Cal. Gov. Code § 25581
The board of supervisors of any county may abandon all or any portion of a park dedicated to the county for park purposes or acquired by the county by deed limiting its use to park purposes, and may sell the land comprising it pursuant to this article or use the land for other county purposes, if it finds that all of the park, where all is to be abandoned, or the portion to be abandoned is not being used by the public for park purposes and that all of said park, or the portion to be abandoned if less than all, is not appropriate, convenient or necessary for park purposes.
The transfer of a park or any portion thereof by the county to any city or district of the county, whether by lease or conveyance, shall not be deemed to be an abandonment of park purposes under this article if such lease or conveyance is expressly made subject to the condition that the property transferred be used only for park purposes.
Ca. Gov. Code § 25581