EXHIBIT 99.1

CERTIFICATION PURSUANT TO

18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of Columbia Banking System, Inc. (the “Company”) on Form 10-Q for the period ending March 31, 2003 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), we, Melanie J. Dressel, President and Chief Executive Officer, and Gary R. Schminkey, Executive Vice President and Chief Financial Officer, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that:

(1) the Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

(2) the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

Dated: May 13, 2003

/s/    MELANIE J. DRESSEL        

Melanie J. Dressel

President and Chief Executive Officer

/s/    GARY R. SCHMINKEY        

Gary R. Schminkey

Executive Vice President and

Chief Financial Officer