Wash. Rev. Code § 36.83.100
If the county legislative authority establishes a road and bridge service district, it shall promptly appoint three persons who are residents of the territory included in that service district to serve as the commissioners of the service district. For counties having an elected executive, the executive shall appoint those commissioners subject to confirmation by the legislative authority of the county. The commissioners first appointed shall be designated to serve for terms of one, two, and three years, respectively, from the date of their appointment. Thereafter, service district commissioners shall be appointed for a term of office of five years. Vacancies must be filled for any unexpired term in the same manner as the original appointment. No member of the legislative authority of the county in which a service district is created may be a commissioner of that service district, except that, if the boundaries of the service district are included within or coterminous with the boundaries of a county commissioner or council district, the county commissioner or councilmember elected from that commissioner or council district may be appointed to serve as a commissioner of the service district. A commissioner shall hold office until his or her successor has been appointed and qualified, unless sooner removed from office for cause in accordance with this chapter or removed by referendum in accordance with RCW 36.83.110. A certificate of the appointment or reappointment of any commissioner must be filed with the county auditor, and such certificate is conclusive evidence of the due and proper appointment of the commissioner. The commissioners of the service district shall receive no compensation for their services, in any capacity, but are entitled to reimbursement for reasonable and necessary expenses, including travel expenses, incurred in the discharge of their duties.
The powers of each service district are vested in the commissioners of the service district. Two commissioners constitute a quorum of the service district for the purpose of conducting its business and exercising its powers and for all other purposes. The commissioners of the service district shall organize itself and select its chair, vice chair, and secretary, who shall serve one-year terms but may be selected for additional terms. When the office of any officer becomes vacant, the commissioners of the service district shall select a new officer from among the commissioners for the balance of the term of office.
RCW 36.83.100