Wash. Rev. Code § 87.80.090
If the board of county commissioners determine[s] that the creation of a board of joint control is in the public interest, of benefit to the irrigation entities and individual water uses within those entities concerned, and will not be detrimental to water right interests outside the proposed board of joint control area of jurisdiction: Then the county board shall so find and adopt a resolution creating the board of joint control, designating it (name of county) County Joint Control Board No. (specify number), and the county board at the same time shall appoint the first members of the board of joint control based on the board composition proposed in the petition and the board of joint control shall consist of this membership. A copy of the resolution creating the board of joint control certified by the clerk of the county board shall be filed with the county assessor of the county in which the board of joint control was created and with the county assessor in any other county in the state in which any lands involved are situated, within five days after the resolution is adopted.
RCW 87.80.090