California Code, Public Resources Code - PRC § 30313

(a) Vacancies that occur shall be filled within 30 days after the occurrence of the vacancy, and shall be filled in the same manner in which the vacating member was selected or appointed.

(b) The executive director of the commission shall notify the appropriate appointing authority of any expected vacancies on the commission.  If the expected vacancy is a person qualified for membership because he or she holds a specified office as a locally elected official, whose term of office as a locally elected official is expected to expire or has expired, then the appointing authority shall notify the boards of supervisors and city selection committees of each county within the affected region of the expected vacancy.