The board of supervisors may provide by ordinance for the organization and government of districts for the following purposes:

1. (a) To protect and preserve the banks of rivers and streams and lands lying contiguous thereto from injury by overflow or washing.
2. (b) To provide for the improvement of rivers and streams.
3. (c) To prevent the obstruction of rivers and streams.
4. (d) To assess, levy and collect within each district a tax for the district.