Wash. Rev. Code § 43.09.285
Whenever by law, two or more municipal corporations or political subdivisions of the state are permitted by law to engage in a joint operation, the funds of such joint operation shall be deposited in the public treasury of the municipal corporation or political subdivision embracing the largest population or the public treasury of any other as so agreed upon by the parties; and such deposit shall be subject to the same audit and fiscal controls as the public treasury where the funds are so deposited: PROVIDED, That whenever the laws applicable to any particular joint operation specifically state a contrary rule for deposits, the specific rule shall apply in lieu of the provisions of this section: PROVIDED, FURTHER, That nothing contained herein shall be construed as limiting the power or authority of the disbursing officer of such joint operation from making disbursements in accordance with the provisions of any contract or agreement entered into between the parties to the joint operation.
RCW 43.09.285