
     Agreement in accordance with its
 
                                       B-5
<PAGE>   157
 
     terms, will have reserved for issuance upon the exercise of the Option,
     that number of shares of Issuer Common Stock equal to the maximum number of
     shares of Issuer Common Stock at any time and from time to time purchasable
     upon exercise of the Option, and all such shares, upon issuance pursuant to
     the Option, will be duly authorized, validly issued, fully paid and
     nonassessable, and will be delivered free and clear of any liens, claims or
     encumbrances (other than those created by this Agreement), and not subject
     to any preemptive rights.
 
          (d) No Violations.  The execution, delivery and performance of this
     Agreement does not and will not, and the consummation by Issuer of any of
     the transactions contemplated hereby will not, constitute or result in (A)
     a breach or violation of, or a default under, its articles of incorporation
     or by-laws, or the comparable governing instruments of any of its
     subsidiaries, or (B) a breach or violation of, or a default under, any
     agreement, lease, contract, note, mortgage, indenture, arrangement or other
     obligation of it or any of its subsidiaries (with or without the giving of
     notice, the lapse of time or both) or under any law, rule, ordinance or
     regulation or judgment, decree, order, award or governmental or
     nongovernmental permit or license to which it or any of its subsidiaries is
     subject, that would, in any case give any other person the ability to
     prevent or enjoin Issuer's performance under this Agreement in any material
     respect.
 
          (e) SierraWest Rights Agreement Amendment.  The SierraWest Rights
     Agreement has been amended to provide that Grantee will not become an
     "Acquiring Person" and that no "Triggering Event," "Stock Acquisition Date"
     or "Distribution Date" (as such terms are defined in the SierraWest Rights
     Agreement) will occur as a result of the approval, execution or delivery of
     this Agreement or the Plan or the consummation of the transactions
     contemplated hereby and thereby, including the acquisition of shares of
     Issuer Common Stock by Grantee or Holder pursuant to this Agreement.
 
     6.  Representations and Warranties of Grantee.  Grantee hereby represents
and warrants to Issuer as follows:
 
          (a) Corporate Authority.  Grantee has full corporate power and
     authority to enter into this Agreement and, subject to obtaining the
     approvals referred to in this Agreement, to consummate the transactions
     contemplated by this Agreement; the execution and delivery of this
     Agreement and the consummation of the transactions contemplated hereby have
     been duly authorized by all necessary corporate action on the part of
     Grantee; and this Agreement has been duly executed and delivered by
     Grantee.
 
          (b) Purchase Not for Distribution.  Any Option Shares or other
     securities acquired by Grantee or Holder upon exercise of the Option will
     not be taken with a view to the public distribution thereof and will not be
     transferred or otherwise disposed of except in a transaction registered or
     exempt from registration under the Securities Act.
 
     7.  Adjustment upon Changes in Issuer Capitalization, Etc.
 
     (a) In the event of any change in Issuer Common Stock by reason of a stock
dividend, stock split, split-up, recapitalization, combination, exchange of
shares, exercise of the SierraWest Rights or similar transaction, the type and
number of shares or securities subject to the Option, and the Purchase Price
therefor, shall be adjusted appropriately, and proper provision shall be made in
the agreements governing such transaction so that
 
                                       B-6
<PAGE>   158
 
Holder shall receive, upon exercise of the Option, the number and class of
shares or other securities or property that Holder would have received in
respect of Issuer Common Stock if the Option had been exercised immediately
prior to such event, or the record date therefor, as applicable. If any
additional shares of Issuer Common Stock are issued after the date of this
Agreement (other than pursuant to an event described in the first sentence of
this Section 7(a)), upon exercise of any option to purchase Issuer Common Stock
outstanding on the date hereof, the number of shares of Issuer Common Stock
subject to the Option shall be adjusted so that, after such issuance, it,
together with any           shares of Issuer Common Stock previously issued
pursuant hereto, equals 19.9% of the number of shares of Issuer Common Stock
then issued and outstanding, without giving effect to any shares subject to or
issued pursuant to the Option. No provision of this Section 7 shall be deemed to
affect or change, or constitute authorization for any violation of, any of the
covenants or representations in the Plan.
 
     (b) In the event that Issuer shall enter into an agreement (i) to
consolidate with or merge into any person, other than Grantee or one of its
subsidiaries, and shall not be the continuing or surviving corporation of such
consolidation or merger, (ii) to permit any person, other than Grantee or one of
its subsidiaries, to merge into Issuer and Issuer shall be the continuing or
surviving corporation, but, in connection with such merger, the then outstanding
shares of Issuer Common Stock shall be changed into or exchanged for stock or
other securities of Issuer or any other person or cash or any other property or
the outstanding shares of Issuer Common Stock immediately prior to such merger
shall after such merger represent less than 50% of the outstanding shares and
share equivalents of the merged company, or (iii) to sell or otherwise transfer
all or substantially all of its assets or deposits to any person, other than
Grantee or one of its subsidiaries, then, and in each such case, the agreement
governing such transaction shall make proper provisions so that the Option
shall, upon the consummation of any such transaction and upon the terms and
conditions set forth herein, be converted into, or exchanged for, an option (the
"Substitute Option"), at the election of Holder, of either (x) the Acquiring
Corporation (as hereinafter defined), (y) any person that controls the Acquiring
Corporation, or (z) in the case of a merger described in clause (ii), Issuer
(such person being referred to as "Substitute Option Issuer").
 
     (c) The Substitute Option shall have the same terms as the Option,
provided, that, if the terms of the Substitute Option cannot, for legal reasons,
be the same as the Option, such terms shall be as similar as possible and in no
event less advantageous to Holder. Substitute Option Issuer shall also enter
into an agreement with Holder in substantially the same form as this Agreement,
which shall be applicable to the Substitute Option.
 
     (d) The Substitute Option shall be exercisable for such number of shares of
Substitute Common Stock (as hereinafter defined) as is equal to the Assigned
Value (as hereinafter defined) multiplied by the number of shares of Issuer
Common Stock for which the Option was theretofore exercisable, divided by the
Average Price (as hereinafter defined). The exercise price of the Substitute
Option per share of Substitute Common Stock (the "Substitute Option Price")
shall be equal to the Purchase Price multiplied by a fraction in which the
numerator is the number of shares of Issuer Common Stock for which the option
was theretofore exercisable and the denominator is the number of shares of the
Substitute Common Stock for which the Substitute Option is exercisable.
 
     (e) The following terms have the meanings indicated:
 
          (i) "Acquiring Corporation" shall mean (x) the continuing or surviving
     corporation of a consolidation or merger with Issuer (if other than
     Issuer), (y) Issuer
 
                                       B-7
<PAGE>   159
 
     in a merger in which Issuer is the continuing or surviving person, or (z)
     the transferee of all or substantially all of Issuer's assets (or a
     substantial part of the assets of its subsidiaries taken as a whole).
 
          (ii) "Substitute Common Stock" shall mean the shares of capital stock
     (or similar equity interest) with the greatest voting power in respect of
     the election of directors (or persons similarly responsible for the
     direction of the business and affairs) of the Substitute Option Issuer.
 
          (iii) "Assigned Value" shall mean the highest of (w) the price per
     share of Issuer Common Stock at which a Tender Offer or an Exchange Offer
     therefor has been made, (x) the price per share of Issuer Common Stock to
     be paid by any third party pursuant to an agreement with Issuer, (y) the
     highest closing price for shares of Issuer Common Stock within the
     six-month period immediately preceding the consolidation, merger, or sale
     in question and (z) in the event of a sale of all or substantially all of
     Issuer's assets or deposits an amount equal to (I) the sum of the price
     paid in such sale for such assets (and/or deposits) and the current market
     value of the remaining assets of Issuer, as determined by a nationally
     recognized investment banking firm selected by Holder divided by (II) the
     number of shares of Issuer Common Stock outstanding at such time. In the
     event that a Tender Offer or an Exchange Offer is made for Issuer Common
     Stock or an agreement is entered into for a merger or consolidation
     involving consideration other than cash, the value of the securities or
     other property issuable or deliverable in exchange for Issuer Common Stock
     shall be determined by a nationally recognized investment banking firm
     selected by Holder.
 
          (iv) "Average Price" shall mean the average closing price of a share
     of Substitute Common Stock for the one year immediately preceding the
     consolidation, merger, or sale in question, but in no event higher than the
     closing price of the shares of Substitute Common Stock on the day preceding
     such consolidation, merger or sale; provided that if Issuer is the issuer
     of the Substitute Option, the Average Price shall be computed with respect
     to a share of common stock issued by Issuer, the person merging into Issuer
     or by any company which controls such person, as Holder may elect.
 
     (f) In no event, pursuant to any of the foregoing paragraphs, shall the
Substitute Option be exercisable for more than 19.9% of the aggregate of the
shares of Substitute Common Stock outstanding prior to exercise of the
Substitute Option. In the event that the Substitute Option would be exercisable
for more than 19.9% of the aggregate of the shares of Substitute Common Stock
but for the limitation in the first sentence of this Section 7(f), Substitute
Option Issuer shall make a cash payment to Holder equal to the excess of (i) the
value of the Substitute Option without giving effect to the limitation in the
first sentence of this Section 7(f) over (ii) the value of the Substitute Option
after giving effect to the limitation in the first sentence of this Section
7(f). This difference in value shall be determined by a nationally-recognized
investment banking firm selected by Holder.
 
     (g) Issuer shall not enter into any transaction described in Section 7(b)
unless the Acquiring Corporation and any person that controls the Acquiring
Corporation assume in writing all the obligations of Issuer hereunder and take
all other actions that may be necessary so that the provisions of this Section 7
are given full force and effect (including, without limitation, any action that
may be necessary so that the holders of the other shares of common stock issued
by Substitute Option Issuer are not entitled to exercise any rights
 
                                       B-8
<PAGE>   160
 
by reason of the issuance or exercise of the Substitute Option and the shares of
Substitute Common Stock are otherwise in no way distinguishable from or have
lesser economic value (other than any diminution in value resulting from the
fact that the Substitute Common Stock are restricted securities, as defined in
Rule 144 under the Securities Act or any successor provision) than other shares
of common stock issued by Substitute Option Issuer).
 
     8.  Repurchase at the Option of Holder.
 
     (a) At the request of Holder at any time (i) commencing upon the first
occurrence of a Repurchase Event (as defined in Section 8(d)) and ending 18
months immediately thereafter and (ii) for 30 business days following the
occurrence of either of the events set forth in clauses (i) and (ii) of Section
3(f) (but solely as to the shares of Issuer Common Stock with respect to which
the required approval was not received, Issuer (or any successor) shall
repurchase from Holder (x) the Option and (y) all shares of Issuer Common Stock
purchased by Holder pursuant hereto with respect to which Holder then has
beneficial ownership. The date on which Holder exercises its rights under this
Section 8 is referred to as the "Request Date". Such repurchase shall be at an
aggregate price (the "Section 8 Repurchase Consideration") equal to the sum of:
 
          (i) the aggregate Purchase Price paid by Holder for any shares of
     Issuer Common Stock acquired pursuant to the Option with respect to which
     Holder then has beneficial ownership;
 
          (ii) the excess, if any, of (x) the Applicable Price (as defined
     below) for each share of Issuer Common Stock over (y) the Purchase Price
     (subject to adjustment pursuant to Section 7), multiplied by the number of
     shares of Issuer Common Stock with respect to which the Option has not been
     exercised; and
 
          (iii) the excess, if any, of the Applicable Price over the Purchase
     Price (subject to adjustment pursuant to Section 7) paid (or, in the case
     of Option Shares with respect to which the Option has been exercised but
     the Closing Date has not occurred, payable) by Holder for each share of
     Issuer Common Stock with respect to which the Option has been exercised and
     with respect to which Holder then has beneficial ownership, multiplied by
     the number of such shares.
 
     (b) If Holder exercises its rights under this Section 8, Issuer shall,
within 10 business days after the Request Date, pay the Section 8 Repurchase
Consideration to Holder in immediately available funds, and contemporaneously
with such payment, Holder shall surrender to Issuer the Option and the
certificates evidencing the shares of Issuer Common Stock purchased thereunder
with respect to which Holder then has beneficial ownership, and Holder shall
warrant that it has sole record and beneficial ownership of such shares and that
the same are then free and clear of all Liens. Notwithstanding the foregoing, to
the extent that prior notification to or approval of any Governmental Entity is
required in connection with the payment of all or any portion of the Section 8
Repurchase Consideration, Holder shall have the ongoing option to revoke its
request for repurchase pursuant to Section 8, in whole or in part, or to require
that Issuer deliver from time to time that portion of the Section 8 Repurchase
Consideration that it is not then so prohibited from paying and promptly file
the required notice or application for approval and expeditiously process the
same (and each party shall cooperate with the other in the filing of any such
notice or application and the obtaining of any such approval) and the period of
time that would otherwise run pursuant to the preceding sentence for the payment
of the portion of the Section 8 Repurchase Consideration shall run instead from
 
                                       B-9
<PAGE>   161
 
the date on which, as the case may be, (i) any required notification period has
expired or been terminated or (ii) such approval has been obtained and, in
either event, any requisite waiting period shall have passed. If any
Governmental Entity disapproves of any part of Issuer's proposed repurchase
pursuant to this Section 8, Issuer shall promptly give notice of such fact to
Holder. If any Governmental Entity prohibits the repurchase in part but not in
whole, then Holder shall have the right (i) to revoke the repurchase request or
(ii) to the extent permitted by such Governmental Entity, determine whether the
repurchase should apply to the Option and/or Option Shares and to what extent to
each, and Holder shall thereupon have the right to exercise the Option as to the
number of Option Shares for which the Option was exercisable at the Request Date
less the sum of the number of shares covered by the Option in respect of which
payment has been made pursuant to Section 8(a)(ii) and the number of shares
covered by the portion of the Option (if any) that has been repurchased;
provided that if the Option shall have terminated prior to the date of such
notice or shall be scheduled to terminate at any time before the expiration of a
period ending on the thirtieth business day after such date, Grantee shall
nonetheless have the right so to exercise the Option or exercise its rights
under Section 9 until the expiration of such period of 30 business days. Holder
shall notify Issuer of its determination under the preceding sentence within
five (5) business days of receipt of notice of disapproval of the repurchase.
 
     (c) For purposes of this Agreement, the "Applicable Price" means the
highest of (i) the highest price per share of Issuer Common Stock paid for any
such share by the person or groups described in Section 8(d)(i), (ii) the price
per share of Issuer Common Stock received by holders of Issuer Common Stock in
connection with any merger or other business combination transaction described
in Section 7(b)(i), 7(b)(ii) or 7(b)(iii), or (iii) the highest closing sales
price per share of Issuer Common Stock quoted on the Nasdaq National Market
System during the 40 business days preceding the Request Date; provided,
however, that in the event of a sale of less than all of Issuer's assets, the
Applicable Price shall be the sum of the price paid in such sale for such assets
and the current market value of the remaining assets of Issuer as determined by
a nationally recognized investment banking firm selected by Holder, divided by
the number of shares of the Issuer Common Stock outstanding at the time of such
sale. If the consideration to be offered, paid or received pursuant to either of
the foregoing clauses (i) or (ii) shall be other than in cash, the value of such
consideration shall be determined in good faith by an independent nationally
recognized investment banking firm selected by Holder and reasonably acceptable
to Issuer, which determination shall be conclusive for all purposes of this
Agreement.
 
     (d) As used herein, "Repurchase Event" shall occur if (i) any person (other
than Grantee or any Subsidiary of Grantee) shall have acquired beneficial
ownership of (as such term is defined in Rule 13d-3 promulgated under the
Exchange Act), or the right to acquire beneficial ownership of, or any "group"
(as such term is defined under the Exchange Act) shall have been formed which
beneficially owns or has the right to acquire beneficial ownership of, 25% or
more of the then outstanding shares of Issuer Common stock, or (ii) Issuer has
entered into an agreement pursuant to which any of the transactions described in
Section 7(b)(i), 7(b)(ii) or 7(b)(iii) could or will be consummated.
 
     9.  Registration Rights.
 
     (a) Demand Registration Rights.  Issuer shall, subject to the conditions of
Section 9(c) below, if requested by any Holder, including Grantee and any
permitted
 
                                      B-10
<PAGE>   162
 
transferee ("Selling Shareholder"), as expeditiously as possible prepare and
file a registration statement under the Securities Act if such registration is
necessary in order to permit the sale or other disposition of any or all shares
of Issuer Common Stock or other securities that have been acquired by or are
issuable to the Selling Shareholder upon exercise of the Option in accordance
with the intended method of sale or other disposition stated by the Selling
Shareholder in such request, including without limitation a "shelf" registration
statement under Rule 415 under the Securities Act or any successor provision,
and Issuer shall use its best efforts to qualify such shares or other securities
for sale under any applicable state securities laws.
 
     (b) Additional Registration Rights.  If Issuer at any time after the
exercise of the Option proposes to register any shares of Issuer Common Stock
under the Securities Act in connection with an underwritten public offering of
such Issuer Common Stock, Issuer will promptly give written notice to the
Selling Shareholders of its intention to do so and, upon the written request of
any Selling Shareholder given within 30 days after receipt of any such notice
(which request shall specify the number of shares of Issuer Common Stock
intended to be included in such underwritten public offering by the Selling
Shareholder), Issuer will cause all such shares for which a Selling Shareholder
requests participation in such registration, to be so registered and included in
such underwritten public offering; provided, however, that Issuer may elect to
not cause any such shares to be so registered (i) if the underwriters in good
faith object for valid business reasons, or (ii) in the case of a registration
solely to implement an employee benefit plan or a registration filed on Form S-4
of the Securities Act or any successor Form; provided, further, however, that
such election pursuant to (i) may only be made two times. If some but not all
the shares of Issuer Common Stock with respect to which Issuer shall have
received requests for registration pursuant to this Section 9(b) shall be
excluded from such registration, Issuer shall make appropriate allocation of
shares to be registered among the Selling Shareholders desiring to register
their shares pro rata in the proportion that the number of shares requested to
be registered by each such Selling Shareholder bears to the total number of
shares requested to be registered by all such Selling Shareholders then desiring
to have Issuer Common Stock registered for sale.
 
     (c) Conditions to Required Registration.  Issuer shall use all reasonable
efforts to cause each registration statement referred to in Section 9(a) above
to become effective and to obtain all consents or waivers of other parties which
are required therefor and to keep such registration statement effective;
provided, however, that Issuer may delay any registration of Option Shares
required pursuant to Section 9(a) above for a period not exceeding 90 days
provided Issuer shall in good faith determine that any such registration would
adversely affect an offering or contemplated offering of other securities by
Issuer, and Issuer shall not be required to register Option Shares under the
Securities Act pursuant to Section 9(a) above:
 
          (i) prior to the earliest of (a) termination of the Plan pursuant to
     Article VII thereof, (b) failure to obtain the requisite shareholder
     approval pursuant to Section 6.1(a) of the Plan, and (c) a Purchase Event
     or a Preliminary Purchase Event;
 
          (ii) on more than one occasion during any calendar year;
 
          (iii) within 90 days after the effective date of a registration
     referred to in Section 9(b) above pursuant to which the Selling Shareholder
     or Selling Shareholders concerned were afforded the opportunity to register
     such shares under the Securities Act and such shares were registered as
     requested; and
 
                                      B-11
<PAGE>   163
 
          (iv) unless a request therefor is made to Issuer by Selling
     Shareholders that hold at least 25% or more of the aggregate number of
     Option Shares (including shares of Issuer Common Stock issuable upon
     exercise of the Option) then outstanding.
 
In addition to the foregoing, Issuer shall not be required to maintain the
effectiveness of any registration statement after the expiration of nine months
from the effective date of such registration statement. Issuer shall use all
reasonable efforts to make any filings, and take all steps, under all applicable
state securities laws to the extent necessary to permit the sale or other
disposition of the Option Shares so registered in accordance with the intended
method of distribution for such shares; provided, however, that Issuer shall not
be required to consent to general jurisdiction or qualify to do business in any
state where it is not otherwise required to so consent to such jurisdiction or
to so qualify to do business.
 
     (d) Expenses.  Except where applicable state law prohibits such payments,
Issuer will pay all expenses (including without limitation registration fees,
qualification fees, blue sky fees and expenses (including the fees and expenses
of counsel), legal expenses, including the reasonable fees and expenses of one
counsel to the holders whose Option Shares are being registered, printing
expenses and the costs of special audits or "cold comfort" letters, expenses of
underwriters, excluding discounts and commissions but including liability
insurance if Issuer so desires or the underwriters so require, and the
reasonable fees and expenses of any necessary special experts) in connection
with each registration pursuant to Section 9(a) or 9(b) above (including the
related offerings and sales by holders of Option Shares) and all other
qualifications, notifications or exemptions pursuant to Section 9(a) or 9(b)
above.
 
     (e) Indemnification.  In connection with any registration under Section
9(a) or 9(b) above Issuer hereby indemnifies the Selling Shareholders, and each
underwriter thereof, including each person, if any, who controls such holder or
underwriter within the meaning of Section 15 of the Securities Act, against all
expenses, losses, claims, damages and liabilities caused by any untrue, or
alleged untrue, statement of a material fact contained in any registration
statement or prospectus or notification or offering circular (including any
amendments or supplements thereto) or any preliminary prospectus, or caused by
any omission, or alleged omission, to state therein a material fact required to
be stated therein or necessary to make the statements therein not misleading,
except insofar as such expenses, losses, claims, damages or liabilities of such
indemnified party are caused by any untrue statement or alleged untrue statement
that was included by Issuer in any such registration statement or prospectus or
notification or offering circular (including any amendments or supplements
thereto) in reliance upon and in conformity with, information furnished in
writing to Issuer by such indemnified party expressly for use therein, and
Issuer and each officer, director and controlling person of Issuer shall be
indemnified by such Selling Shareholders, or by such underwriter, as the case
may be, for all such expenses, losses, claims, damages and liabilities caused by
any untrue, or alleged untrue, statement, that was included by Issuer in any
such registration statement or prospectus or notification or offering circular
(including any amendments or supplements thereto) in reliance upon, and in
conformity with, information furnished in writing to Issuer by such holder or
such underwriter, as the case may be, expressly for such use.
 
     Promptly upon receipt by a party indemnified under this Section 9(e) of
notice of the commencement of any action against such indemnified party in
respect of which indemnity or reimbursement may be sought against any
indemnifying party under this Section 9(e), such indemnified party shall notify
the indemnifying party in writing of the commencement of such action, but the
failure so to notify the indemnifying party shall not relieve it of any
 
                                      B-12
<PAGE>   164
 
liability which it may otherwise have to any indemnified party under this
Section 9(e) unless the failure so to notify the indemnified party results in
substantial prejudice thereto. In case notice of commencement of any such action
shall be given to the indemnifying party as above provided, the indemnifying
party shall be entitled to participate in and, to the extent it may wish,
jointly with any other indemnifying party similarly notified, to assume the
defense of such action at its own expense, with counsel chosen by it and
satisfactory to such indemnified party. The indemnified party shall have the
right to employ separate counsel in any such action and participate in the
defense thereof, but the fees and expenses of such counsel (other than
reasonable costs of investigation) shall be paid by the indemnified party unless
(i) the indemnifying party either agrees to pay the same, (ii) the indemnifying
party fails to assume the defense of such action with counsel satisfactory to
the indemnified party, or (iii) the indemnified party has been advised by
counsel that one or more legal defenses may be available to the indemnifying
party that may be contrary to the interest of the indemnified party, in which
case the indemnifying party shall be entitled to assume the defense of such
action notwithstanding its obligation to bear fees and expenses of such counsel.
No indemnifying party shall be liable for any settlement entered into without
its consent, which consent may not be unreasonably withheld.
 
     If the indemnification provided for in this Section 9(e) is unavailable to
a party otherwise entitled to be indemnified in respect of any expenses, losses,
claims, damages or liabilities referred to herein, then the indemnifying party,
in lieu of indemnifying such party otherwise entitled to be indemnified, shall
contribute to the amount paid or payable by such party to be indemnified as a
result of such expenses, losses, claims, damages or liabilities in such
proportion as is appropriate to reflect the relative benefits received by
Issuer, the Selling Shareholders and the underwriters from the offering of the
securities and also the relative fault of Issuer, the Selling Shareholders and
the underwriters in connection with the statements or omissions which resulted
in such expenses, losses, claims, damages or liabilities, as well as any other
relevant equitable considerations. The amount paid or payable by a party as a
result of the expenses, losses, claims, damages and liabilities referred to
above shall be deemed to include any legal or other fees or expenses reasonably
incurred by such party in connection with investigating or defending any action
or claim; provided, however, that in no case shall any Selling Shareholder be
responsible, in the aggregate, for any amount in excess of the net offering
proceeds attributable to its Option Shares included in the offering. No person
guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of
the Securities Act) shall be entitled to contribution from any person who was
not guilty of such fraudulent misrepresentation. Any obligation by any holder to
indemnify shall be several and not joint with other holders.
 
     In connection with any registration pursuant to Section 9(a) or 9(b) above,
Issuer and each Selling Shareholder (other than Grantee) shall enter into an
agreement containing the indemnification provisions of this Section 9(e).
 
     (f) Miscellaneous Reporting.  Issuer shall comply with all reporting
requirements and will do all such other things as may be necessary to permit the
expeditious sale at any time of any Option Shares by the Selling Shareholders
thereof in accordance with and to the extent permitted by any rule or regulation
promulgated by the SEC from time to time, including, without limitation, Rule
144. Issuer shall at its expense provide the Selling Shareholders with any
information necessary in connection with the completion and filing of any
reports or forms required to be filed by them under the Securities Act or the
Exchange Act, or required pursuant to any state securities laws or the rules of
any stock exchange.
 
                                      B-13
<PAGE>   165
 
     10.  Quotation; Listing.  If Issuer Common Stock or any other securities to
be acquired in connection with the exercise of the Option are then authorized
for quotation or trading or listing on the NYSE, the Nasdaq National Market
System or any securities exchange, Issuer, upon the request of Holder, will
promptly file an application, if required, to authorize for quotation or trading
or listing the shares of Issuer Common Stock or other securities to be acquired
upon exercise of the Option on the NYSE, the Nasdaq National Market System or
such other securities exchange and will use its best efforts to obtain approval,
if required, of such quotation or listing as soon as practicable.
 
     11.  Division of Option.  This Agreement (and the Option granted hereby)
are exchangeable, without expense, at the option of Holder, upon presentation
and surrender of this Agreement at the principal office of Issuer for other
Agreements providing for Options of different denominations entitling the holder
thereof to purchase in the aggregate the same number of shares of Issuer Common
Stock purchasable hereunder. The terms "Agreement" and "Option" as used herein
include any other Agreements and related Options for which this Agreement (and
the Option granted hereby) may be exchanged. Upon receipt by Issuer of evidence
reasonably satisfactory to it of the loss, theft, destruction or mutilation of
this Agreement, and (in the case of loss, theft or destruction) of reasonably
satisfactory indemnification, and upon surrender and cancellation of this
Agreement, if mutilated, Issuer will execute and deliver a new Agreement of like
tenor and date. Any such new Agreement executed and delivered shall constitute
an additional contractual obligation on the part of Issuer, whether or not the
Agreement so lost, stolen, destroyed or mutilated shall at any time be
enforceable by anyone.
 
     12.  Miscellaneous.
 
     (a) Expenses.  Each of the parties hereto shall bear and pay all costs and
expenses incurred by it or on its behalf in connection with the transactions
contemplated hereunder, including fees and expenses of its own financial
consultants, investment bankers, accountants and counsel; provided, however,
that Issuer will pay all stamp taxes in connection with the issuance and the
sale of the Option Shares and in connection with the exercise of the Option, and
will save the Selling Shareholders harmless, without limitation as to time,
against any and all liabilities, with respect to all such taxes.
 
     (b) Waiver and Amendment.  Any provision of this Agreement may be waived at
any time by the party that is entitled to the benefits of such provision. This
Agreement may not be modified, amended, altered or supplemented except upon the
execution and delivery of a written agreement executed by the parties hereto.
 
     (c) Entire Agreement; No Third-Party Beneficiaries; Severability.  This
Agreement, together with the Plan and the other documents and instruments
referred to herein and therein, between Grantee and Issuer (i) constitutes the
entire agreement and supersedes all prior agreements and understandings, both
written and oral, between the parties with respect to the subject matter hereof
and (ii) is not intended to confer upon any person other than the parties hereto
(other than the indemnified parties under Section 9(e) and any transferees of
the Option Shares or any permitted transferee of this Agreement pursuant to
Section 12(h)) any rights or remedies hereunder. If any term, provision,
covenant or restriction of this Agreement is held by a court of competent
jurisdiction or Governmental Entity to be invalid, void or unenforceable, the
remainder of the terms, provisions, covenants and restrictions of this Agreement
shall remain in full force and effect and shall in no way be affected, impaired
or invalidated. If for any reason such court or Governmental Entity determines
that the Option does not permit Holder to acquire, or does not require Issuer to
repurchase, the full number of shares of Issuer Common Stock
 
                                      B-14
<PAGE>   166
 
as provided in Section 2 (as may be adjusted herein), it is the express
intention of Issuer to allow Holder to acquire or to require Issuer to
repurchase such lesser number of shares as may be permissible without any
amendment or modification hereof.
 
     (d) Governing Law.  This Agreement shall be governed and construed in
accordance with the laws of the State of California without regard to any
applicable conflicts of law rules.
 
     (e) Descriptive Headings.  The descriptive headings contained herein are
for convenience of reference only and shall not affect in any way the meaning or
interpretation of this Agreement.
 
     (f) Notices.  All notices and other communications hereunder shall be in
writing and shall be deemed given if delivered personally, telecopied (with
confirmation) or mailed by registered or certified mail (return receipt
requested) to the parties at the addresses set forth in the Plan (or at such
other address for a party as shall be specified by like notice).
 
     (g) Counterparts.  This Agreement and any amendments hereto may be executed
in two counterparts, each of which shall be considered one and the same
agreement and shall become effective when both counterparts have been signed, it
being understood that both parties need not sign the same counterpart.
 
     (h) Assignment.  Neither this Agreement nor any of the rights, interests or
obligations hereunder or under the Option shall be assigned by any of the
parties hereto (whether by operation of law or otherwise) without the prior
written consent of the other party, except that Holder may assign this Agreement
to a wholly-owned subsidiary of Holder and Holder may assign its rights
hereunder in whole or in part after the occurrence of a Purchase Event. Subject
to the preceding sentence, this Agreement shall be binding upon, inure to the
benefit of and be enforceable by the parties and their respective successors and
assigns.
 
     (i) Further Assurances.  In the event of any exercise of the Option by the
Holder, Issuer and the Holder shall execute and deliver all other documents and
instruments and take all other action that may be reasonably necessary in order
to consummate the transactions provided for by such exercise.
 
     (j) Specific Performance.  The parties hereto agree that this Agreement may
be enforced by either party through specific performance, injunctive relief and
other equitable relief. Both parties further agree to waive any requirement for
the securing or posting of any bond in connection with the obtaining of any such
equitable relief and that this provision is without prejudice to any other
rights that the parties hereto may have for any failure to perform this
Agreement.
 
                                      B-15
<PAGE>   167
 
     IN WITNESS WHEREOF, Issuer and Grantee have caused this Stock Option
Agreement to be signed by their respective officers thereunto duly authorized,
all as of the day and year first written above.
 
                                          SIERRAWEST BANCORP
 
                                          By       /s/ WILLIAM T. FIKE
                                            ------------------------------------
                                          Name: William T. Fike
                                          Title: President and Chief Executive
                                                 Officer
 
                                          BANCWEST CORPORATION
 
                                          By     /s/ WALTER A. DODS, JR.
                                            ------------------------------------
                                          Name: Walter A. Dods, Jr.
                                          Title: Chairman and Chief Executive
                                                 Officer
 
                                      B-16
<PAGE>   168
 
                                                                      APPENDIX C
                    [NationsBanc Montgomery Securities Logo]
 
February 25, 1999
 
Board of Directors
SierraWest Bancorp
10181 Truckee Tahoe Airport Road
Truckee, CA 96160-9010
 
Gentlemen:
 
     We understand that SierraWest Bancorp, a California corporation
("SierraWest"), and BancWest Corporation, a Delaware corporation ("BWE"),
propose to enter into an Agreement and Plan of Merger to be dated on or about
February 25, 1999 (the "Merger Agreement"), pursuant to which SierraWest will be
merged with and into a wholly-owned subsidiary of BWE, which will be the
surviving entity (the "Merger"). Pursuant to the Merger, as more fully described
in the February 21, 1999 draft of the Merger Agreement provided to us and as
further described to us by management of SierraWest, we understand that each
outstanding share of the common stock, no par value per share ("SierraWest
Common Stock"), of SierraWest will be converted into 0.82 of a fully paid and
non-assessable share of the common stock, $1.00 par value per share ("BWE Common
Stock"), of BWE, subject to certain adjustments (the "Consideration"). The terms
and conditions of the Merger are set forth in more detail in the Merger
Agreement.
 
     You have asked for our opinion as investment bankers as to whether the
Consideration to be received by the shareholders of SierraWest pursuant to the
Merger is fair to such shareholders from a financial point of view, as of the
date hereof.
 
     In connection with our opinion, we have, among other things: (i) reviewed
certain publicly available financial and other data with respect to SierraWest
and BWE, including the consolidated financial statements for recent years and
interim periods to December 31, 1998, and certain other relevant financial and
operating data relating to SierraWest and BWE made available to us from
published sources and from the internal records of SierraWest and BWE; (ii)
reviewed the February 21, 1999 draft of the Merger Agreement; (iii) reviewed
certain publicly available information concerning the trading of, and the
trading market for, SierraWest Common Stock and BWE Common Stock; (iv) compared
certain financial data of SierraWest and BWE with those of certain other
companies in the banking industry which we deemed to be relevant; (v) considered
the financial terms, to the extent publicly available, of selected recent
business combinations of companies in the banking industry which we deemed to be
comparable, in whole or in part, to the Merger; (vi) conducted discussions with
representatives of the senior management of SierraWest and BWE concerning their
respective businesses and prospects; (vii) reviewed certain information,
including financial forecasts and related assumptions, furnished to us by
SierraWest and BWE, respectively; and (viii) performed such other analyses and
examinations as we have deemed appropriate.
 
     In connection with our review, we have not assumed any obligation
independently to verify the foregoing information and have relied on its being
accurate and complete in all
 
                            [NATIONSBANC LOGO SMALL]
                                       C-1
<PAGE>   169
 
material respects. With respect to the financial forecasts for SierraWest and
BWE provided to us by their respective management, upon their advice and with
your consent we have assumed for purposes of our opinion that the forecasts have
been reasonably prepared on bases reflecting the best available estimates and
judgements of their respective management at the time of preparation as to the
future financial performance of SierraWest and BWE and that they provide a
reasonable basis upon which we can form our opinion. We have relied upon
assurances of the management of SierraWest and BWE, respectively, that there
have been no material changes in SierraWest's or BWE's assets, financial
condition, results of operations, business or prospects since the respective
dates of their last financial statements made available to us. We have also
relied on advice of counsel to SierraWest as to all legal matters with respect
to SierraWest, the Merger and the Merger Agreement. We have assumed that the
Merger will be consummated in a manner that complies in all respects with the
applicable provisions of the Securities Act of 1933 (the "Securities Act"), the
Securities Exchange Act of 1934 and all other applicable federal and state
statutes, rules and regulations. In addition, we have not assumed responsibility
for reviewing any individual credit files, or making an independent evaluation,
appraisal or physical inspection of any of the assets or liabilities (contingent
or otherwise) of SierraWest or BWE, nor have we been furnished with any such
appraisals. We are not experts in the evaluation of loan portfolios for purposes
of assessing the adequacy of the allowances for losses with respect thereto and
have assumed, with your consent, that such allowances for each of SierraWest and
BWE are in the aggregate adequate to cover such losses. You have informed us,
and we have assumed, that the Merger will be recorded as a pooling of interests
under generally accepted accounting principles. Our opinion is based on
economic, monetary and market and other conditions as in effect on, and the
information made available to us as of, the date hereof. Accordingly, although
subsequent developments may affect this opinion, we have not assumed any
obligation to update, revise or reaffirm this opinion.
 
     We have further assumed with your consent that the Merger will be
consummated in accordance with the terms described in the February 21, 1999
draft of the Merger Agreement, without any further material revisions thereto,
and without waiver by SierraWest of any of the conditions to its obligations
thereunder.
 
     We have acted as the financial advisor to SierraWest in connection with the
Merger and will receive a fee for our services, including rendering this
opinion, a significant portion of which is contingent upon the consummation of
the Merger.
 
     Based upon the foregoing and in reliance thereon, it is our opinion as
investment bankers that the Consideration to be received by the shareholders of
SierraWest pursuant to the Merger is fair to such shareholders from a financial
point of view, as of the date hereof.
 
     This opinion is directed to the Board of Directors of SierraWest in its
consideration of the Merger and is not a recommendation to any shareholder as to
how such shareholder should vote with respect to the Merger. Further, this
opinion addresses only the financial fairness of the Consideration to the
shareholders and does not address the relative merits of the Merger and any
alternatives to the Merger, SierraWest's underlying decision to proceed with or
effect the Merger, or any other aspect of the Merger. This opinion may not be
used or referred to by SierraWest, or quoted or disclosed to any person in any
manner, without our prior written consent, which consent is hereby given to the
inclusion of this opinion in any proxy statement, prospectus or registration
statement filed with the Securities and Exchange Commission in connection with
the Merger. In furnishing this
 
                                       C-2
<PAGE>   170
 
opinion, we do not admit that we are experts within the meaning of the term
"experts" as used in the Securities Act and the rules and regulations
promulgated thereunder, nor do we admit that this opinion constitutes a report
or valuation within the meaning of Section 11 of the Securities Act.
 
                                Very truly yours,
                                [NationsBanc Montgomery Securities LLC
                                Signature]
                                NATIONSBANC MONTGOMERY
                                  SECURITIES LLC
 
                                       C-3
<PAGE>   171
 
                                                                      APPENDIX D
 
                        CHAPTER 13.   DISSENTERS' RIGHTS
 
     1300 [SHORT FORM MERGER; PURCHASE OF SHARES AT FAIR MARKET VALUE;
"DISSENTING SHARES" AND DISSENTING SHAREHOLDER]. -- (a) If the approval of the
outstanding shares (Section 152) of a corporation is required for a
reorganization under subdivisions (a) and (b) or subdivision (e) or (f) of
Section 1201, each shareholder of the corporation entitled to vote on the
transaction and each shareholder of a subsidiary corporation in a short-form
merger may, by complying with this chapter, require the corporation in which the
shareholder holds shares to purchase for cash at their fair market value the
shares owned by the shareholder which are dissenting shares as defined in
subdivision (b). The fair market value shall be determined as of the day before
the first announcement of the terms of the proposed reorganization or short-form
merger, excluding any appreciation or depreciation on consequence of the
proposed action, but adjusted for any stock split, reverse stock split, or share
dividend which becomes effective thereafter.
 
     (b) As used in this chapter, "dissenting shares" means shares which come
within all of the following descriptions:
 
          (1) Which were not immediately prior to the reorganization or
     short-form merger either (A) listed on any national securities exchange
     certified by the Commissioner of Corporations under subdivision (o) of
     Section 25100 or (B) listed on the list of OTC margin stocks issued by the
     Board of Governors of the Federal Reserve System, and the notice of meeting
     of shareholders to act upon the reorganization summarizes this section and
     Sections 1301, 1302, 1303 and 1304; provided, however, that this provision
     does not apply to any shares with respect to which there exists any
     restriction on transfer imposed by the corporation or by any law or
     regulation; and provided, further, that this provision does not apply to
     any class of shares described in subparagraph (A) or (B) if demands for
     payment are filed with respect to 5 percent or more of the outstanding
     shares of that class.
 
          (2) Which were outstanding on the date for the determination of
     shareholders entitled to vote on the reorganization and (A) were not voted
     in favor of the reorganization or, (B) if described in subparagraph (A) or
     (B) of paragraph (1) (without regard to the provisos in that paragraph),
     were voted against the reorganization, or which were held of record on the
     effective date of a short-form merger; provided, however, that subparagraph
     (A) rather than subparagraph (B) of this paragraph applies in any case
     where the approval required by Section 1201 is sought by written consent
     rather than at a meeting.
 
          (3) Which the dissenting shareholder has demanded that the corporation
     purchase at their fair market value, in accordance with Section 1301.
 
          (4) Which the dissenting shareholder has submitted for endorsement, in
     accordance with Section 1302.
 
     (c) As used in this chapter, "dissenting shareholder" means the
recordholder of dissenting shares and includes a transferee of record.
 
     1301 [DISSENTER'S RIGHTS; DEMAND ON CORPORATION FOR PURCHASE OF
SHARES]. -- (a) If, in the case of reorganization, any shareholders of a
corporation have a right under Section 1300, subject to compliance with
paragraphs
 
                                       D-1
<PAGE>   172
 
(3) and (4) of subdivision (b) thereof, to require the corporation to purchase
their shares for cash, such corporation shall mail to each such shareholder a
notice of the approval of the reorganization by its outstanding shares (Section
152) within 10 days after the date of such approval, accompanied by a copy of
Sections 1300, 1302, 1303, 1304 and this section, a statement of the price
determined by the corporation to represent the fair market value of the
dissenting shares, and a brief description of the procedure to be followed if
the shareholder desires to exercise the shareholder's right under such sections.
The statement of price constitutes an offer by the corporation to purchase at
the price stated any dissenting shares as defined in subdivision (b) of Section
1300, unless they lose their status as dissenting shares under Section 1309.
 
     (b) Any shareholder who has a right to require the corporation to purchase
the shareholder's shares for cash under Section 1300, subject to compliance with
paragraphs (3) and (4) of subdivision (b) thereof, and who desires the
corporation to purchase such shares shall make written demand upon the
corporation for the purchase of such shares and payment to the shareholder in
cash of their fair market value. The demand is not effective for any purpose
unless it is received by the corporation or any transfer agent thereof (1) in
the case of shares described in clause (i) or (ii) of paragraph (1) of
subdivision (b) of Section 1300 (without regard to the provisos in that
paragraph), not later than the date of the shareholders' meeting to vote upon
the reorganization, or (2) in any other case within 30 days after the date on
which the notice of the approval by the outstanding shares pursuant to
subdivision (a) or the notice pursuant to subdivision (i) of Section 1110 was
mailed to shareholder.
 
     (c) The demand shall state the number and class of the shares held of
record by the shareholder which the shareholder demands that the corporation
purchase and shall contain a statement of what such shareholder claims to be the
fair market value of those shares as of the day before the announcement of the
proposed reorganization or shortform merger. The statement of fair market value
constitutes an offer by the shareholder to sell the shares at such price.
 
     1302 [DISSENTING SHARES, STAMPING OR ENDORSING]. -- Within 30 days after
the date on which notice of the approval by the outstanding shares or the notice
pursuant to subdivision (i) of Section 1110 was mailed to the shareholder, the
shareholder shall submit to the corporation at its principal office or at the
office of any transfer agent thereof, (a) if the shares are certificated
securities, the shareholder's certificates representing any shares which the
shareholder demands that the corporation purchase, to be stamped or endorsed
with a statement that the shares are dissenting shares or to be exchanged for
certificates of appropriate denomination so stamped or endorsed or (b) if the
shares are uncertificated securities, written notice of the number of shares
which the shareholder demands that the corporation purchase. Upon subsequent
transfers of the dissenting shares on the books of the corporation the new
certificates, initial transaction statement, and other written statements issued
therefor shall bear a like statement, together with the name of the original
dissenting holder of the shares.
 
     1303 [DISSENTING SHAREHOLDER ENTITLED TO AGREED PRICE WITH INTEREST; TIME
OF PAYMENT]. -- (a) If the corporation and the shareholder agree that the shares
are dissenting shares and agree upon the price of the shares, the dissenting
shareholder is entitled to the agreed price with interest thereon at the legal
rate on judgments from the date of the agreement. Any agreements fixing the fair
market value of any dissenting shares as between the corporation and the holders
thereof shall be filed with the secretary of the corporation.
 
                                       D-2
<PAGE>   173
 
     (b) Subject to the provisions of Section 1306, payment of the fair market
value of dissenting shares shall be made within 30 days after the amount thereof
has been agreed or within 30 days after any statutory or contractual conditions
to the reorganization are satisfied, whichever is later, and in the case of
certificated securities, subject to surrender of the certificates therefor,
unless provided otherwise by agreement.
 
     1304 [DISSENTERS ACTIONS; JOINDER; CONSOLIDATION; APPOINTMENT OF
APPRAISERS]. -- (a) If the corporation denies that the shares are dissenting
shares, or the corporation and the shareholder fail to agree upon the fair
market values of the shares, then the shareholder demanding purchase of such
shares as dissenting shares or any interested corporation, within six months
after the date on which notice of the approval by the outstanding shares
(Section 152) or notice pursuant to subdivision (i) of Section 1110 was mailed
to the shareholder, but not thereafter, may file a complaint in the superior
court of the proper county praying the court to determine whether the shares are
dissenting shares or the fair market value of the dissenting shares or both or
may intervene in any action pending on such a complaint.
 
     (b) Two or more dissenting shareholders may join as plaintiffs or be joined
as defendants in any such action and two or more such actions may be
consolidated.
 
     (c) On the trial of the action, the court shall determine the issues. If
the status of the shares as dissenting shares is in issue, the court shall first
determine that issue. If the fair market value of the dissenting shares is in
issue, the court shall determine, or shall appoint one or more impartial
appraisers to determine, the fair market value of the shares.
 
     1305 [APPRAISERS DUTY AND REPORT; COURT JUDGEMENT; PAYMENT; APPEAL; COSTS
OF ACTION]. -- (a) If the court appoints an appraiser or appraisers, they shall
proceed forthwith to determine the fair market value per share. Within the time
fixed by the court, the appraisers, or a majority of them, shall make and file a
report in the office of the clerk of the court. Thereupon, on the motion of any
party, the report shall be submitted to the court and considered on such
evidence as the court considers relevant. If the court finds the report
reasonable, the court may confirm it.
 
     (b) If a majority of the appraisers appointed fail to make and file a
report within 10 days from the date of their appointment or within such further
time as may be allowed by the court or the report is not confirmed by the court,
the court shall determine the fair market value of the dissenting shares.
 
     (c) Subject to the provisions of Section 1306, judgment shall be rendered
against the corporation for payment of an amount equal to the fair market value
of each dissenting share multiplied by the number of dissenting shares which any
dissenting shareholder who is a party, or who has intervened, is entitled to
require the corporation to purchase, with interest thereon at the legal rate
from the date on which judgment was entered.
 
     (d) Any such judgment shall be payable forthwith with respect to
uncertificated securities and, with respect to certificated securities, only
upon the endorsement and delivery to the corporation of the certificates for the
shares described in the judgment. Any party may appeal from the judgment.
 
     (e) The costs of the action, including reasonable compensation to the
appraisers to be fixed by the court, shall be assessed or apportioned as the
court considers equitable, but, if the appraisal exceeds the price offered by
the corporation, the corporation shall pay the costs (including in the
discretion of the court attorneys' fees, fees of expert witnesses and interest
at the legal rate on judgments from the date of compliance with Sections 1300,
 
                                       D-3
<PAGE>   174
 
1301 and 1302 if the value awarded by the court for the shares is more than 125
percent of the price offered by the corporation under subdivision (a) of Section
1301).
 
     1306 [DISSENTING SHAREHOLDERS: EFFECT OF PREVENTION OF PAYMENT OF FAIR
MARKET VALUE]. -- To the extent that the provisions of Chapter 5 prevent the
payment to any holders of dissenting shares of their fair market value, they
shall become creditors of the corporation for the amount thereof together with
interest at the legal rate on judgments until the date of payment, but
subordinate to all other creditors in any liquidation proceeding, such debt to
be payable when permissible under the provisions of Chapter 5.
 
     1307 [DISSENTING SHARES, DISPOSITION OF DIVIDENDS]. -- Cash dividends
declared and paid by the corporation upon the dissenting shares after the date
of approval of the reorganization by the outstanding shares (Section 152) and
prior to payment for the shares by the corporation shall be credited against the
total amount to be paid by the corporation therefor.
 
     1308 [DISSENTING SHARES, RIGHTS AND PRIVILEGES]. -- Except as expressly
limited in this chapter, holders of dissenting shares continue to have all the
rights and privileges incident to their shares, until the fair market value of
their share is agreed upon or determined. A dissenting shareholder may not
withdraw a demand for payment unless the corporation consents thereto.
 
     1309 [DISSENTING SHARES, LOSS OF STATUS]. -- Dissenting shares lose their
status as dissenting shares and the holders thereof cease to be dissenting
shareholders and cease to be entitled to require the corporation to purchase
their shares upon the happening of any of the following:
 
     (a) The corporation abandons the reorganization. Upon abandonment of the
reorganization, the corporation shall pay on demand to any dissenting
shareholder who has initiated proceedings in good faith under this chapter all
necessary expenses incurred in such proceedings and reasonable attorneys' fees.
 
     (b) The shares are transferred prior to their submission for endorsement in
accordance with Section 1302 or are surrendered for conversion into shares of
another class in accordance with the articles.
 
     (c) The dissenting shareholder and the corporation do not agree upon the
status of the shares as dissenting shares or upon the purchase price of the
shares, and neither files a complaint or intervenes in a pending action as
provided in Section 1304, within six months after the date on which notice of
the approval by the outstanding shares or notice pursuant to subdivision (i) of
Section 1110 was mailed to the shareholder.
 
     (d) The dissenting shareholder, with the consent of the corporation,
withdraws the shareholder's demand for purchase of the dissenting shares.
 
     1310 [SUSPENSION OF CERTAIN PROCEEDINGS WHILE LITIGATION IS PENDING]. -- If
litigation is instituted to test the sufficiency or regularity of the votes of
the shareholders in authorizing a reorganization, any proceedings under Sections
1304 and 1305 shall be suspended until final determination of such litigation.
 
     1311 [CHAPTER INAPPLICABLE TO CERTAIN CLASSES OF SHARES]. -- This chapter,
except Section 1312, does not apply to classes of shares whose terms and
provisions specifically set forth the amount to be paid in respect to such
shares in the event of a reorganization or merger.
 
                                       D-4
<PAGE>   175
 
     1312 [VALIDITY OF REORGANIZATION OR SHORT FORM MERGER, ATTACK ON;
SHAREHOLDERS' RIGHTS; BURDEN OF PROOF]. -- (a) No shareholder of a corporation
who has a right under this chapter to demand payment of cash for the shares held
by the shareholder shall have any right at law or in equity to attack the
validity of the reorganization or short-form merger, or to have the
reorganization or short-form merger set aside or rescinded, except in an action
to test whether the number of shares required to authorize or approve the
reorganization have been legally voted in favor thereof; but any holder of
shares of a class whose terms and provisions specifically set forth the amount
to be paid in respect to them in the event of a reorganization or short-form
merger is entitled to payment in accordance with those terms and provisions or,
if the principal terms of the reorganization are approved pursuant to
subdivision (b) of Section 1202, is entitled to payment in accordance with the
terms and provisions of the approved reorganization.
 
     (b) If one of the parties to a reorganization or short-form merger is
directly or indirectly controlled by, or under common control with, another
party to the reorganization or short-form merger, subdivision (a) shall not
apply to any shareholder of such party who has not demanded payment for cash for
such shareholder's shares pursuant to this chapter; but if the shareholder
institutes any action to attack the validity of the reorganization of short-form
merger or to have the reorganization or short-form merger set aside or
rescinded, the shareholder shall not thereafter have any right to demand payment
of cash for the shareholder's shares pursuant to this chapter. The court in any
action attacking the validity of the reorganization or short-form merger or to
have the reorganization or short-form merger set aside or rescinded shall not
restrain or enjoin the consummation of the transaction except upon 10 days,
prior notice to the corporation and upon a determination by the court that
clearly no other remedy will adequately protect the complaining shareholder or
the class of shareholders of which such shareholder is a member.
 
     (c) If one of the parties to a reorganization or short-form merger is
directly or indirectly controlled by, or under common control with, another
party to the reorganization or short-form merger, in any action to attack the
validity of the reorganization or short-form merger or to have the
reorganization or short-form merger set aside or rescinded, (1) a party to a
reorganization or short-form merger which controls another party to the
reorganization or short-form merger shall have the burden of proving that the
transaction is just and reasonable as to the shareholders of the controlled
party, and (2) a person who controls two or more parties to a reorganization
shall have the burden of proving that the transaction is just and reasonable as
to the shareholders of any party so controlled.
 
                                       D-5
<PAGE>   176
 
                                    PART II
                   INFORMATION NOT REQUIRED IN THE PROSPECTUS
 
ITEM 20.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.
 
     The Registrant is incorporated under the laws of Delaware. Section 145 of
the Delaware General Corporation Law authorizes indemnification of directors and
officers of a Delaware corporation under certain circumstances against expenses,
judgments and the like in connection with an action, suit or proceeding. Article
X of the Registrant's Amended and Restated Bylaws provides for indemnification
of directors and officers under certain circumstances. The Registrant has
purchased a standard liability policy, which, subject to any limitations set
forth in the policy, indemnifies the Registrant's directors and officers for
damages that they become legally obligated to pay as a result of any negligent
act, error or omission committed while serving in their official capacity.
 
     Banque Nationale de Paris, the holder of 100% of the Class A Common Stock
of the Registrant, has agreed to indemnify the persons who serve as Class A
directors of the Registrant (who are elected by the holders of the Class A
Common Stock) under certain circumstances against, judgments and the like in
connection with an action, suit or proceeding by reason of the fact that the
person is or was a Class A director.
 
ITEM 21.  EXHIBITS AND FINANCIAL STATEMENT SCHEDULES.
 
     (a) Exhibits.
 
<TABLE>
<CAPTION>
EXHIBIT
NUMBER                            DESCRIPTION                           PAGE
- -------                           -----------                           ----
<C>       <S>                                                           <C>
  2.1     Agreement and Plan of Merger, dated as of February 25, 1999,
          as amended by the First Amendment dated as of March 24,
          1999, among BancWest Corporation, Bank of the West and
          SierraWest Bancorp, included as Appendix A to the Proxy
          Statement-Prospectus included in this Registration Statement
  3.1     Certificate of Incorporation of BancWest Corporation
          (formerly known as First Hawaiian, Inc.) (incorporated by
          reference to Exhibit 3(i) of BancWest Corporation's Current
          Report on Form 8-K filed with the SEC on November 5, 1998)
  3.2     Amended and Restated Bylaws of BancWest Corporation
          (formerly known as First Hawaiian, Inc.) (incorporated by
          reference to Exhibit 3(ii) of BancWest Corporation's Current
          Report on Form 8-K filed with the SEC on November 5, 1998)
  4.1     Standstill and Governance Agreement between First Hawaiian,
          Inc. and Banque Nationale de Paris, dated as of November 1,
          1998 (incorporated by reference to BancWest Corporation's
          Current Report on Form 8-K filed with the SEC on November 5,
          1998)
  5.1     Opinion of Simpson Thacher & Bartlett as to the legality of
          the securities being registered
  8.1     Opinion of Pillsbury Madison & Sutro LLP as to tax matters
  8.2     Opinion of McCutchen, Doyle, Brown & Enersen LLP as to tax
          matters
</TABLE>
 
                                      II-1
<PAGE>   177
 
<TABLE>
<CAPTION>
EXHIBIT
NUMBER                            DESCRIPTION                           PAGE
- -------                           -----------                           ----
<C>       <S>                                                           <C>
 23.1     Consent of NationsBanc Montgomery Securities LLC, included
          in Appendix C to the Proxy Statement-Prospectus included in
          this Registration Statement
 23.2     Consent of Simpson Thacher & Bartlett, included in the
          opinion filed as Exhibit 5.1
 23.3     Consent of Pillsbury Madison & Sutro LLP, included in the
          opinion filed as Exhibit 8.1
 23.4     Consent of McCutchen, Doyle, Brown & Enersen LLP, included
          in the opinion filed as Exhibit 8.2
 23.5     Consent of PricewaterhouseCoopers LLP (BancWest
          Corporation's independent accountants)
 23.6     Consent of Deloitte & Touche LLP (SierraWest Corporation's
          independent accountants)
 23.7     Consent of PricewaterhouseCoopers LLP (Old BancWest
          Corporation's independent accountants) (included in Exhibit
          23.6)
 23.8     Consent of Deloitte & Touche LLP (Old BancWest Corporation's
          independent accountants)
 24.1     Powers of Attorney
 99.1     Stock Option Agreement, dated as of February 25, 1999 by and
          between SierraWest Bancorp and BancWest Corporation,
          included as Appendix B to the Proxy Statement-Prospectus
          included in this Registration Statement
 99.2     Form of Proxy for Annual Meeting of Shareholders of
          SierraWest Bancorp
 99.3     Opinion of NationsBanc Montgomery Securities LLC, included
          as Appendix C to the Proxy Statement-Prospectus included in
          this Registration Statement
</TABLE>
 
     (b) Financial Statement Schedules.
 
ITEM 22.  UNDERTAKINGS.
 
     (a) The undersigned Registrant hereby undertakes:
 
     (1) To file, during any period in which offers or sales are being made, a
         post-effective amendment to this registration statement:
 
        (i)  To include any prospectus required by Section 10(a)(3) of the
             Securities Act of 1933;
 
        (ii)  To reflect in the prospectus any fact or events arising after the
              effective date of the registration statement (or the most recent
              post-effective amendment thereof) which, individually or in the
              aggregate, represent a fundamental change on the information set
              forth in the registration statement. Notwithstanding the
              foregoing, any increase or decrease in volume of securities
              offered (if the total dollar value of securities offered would not
              exceed that which was registered) and any deviation from the low
              or high end of the estimated maximum offering range may be
              reflected in the form of prospectus filed with the Commission
              pursuant to Rule 424(b) if, in the aggregate, the changes in
              volume and price represent no more than
 
                                      II-2
<PAGE>   178
 
              20 percent change in the maximum aggregate offering price set
              forth in the "Calculation of Registration Fee" table in the
              effective registration statement.
 
        (iii) To include any material with respect to the plan of distribution
              not previously disclosed in the registration statement or any
              material change to such information in the registration statement.
 
     (2) That, for the purpose of determining any liability under Securities Act
         of 1933, each such post-effective amendment shall be deemed to be a new
         registration statement relating to the securities offered therein, and
         the offering of such securities at that time shall be deemed to be the
         initial bona fide offering thereof.
 
     (3) To remove from registration by means of a post-effective amendment any
         of the securities being registered which remain unsold at the
         termination of the offering.
 
     (b) The undersigned Registrant hereby undertakes that, for the purposes of
determining any liability under the Securities Act of 1933, each filing of the
Registrant's annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable, each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the registration statement shall be
deemed to be a new registration statement relating to the securities offered
therein, and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.
 
     (c) The undersigned Registrant hereby undertakes as follows: that prior to
any public reoffering of the securities registered hereunder through use of a
prospectus which is part of this registration statement, by any person or party
who is deemed to be an underwriter within the meaning of Rule 145(c), the issuer
undertakes that such reoffering prospectus will contain the information called
for by the applicable registration form with respect to reofferings by persons
who may be deemed underwriters, in addition to the information called for by the
other items of the applicable form.
 
     (d) The Registrant undertakes that every prospectus: (i) that is filed
pursuant to paragraph (c) immediately preceding, or (ii) that purports to meet
the requirements of Section 10(a)(3) of the Act and is used in connection with
an offering of securities subject to Rule 415, will be filed as a part of an
amendment to the registration statement and will not be used until such
amendment is effective, and that, for purposes of determining any liability
under the Securities Act of 1933, each such post-effective amendment shall be
deemed to be a new registration statement relating to the securities offered
therein, and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.
 
     (e) Insofar as indemnification for liabilities arising under Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
Registrant pursuant to the foregoing provisions, or otherwise, the Registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as expressed in the Act and is,
therefore, unenforceable. In the event that a claim for indemnification against
such liabilities (other than payment by the Registrant of successful defense of
any action, suit or proceeding) is asserted by such director, officer or
controlling person in connection with the securities being registered, the
Registrant will, unless in the opinion of its counsel the matter has been
settled by controlling precedent, submit to a court of appropriate jurisdiction
the question whether such indemnification by it is against
 
                                      II-3
<PAGE>   179
 
public policy as expressed in the act and will be governed by the final
adjudication of such issue.
 
     (f) The undersigned Registrant hereby undertakes to respond to request for
information that is incorporated by reference into the prospectus pursuant to
Item 4, 10(b), 11, or 13 of this form, within one business day of receipt of
such request, and to send the incorporated documents by first class mail or
other equally prompt means. This includes information contained in documents
filed subsequent to the effective date of the registration statement through the
date of responding to the request.
 
     (g) The undersigned Registrant hereby undertakes to supply by means of a
post-effective amendment all information concerning a transaction, and the
company being acquired involved therein, that was not the subject of and
included in the registration statement when it became effective.
 
                                      II-4
<PAGE>   180
 
                                   SIGNATURES
 
     Pursuant to requirements of the Securities Act of 1933, as amended, the
Registrant has duly caused this registration statement to be signed on its
behalf by the undersigned, thereunto duly authorized, in the city and county of
Honolulu, state of Hawaii, on April 14, 1999.
 
                                          BANCWEST CORPORATION
 
                                          By: /s/ HOWARD H. KARR
                                             -----------------------------------
                                              Name: Howard H. Karr
                                              Title: Executive Vice President
                                              and
                                                     Chief Financial Officer
 
     Pursuant to the requirements of the Securities Act of 1933, as amended,
this registration statement has been signed by the following persons in the
capacities and on the dates indicated.
 
<TABLE>
<CAPTION>
                SIGNATURE                               TITLE                    DATE
                ---------                               -----                    ----
<C>                                         <S>                             <C>
 
                    *                       Chairman, Chief Executive       April 14, 1999
- ------------------------------------------    Officer and
           Walter A. Dods, Jr.                Director(Principal Executive
                                              Officer)
 
            /s/ HOWARD H. KARR              Executive Vice President and    April 14, 1999
- ------------------------------------------    Chief Financial Officer
              Howard H. Karr                  (Principal Financial and
                                              Accounting Officer)
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
              Jacques Ardant
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
            John W. A. Buyers
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
          Dr. Julia Ann Frohlich
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
            Robert A. Fuhrman
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
            Paul Mullin Ganley
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
              David M. Haig
</TABLE>
 
                                      II-5
<PAGE>   181
 
<TABLE>
<CAPTION>
                SIGNATURE                               TITLE                    DATE
                ---------                               -----                    ----
<C>                                         <S>                             <C>
                    *                       Director                        April 14, 1999
- ------------------------------------------
               John A. Hoag
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
          Bert T. Kobayashi, Jr.
 
                                            Director
- ------------------------------------------
             Michel Larrouilh
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
           Vivien Levy-Garboua
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
             Yves Martrenchar
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
            Dr. Fujio Matsuda
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
              Don J. McGrath
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
              Rodney R. Peck
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
               Joel Sibrac
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
               John K. Tsui
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
            Jacques Henri Wahl
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
          General Fred C. Weyand
 
                    *                       Director                        April 14, 1999
- ------------------------------------------
               Robert C. Wo
 
          * By Power of Attorney
 
            /s/ HOWARD H. KARR              Attorney-in-Fact
- ------------------------------------------
</TABLE>
 
                                      II-6
<PAGE>   182
 
                                 EXHIBIT INDEX
 
<TABLE>
<CAPTION>
EXHIBIT
NUMBER                            DESCRIPTION                           PAGE
- -------                           -----------                           ----
<C>       <S>                                                           <C>
  2.1     Agreement and Plan of Merger, dated as of February 25, 1999,
          as amended by the First Amendment dated as of March 24,
          1999, among BancWest Corporation, Bank of the West and
          SierraWest Bancorp, included as Appendix A to the Proxy
          Statement-Prospectus included in this Registration Statement
  3.1     Certificate of Incorporation of BancWest Corporation
          (formerly known as First Hawaiian, Inc.) (incorporated by
          reference to Exhibit 3(i) of BancWest Corporation's Current
          Report on Form 8-K filed with the SEC on November 5, 1998)
  3.2     Amended and Restated Bylaws of BancWest Corporation
          (formerly known as First Hawaiian, Inc.) (incorporated by
          reference to Exhibit 3(ii) of BancWest Corporation's Current
          Report on Form 8-K filed with the SEC on November 5, 1998)
  4.1     Standstill and Governance Agreement between First Hawaiian,
          Inc. and Banque Nationale de Paris, dated as of November 1,
          1998 (incorporated by reference to BancWest Corporation's
          Current Report on Form 8-K filed with the SEC on November 5,
          1998)
  5.1     Opinion of Simpson Thacher & Bartlett as to the legality of
          the securities being registered
  8.1     Opinion of Pillsbury Madison & Sutro LLP as to tax matters
  8.2     Opinion of McCutchen, Doyle, Brown & Enersen LLP as to tax
          matters
 23.1     Consent of NationsBanc Montgomery Securities LLC, included
          in Appendix C to the Proxy Statement-Prospectus included in
          this Registration Statement
 23.2     Consent of Simpson Thacher & Bartlett, included in the
          opinion filed as Exhibit 5.1
 23.3     Consent of Pillsbury Madison & Sutro LLP, included in the
          opinion filed as Exhibit 8.1
 23.4     Consent of McCutchen, Doyle, Brown & Enersen LLP, included
          in the opinion filed as Exhibit 8.2
 23.5     Consent of PricewaterhouseCoopers LLP (BancWest
          Corporation's independent accountants)
 23.6     Consent of Deloitte & Touche LLP (SierraWest Corporation's
          independent accountants)
 23.7     Consent of PricewaterhouseCoopers LLP (Old BancWest
          Corporation's independent accountants)
 23.8     Consent of Deloitte & Touche LLP (Old BancWest Corporation's
          independent accountants) (included in Exhibit 23.6)
 24.1     Powers of Attorney
 99.1     Stock Option Agreement, dated as of February 25, 1999 by and
          between SierraWest Bancorp and BancWest Corporation,
          included as Appendix B to the Proxy Statement-Prospectus
          included in this Registration Statement
</TABLE>
<PAGE>   183
 
<TABLE>
<CAPTION>
EXHIBIT
NUMBER                            DESCRIPTION                           PAGE
- -------                           -----------                           ----
<C>       <S>                                                           <C>
 99.2     Form of Proxy for Annual Meeting of Shareholders of
          SierraWest Bancorp
 99.3     Opinion of NationsBanc Montgomery Securities LLC, included
          as Appendix C to the Proxy Statement-Prospectus included in
          this Registration Statement
</TABLE>

<PAGE>   1
                                                                     EXHIBIT 5.1

                   [Letterhead of Simpson Thacher & Bartlett]


                                            April 14, 1999






BancWest Corporation
999 Bishop Street
Honolulu, Hawaii 96813


Ladies and Gentlemen:

         We have acted as counsel to BancWest Corporation, a Delaware
corporation (the "Company"), in connection with the merger (the "Merger") of
SierraWest Corporation ("SierraWest") with and into Bank of the West, a wholly
owned subsidiary of the Company, pursuant to the Agreement and Plan of Merger,
dated as of February 25, 1999, as amended by Amendment No.1, dated March 24,
1999 (the "Merger Agreement") among the Company, Bank of the West and
SierraWest. This opinion letter is furnished to you in connection with a
registration statement on Form S-4 (the "Registration Statement") filed with the
Securities and Exchange Commission (the "Commission") under the Securities Act
of 1933, as amended, for the registration of up to 4,624,004 shares of common
stock, $1.00 par value per share (the "Shares"), of the Company to be issued in
the Merger in accordance with the terms of the Merger Agreement.
<PAGE>   2
BancWest Corporation                   2                          April 14, 1999


         We have examined, and have relied as to matters of fact upon, an
executed copy of the Merger Agreement and of the Registration Statement, and
originals, or duplicates or certified or conformed copies, of such records,
agreements, instruments and other documents and such certificates of public
officials and of officers and representatives of the Company, and have made such
other and further investigations, as we have deemed relevant and necessary in
connection with the opinion expressed herein.

         In such examination, we have assumed the genuineness of all signatures,
the legal capacity of natural persons, the authenticity of all documents
submitted to us as originals, the conformity to original documents of all
documents submitted to us as duplicates or certified or conformed copies, and
the authenticity of the originals of such latter documents.

         Based upon the foregoing, and subject to the qualifications and
limitations stated herein, we are of the opinion that the Shares have been duly
authorized and, when issued in the Merger in accordance with the terms of the
Merger Agreement, will be validly issued, fully paid and nonassessable.

         We hereby consent to the filing of this opinion letter as an exhibit to
the Registration Statement. We also consent to the reference to this firm
appearing in the Registration Statement under the caption "Legal Matters."
<PAGE>   3
BancWest Corporation                   3                          April 14, 1999


         We are members of the Bar of the State of New York, and we do not
express any opinion herein concerning any law other than the Delaware General
Corporation Law.

                                            Very truly yours,


                                            /s/ Simpson Thacher & Bartlett

                                            SIMPSON THACHER & BARTLETT

<PAGE>   1
                                                                     EXHIBIT 8.1



                 [LETTERHEAD OF PILLSBURY MADISON & SUTRO LLP]


                                             April 14, 1999


BancWest Corporation
999 Bishop Street
Honolulu, HI 96813

Ladies and Gentlemen:

     With reference to the Registration Statement on Form S-4 (the 
"Registration Statement") being filed by BancWest Corporation, a Delaware 
corporation ("BancWest"), with the Securities and Exchange Commission in 
connection with the registration under the Securities Act of 1933, as amended, 
of shares of BancWest's common stock to be issued incident to the merger 
described in the Registration Statement (the "Merger") of SierraWest Bancorp, a 
California corporation, with and into Bank of the  West, a California 
state-chartered bank and a wholly owned subsidiary of BancWest, in our opinion 
the discussion under the caption "The Merger-Certain Federal Income Tax 
Consequences" in the Registration Statement sets forth the material United 
States federal income tax considerations generally applicable to the Merger.

     We hereby consent to the filing of this opinion as Exhibit 8.1 to the 
Registration Statement and to the use of our name in the Registration Statement 
and in the Proxy Statement/Prospectus included therein.

                                             Very truly yours,




                                             /s/ Pillsbury Madison & Sutro LLP











<PAGE>   1
                                                                     EXHIBIT 8.2

             [Letterhead of McCutchen, Doyle, Brown & Enersen, LLP]

April 14, 1999




SierraWest Bancorp
10181 Truckee-Tahoe Airport Road
Truckee, California 96160


               MERGER OF SIERRAWEST BANCORP INTO BANK OF THE WEST


Ladies and Gentlemen:

         We have acted as counsel for SierraWest Bancorp, a California
corporation ("SierraWest"), in connection with the merger of SierraWest with and
into Bank of the West, a California state chartered bank and a wholly-owned
subsidiary of BancWest Corporation, a Delaware corporation ("BancWest"),
pursuant to the Agreement and Plan of Merger dated as of February 25, 1999 as
amended (the "Agreement"). This opinion is delivered to you pursuant to Section
6.3(c) of the Agreement. Capitalized terms used in this letter without
definition have the respective meanings given them in the Agreement.

         The Agreement provides that at the Effective Time, SierraWest will be
merged with and into Bank of the West, pursuant to the California Corporations
Code, with Bank of the West as the surviving corporation. In the Merger, each
share of SierraWest common stock will be exchanged for the right to receive 0.82
shares of BancWest common stock. No fractional shares of BancWest common stock
will be issued in the Merger, but SierraWest shareholders who would otherwise be
entitled to receive fractional shares will receive cash in lieu thereof.

         In rendering the opinions expressed in this letter, we have assumed
that (i) the transactions described in the Agreement will be carried out in all
respects as provided therein; (ii) Bank of the West is a wholly-owned first tier
subsidiary of BancWest; (iii) Bank of the West will acquire substantially all of
the properties of SierraWest in the Merger and Bank of the West has no plan or
intention to sell or otherwise dispose of any of the assets of SierraWest to be
acquired in the Merger, except for dispositions made in the ordinary course of
business; (iv) SierraWest has not redeemed any of its shares or otherwise
disposed of any
<PAGE>   2
April 14, 1999
Page 2


of its assets in contemplation of the Merger, except for dispositions in the
ordinary course of its business; (v) Bank of the West has no plan or intention
to issue additional shares of its stock and BancWest has no plan or intention to
dispose of stock of Bank of the West in a transaction or series of transactions
that would result in BancWest losing control of Bank of the West, within the
meaning of Section 368(c) of the Internal Revenue Code of 1986, as amended (the
"Code"); and (vi) neither BancWest nor any corporation related (within the
meaning of Treasury Regulation section 1.368-1(e)(3)) to BancWest will acquire
any of the BancWest stock to be received in the Merger.

         Based upon our understanding of the transaction as described above and
the above assumptions, and upon existing statutes, regulations, court decisions
and published rulings of the Internal Revenue Service, it is our opinion that,
for Federal income tax purposes:

         1.   The merger of SierraWest into Bank of the West and the issuance of
BancWest common stock in the transaction as described in the Agreement will
qualify as a reorganization under Sections 368(a)(1)(A) and 368(a)(2)(D) of the
Code.

         2.   BancWest and SierraWest will each be a party to that 
reorganization within the meaning of Section 368(b) of the Code.

         3.   No gain or loss will be recognized by holders of SierraWest stock
on the exchange of SierraWest stock for BancWest common stock, except to the
extent gain is recognized with respect to cash received in lieu of fractional
shares.

         4.   The aggregate adjusted tax basis of the shares of BancWest stock
received by a SierraWest shareholder in the Merger (including any fractional
share prior to its conversion into cash) will be the same as the aggregate
adjusted tax basis of the SierraWest shares surrendered in exchange therefor.

         5.   The holding period of BancWest stock received in exchange for
SierraWest stock (including any fractional share prior to its conversion into
cash) will include the holding period of the SierraWest stock for which it is
exchanged, assuming that the shares of SierraWest stock are capital assets in
the hands of the holder at the Effective Time.

         6.   Cash received by a SierraWest shareholder in lieu of a fractional
share of BancWest common stock will, to the extent such share was a capital
asset in the hands of the SierraWest shareholder, result in recognition of
capital gain or loss by such shareholder
<PAGE>   3
April 14, 1999
Page 3



measured by the difference between the amount received and the adjusted tax
basis of such fractional share.

            We hereby consent to the filing of this opinion as an exhibit to the
BancWest Registration Statement on Form S-4 and the reference to the name of our
firm therein and under the captions "Certain Federal Income Tax Consequences"
and "Legal Matters" in the Proxy Statement/Prospectus furnished in connection
with the solicitation of proxies by the Board of Directors of SierraWest.


                                                  Very truly yours,

                                       McCUTCHEN, DOYLE, BROWN & ENERSEN, LLP


                                            By   /s/ Roger D. Ehlers 
                                                 A Member of the Firm

<PAGE>   1
 
                                                                    EXHIBIT 23.5
 
                       CONSENT OF INDEPENDENT ACCOUNTANTS
 
     We hereby consent to the incorporation by reference in the Prospectus
constituting part of this Registration Statement on Form S-4 of BancWest
Corporation (formerly First Hawaiian, Inc.), to be filed on or about April 14,
1999, of our report dated January 21, 1999, appearing on page 50 of BancWest
Corporation's 1998 Annual Report to Shareholders, which is incorporated by
reference in its Annual Report on Form 10-K for the year ended December 31,
1998. We also consent to the reference to our firm under the caption "Experts"
in such Prospectus.
 
                                          /s/ PricewaterhouseCoopers LLP
 
Honolulu, Hawaii
April 14, 1999
 
                                                

<PAGE>   1
 
                                                                    EXHIBIT 23.6
 
                         INDEPENDENT AUDITORS' CONSENT
 
     We consent to the incorporation by reference in this Registration Statement
of BancWest Corporation on Form S-4 of our report dated January 29, 1999,
appearing in the Annual Report on Form 10-K of SierraWest Bancorp for the year
ended December 31, 1998 and of our report dated January 19, 1996, relating to
the financial statements of BancWest Corporation and its subsidiaries appearing
in the current report on Form 8-K/A of BancWest Corporation dated November 1,
1998.
 
     We also consent to the reference to us under the heading "Experts" in the
Prospectus/Proxy Statement, which is part of this Registration Statement.
 
                                              /s/ DELOITTE & TOUCHE LLP
 
Sacramento, California
San Francisco, California
April 14, 1999
 
                                        

<PAGE>   1
 
                                                                    EXHIBIT 23.7
 
                       CONSENT OF INDEPENDENT ACCOUNTANTS
 
     We hereby consent to the incorporation by reference in the Prospectus
constituting part of this Registration Statement on Form S-4 of BancWest
Corporation (formerly First Hawaiian, Inc.), to be filed on or about April 14,
1999, of our report dated January 20, 1998 relating to the financial statements
of BancWest Corporation (old BancWest), which appears as Exhibit 99.1 of Form
8-K/A dated November 1, 1998. We also consent to the reference to our firm under
the caption "Experts" in such Prospectus.
 
                                              /s/ PRICEWATERHOUSECOOPERS LLP
 
Honolulu, Hawaii
April 14, 1999
 
                                        

<PAGE>   1
 
                                                                    EXHIBIT 24.1
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                /s/ WALTER A. DODS, JR.
                                          --------------------------------------
                                          Walter A. Dods, Jr.
                                          Chairman, Chief Executive Officer
                                            and Director (Principal Executive
                                          Officer)
<PAGE>   2
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                  /s/ HOWARD H. KARR
                                          --------------------------------------
                                          Howard H. Karr
                                          Executive Vice President
                                            and Chief Financial Officer
                                            (Principal Financing and
                                            Accounting Officer)
<PAGE>   3
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                  /s/ JACQUES ARDANT
                                          --------------------------------------
                                          Jacques Ardant
                                          Director
<PAGE>   4
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                 /s/ JOHN W.A. BUYERS
                                          --------------------------------------
                                          John W.A. Buyers
                                          Director
<PAGE>   5
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                /s/ JULIA ANN FROHLICH
                                          --------------------------------------
                                          Julia Ann Frohlich
                                          Director
<PAGE>   6
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                /s/ PAUL MULLIN GANLEY
                                          --------------------------------------
                                          Paul Mullin Ganley
                                          Director
<PAGE>   7
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                   /s/ DAVID M. HAIG
                                          --------------------------------------
                                          David M. Haig
                                          Director
<PAGE>   8
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                   /s/ JOHN A. HOAG
                                          --------------------------------------
                                          John A. Hoag
                                          Director
<PAGE>   9
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                 /s/ ROBERT A. FUHRMAN
                                          --------------------------------------
                                          Robert A. Fuhrman
                                          Director
<PAGE>   10
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                              /s/ BERT T. KOBAYASHI, JR.
                                          --------------------------------------
                                          Bert T. Kobayashi, Jr.
                                          Director
<PAGE>   11
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                          /s/     VIVIEN LEVY-GARBOUA
 
                                          --------------------------------------
                                          Vivien Levy-Garboua
                                          Director
<PAGE>   12
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                 /s/ YVES MARTRENCHAR
 
                                          --------------------------------------
                                          Yves Martrenchar
                                          Director
<PAGE>   13
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                   /s/ FUJIO MATSUDA
 
                                          --------------------------------------
                                          Fujio Matsuda
                                          Director
<PAGE>   14
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                  /s/ DON J. MCGRATH
 
                                          --------------------------------------
                                          Don J. McGrath
                                          Director
<PAGE>   15
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                  /s/ RODNEY R. PECK
 
                                          --------------------------------------
                                          Rodney R. Peck
                                          Director
<PAGE>   16
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                    /s/ JOEL SIBRAC
                                          --------------------------------------
                                          Joel Sibrac
                                          Director
<PAGE>   17
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                   /s/ JOHN K. TSUI
                                          --------------------------------------
                                          John K. Tsui
                                          Director
<PAGE>   18
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                /s/ JACQUES HENRI WAHL
                                          --------------------------------------
                                          Jacques Henri Wahl
                                          Director
<PAGE>   19
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                  /s/ FRED C. WEYAND
                                          --------------------------------------
                                          Fred C. Weyand
                                          Director
<PAGE>   20
 
                               POWER OF ATTORNEY
 
     KNOW ALL MEN BY THESE PRESENTS, that the undersigned, being an officer or
director, or both, of BANCWEST CORPORATION (the "Issuer"), in its capacity as
set forth below, hereby constitutes and appoints WALTER A. DODS, JR. and HOWARD
H. KARR, of Honolulu, Hawaii, and DON J. MCGRATH, of San Francisco, California,
and each of them, his true and lawful attorney and agent, to do any and all acts
and all things and to execute any and all instruments which said attorney and
agent may deem necessary or desirable to enable the Issuer to comply with the
Securities Act of 1933, as amended (the "Act"), and any rules, regulations and
requirements of the Securities and Exchange Commission thereunder, in connection
with the registration and requirements of the Securities and Exchange Commission
thereunder, in connection with the registration under the Act of shares of
common stock of the Issuer (the "Shares"), including, without limitation, the
power and authority to sign the name of the undersigned in the capacity
indicated below to the Registration Statement on Form S-4 to be filed with the
Securities and Exchange Commission with respect to such Shares, to any and all
amendments or supplements to such Registration Statement, whether such
amendments or supplements are filed before or after the effective date of such
Registration Statement, to any related Registration Statement filed pursuant to
Rule 462 under the Act, and to any and all instruments or documents filed as
part of or in connection with such Registration Statement or any and all
amendments thereto, whether such amendments are filed before or after the
effective date of such Registration Statement; and the undersigned hereby
ratifies and confirms all that such attorney and agent shall do or cause to be
done by virtue hereof.
 
Date: April 14, 1999
 
                                                   /s/ ROBERT C. WO
                                          --------------------------------------
                                          Robert C. Wo
                                          Director

<PAGE>   1
                                                                    EXHIBIT 99.2


                               FORM OF PROXY CARD

                               SIERRAWEST BANCORP
      PROXY FOR THE ANNUAL MEETING OF SHAREHOLDERS TO BE HELD MAY 27, 1999
           THIS PROXY IS SOLICITED ON BEHALF OF THE BOARD OF DIRECTORS

                                 REVOCABLE PROXY


         The undersigned holder of common stock acknowledges receipt of the
Notice of Annual Meeting of Shareholders of SierraWest Bancorp, a California
corporation ("SierraWest"), and the accompanying Proxy Statement-Prospectus
dated April __, 1999, and revoking any proxy heretofore given, hereby
constitutes and appoints David W. Clark, Jerrold T. Henley, William W.
McClintock and Thomas M. Watson, and each of them, with full power of
substitution, as attorney and proxy to appear and vote all of the shares of
common stock of SierraWest standing in the name of the undersigned which the
undersigned could vote if personally present and acting at the Annual Meeting of
the Shareholders of SierraWest to be held at the North Lake Tahoe Conference
Center, 8318 North Boulevard, Kings Beach, California, on May 27, 1999, at 4:00
p.m. local time or at any adjournments thereof, upon the following items as set
forth in the Notice of Meeting and more fully described in the Proxy
Statement-Prospectus.

         1. PROPOSAL ONE: THE MERGER. To approve the Agreement and Plan of
Merger dated as of February 25, 1999, as amended, among BancWest Corporation,
Bank of the West and SierraWest, providing for the merger of both SierraWest and
its wholly-owned subsidiary, SierraWest Bank, with and into Bank of the West.
The Agreement and Plan of Merger, as amended, is set forth in its entirety as
Appendix A to the accompanying Proxy Statement-Prospectus.

_____ FOR              _____ AGAINST             _____ ABSTAIN

         2. ELECTION OF THIRTEEN DIRECTORS. To elect the following persons as
directors of SierraWest to serve until the merger is completed or until the next
Annual Meeting: David W. Clark, Ralph J. Coppola, William T. Fike, Richard S.
Gaston, Jerrold T. Henley, John J. Johnson, Ronald A. Johnson, A. Morgan Jones,
Jack V. Leonesio, William W. McClintock, Bernard E. Moore, Gary E. Stein, Thomas
M. Watson.

  _____ FOR (EXCEPT AS MARKED TO THE CONTRARY BELOW) _____ AGAINST_____ ABSTAIN

INSTRUCTION: To withhold authority to vote for any individual nominee strike
nominee's name or write that nominee's name in the following space:
___________________________________

         3. OTHER BUSINESS: The proxies are authorized to vote in their
discretion on such other business as may properly come before the meeting or any
adjournments or postponements thereof.

                       _____ FOR_____ AGAINST_____ ABSTAIN

      THIS PROXY IS SOLICITED BY, AND ON BEHALF OF, THE BOARD OF DIRECTORS
                   AND MAY BE REVOKED PRIOR TO ITS EXERCISE.
<PAGE>   2
   THE BOARD OF DIRECTORS RECOMMENDS A VOTE "FOR" EACH OF THE PROPOSALS. THIS
 PROXY, WHEN PROPERLY EXECUTED AND RETURNED TO SIERRAWEST, WILL BE VOTED IN THE
MANNER DIRECTED. IF NO DIRECTION IS MADE, THIS PROXY WILL BE VOTED "FOR" EACH OF
  THE PROPOSALS. IF OTHER BUSINESS IS PRESENTED, THIS PROXY SHALL BE VOTED IN
            ACCORDANCE WITH THE BEST JUDGMENT OF THE PROXY HOLDERS.


                                 SHAREHOLDER(S)

                       __________________________________
                                   (Signature)

                       __________________________________
                                   (Signature)

                       __________________________________
                             (No. of Common Shares)

                       Date________________________, 1999
           I/We do____ or do not ____ expect to attend this meeting.



    Please sign exactly as your name(s) appear(s). When signing as attorney,
  executor, administrator, trustee, officer, partner, or guardian, please give
 full title. If more than one trustee, all should sign. WHETHER OR NOT YOU PLAN
    TO ATTEND THIS MEETING, PLEASE SIGN AND RETURN THIS PROXY AS PROMPTLY AS
                  POSSIBLE IN THE ENCLOSED POST-PAID ENVELOPE.

   To assure a quorum, you are urged to date and sign this Proxy and mail it
   promptly in the enclosed envelope, which requires no additional postage if
                     mailed in the United States or Canada.


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