California Code, Streets and Highways Code - SHC § 8130.1

Upon the completion of the formation of the district, if the district is comprised of two or more cities and unincorporated territory, the board shall appoint a commission composed of five members.  The number of commissioners shall be so divided as to relate the number of commissioners from each unit as closely as possible to the proportion which the assessed valuation of that unit, as shown on the last equalized assessment roll of the county, bears to the total assessed valuation within the district at the time the district is formed, except that each unit shall have at least one commissioner.  For the purposes of this section, all of the unincorporated area in the district shall be considered a single unit.

If the district is comprised of two or more cities, without unincorporated territory, the appointment of commissioners shall be by the city councils of the respective cities, each such council appointing the number determined by the board as provided above.