Cal. Health & Safety Code § 13839

In the case of a district which includes only incorporated territory in more than one city, the district board may be elected or appointed by the city councils in which the district is located. If the district board is appointed, the city councils shall appoint directors according to the proportionate share of population of that portion of each city within the district, provided that each city council shall appoint at least one director. The city council may appoint one or more of its own members to the district board. In no case shall the directors exceed the number permitted pursuant to Section 13842.