Wash. Rev. Code § 35.39.034
Moneys thus determined available for this purpose may be invested on an individual fund basis or may, unless otherwise restricted by law be commingled within one common investment portfolio for investment. All income derived from such investment shall be apportioned and used for the benefit of the various participating funds or for the benefit of the general or current expense fund as the governing body of the city of [or] town shall determine by ordinance or resolution: PROVIDED, That funds derived from the sale of general obligation bonds or revenue bonds or similar instruments of indebtedness shall be invested, or used in such manner as the initiating ordinances, resolutions, or bond covenants may lawfully prescribe.
Any excess or inactive funds on hand in the city treasury not otherwise invested, or required to be invested by this section, as now or hereafter amended, may be invested by the city treasurer in United States government bonds, notes, bills, certificates of indebtedness, or interim financing warrants of a local improvement district which is within the protection of the local improvement guaranty fund law for the benefit of the general or current expense fund.
All previous or outstanding investments of city or town funds for the benefit of the city's or town's general or current expense fund which have been or could be made in accordance with the provisions of this section, as now or hereafter amended, are declared valid.
RCW 35.39.034