California Code, Streets and Highways Code - SHC § 1394

(a) Any county may join with any city located within such county in the acquisition, construction or maintenance of any bridge or viaduct within the county, whether such bridge or viaduct is or is to be located within or without any such city.

(b) The expense of the acquisition, construction or maintenance of any such bridge or viaduct shall be borne by such county and such city in such proportion as the legislative bodies thereof determine by resolution or ordinance and may be paid out of any funds available for such purpose.

(c) The authority and responsibility for the acquisition, construction or maintenance of any such bridge or viaduct shall vest in the county or in the city, or in both, as provided in the ordinances or resolutions apportioning the expense.