Cal. Gov. Code § 25704

The board of supervisors may enter into contracts with cities within the county, water districts within the county of any type or kind, sanitation districts within the county, the State of California, or the Government of the United States, under such terms as may be mutually advantageous, for the acquisition or construction, or both, of the works authorized by this article to be acquired or constructed. Each of the parties to such contract may contribute to the cost of such acquisition, or construction, or both, such sums of money as may be therein agreed upon.