Cal. Civ. Code § 1009
(a) The Legislature finds that:
(f) No use, subsequent to the effective date of this section, by the public of property described in subdivision (e) shall constitute evidence or be admissible as evidence that the public or any governmental body or unit has any right in such property by implied dedication if the owner does any of the following actions:
(3) Enters into a written agreement with any federal, state, or local agency providing for the public use of such land.
After taking any of the actions set forth in paragraph (1), (2), or (3), and during the time such action is effective, the owner shall not prevent any public use which is appropriate under the permission granted pursuant to such paragraphs by physical obstruction, notice, or otherwise.