Cal. Gov. Code § 25682
In connection with flood control work done by the county or by any district therein or agency thereof, highways, bridges, and other public works affected thereby or which will be of public benefit, whether located in the county or wholly or partially in incorporated or unincorporated territory outside the county, may be constructed, reconstructed, remodeled, maintained, repaired, or demolished at the expense of the county doing the flood control work, or in which any district or agency thereof is doing the work, or at the joint expense of the county and the county or city in which the work is done, as may be provided by agreement between the board of supervisors of the county doing the flood control work, or in which any district or agency thereof is doing the work, and the board of supervisors or other legislative body of the county or city in which any of the work is wholly or partially performed.
Ca. Gov. Code § 25682