Wash. Rev. Code § 85.07.110
It shall be the duty of the commissioners of such district annually to levy assessments sufficient to pay interest on such bonds as they fall due. They may at any time levy such additional assessment as they deem best to redeem and retire such bonds. Commencing not less than five years before the due date of such bonds, they shall determine the number of equal annual levies necessary to retire such bonds at maturity, and annually thereafter levy an assessment sufficient to liquidate all of said bonds by maturity. Such levies for interest and redemption of the bonds shall be added to the annual cost of the maintenance of the diking or drainage system of said district. Such assessments shall be collected by the county treasurer and kept as a special fund for the sole purpose of paying interest upon and liquidating said bonds.
RCW 85.07.110
Liberal construction-Severability-1983 c 167: See RCW 39.46.010 and note following.