Public Resources Code § 5502
ARTICLE 3. Regional Park, Park and Open-Space, and Open-Space Districts
(a) Three or more cities, together with any parcel or parcels of city or county territory, whether in the same or different counties, may organize and incorporate. All the territory in the proposed district shall be contiguous.
(b) Notwithstanding subdivision (a), one or more cities, together with any parcel or parcels of city or county territory, whether in the same or different counties, the territory of all of which when combined has a population of at least 50,000, may organize and incorporate. All the territory in the proposed district shall be contiguous.