Cal. Water Code § 8156

Any county within the limits of which there are drainage improvements within any of the classes mentioned in Section 8155 may, by a four-fifths vote of the members of the board of supervisors, transfer and convey all, but not less than all, of the drainage improvements in the county and within such classes to any flood control district in the county. The flood control district shall thereupon assume sole control and jurisdiction over such drainage improvements.