Wash. Rev. Code § 85.08.840
The boards of county commissioners of the counties in which a joint drainage improvement district is situated shall have jurisdiction in joint session to hear, supervise, and conduct the merger proceedings relating to such a district. The auditor of the county in which the greater length of the system of improvements lies shall act as clerk of the joint sessions of the boards of county commissioners, and shall give the notice provided for in RCW 85.08.870. He or she shall furnish to the auditor of the other county duplicate copies of the records of proceedings of the joint sessions. Duplicate records of all proceedings had and papers filed in connection with the merger of a joint drainage improvement district shall be kept with the auditor of each county. The board of county commissioners of the county in which a drainage improvement district or consolidated drainage improvement district is situated shall have exclusive jurisdiction to hear, supervise, and conduct merger proceedings relating to such districts.
RCW 85.08.840