Public Resources Code § 9203

In the case of a district which contains only unincorporated territory in more than one county, the district board may be elected or may be appointed by the boards of supervisors of the counties in which the district is located. If the district board is appointed by the boards of supervisors, they shall appoint directors according to the proportionate share of population of that portion of each county within the district, provided that each board of supervisors shall appoint at least one director.