Oregon Revised Statutes Water Resources: Irrigation, Drainage, Flood Control, Reclamation § 547.112

(a) To change the number of supervisors on the board, the board shall notify the county clerk of the county in which the petition to form the district was filed and the Secretary of State at least six months prior to the annual meeting at which one or more supervisors will be added to or removed from the board or within 30 days of taking action under subsection (1) of this section, whichever is later.

(b) To reduce the number of supervisors from five to three, the board shall phase in the change, beginning at the next annual meeting, in a manner that allows each supervisor to serve the full term to which the supervisor was elected. If the decision is made to reduce the number of supervisors from five to three at an annual meeting at which only one supervisor is scheduled to be elected, the election must be canceled and the supervisor whose term is ending shall continue in office until the following annual meeting. At the following annual meeting, the terms of two more supervisors will end, and only one supervisor will be elected.