Wash. Rev. Code § 87.68.110
Each irrigation district which has or hereafter may enter into a contract with the United States providing for the operation and maintenance, by means of a board of control, of irrigation works used in common with other districts, shall include in the annual levy of assessments a sufficient amount to pay the annual estimated pro rata proportion of the costs chargeable to such district and also such reserve fund as may be fixed by the contract: PROVIDED, That any district may appropriate moneys from other funds to pay said costs.
When assessments are paid to the county treasurer for the board of control fund, they shall be deposited in a special fund, known as the "Board of Control Fund," and when assessments are paid to the county treasurer for the board of control reserve fund they shall be deposited in a special fund known as the "Board of Control Reserve Fund," and said funds may be disbursed only upon vouchers approved by a majority of the voting power of the members of the board of control, and the county auditor shall issue warrants for the payments of such claims which shall be payable out of the funds on which the same are drawn.
Any moneys in the "Board of Control Reserve Fund," when so requested by the board of control, shall be invested by the treasurer of said county and under the direction of said board of control in U.S. bonds or bonds of the state or any bonds pronounced by the treasurer of the state as valid securities for the deposit of public funds.
RCW 87.68.110