Cal. Gov. Code § 25684

The board of supervisors may provide by ordinance for the organization and government of districts: (a) to protect and preserve the banks of rivers and streams and lands lying contiguous thereto from injury by overflow or the washing thereof, (b) to provide for the improvement of such rivers and streams, and (c) to prevent their obstruction. The board may provide for the assessment, levy, and collection within such districts of a tax for the purposes of this section.