Cal. Gov. Code § 25681

The board may appropriate and expend money from the general or other appropriate funds of the county for the construction of works, improvements, levees, or check dams to prevent the overflow and flooding of streams and rivers in the county, and may construct works, improvements, levees, or check dams outside the county for such purposes upon channels, streams, or rivers which flow or lie in or through more than one county, or where any of such work is reasonably necessary for the control of flood waters in the county, although the channel, river, or stream does not lie or flow in or through two or more counties.