Cal. Water Code § 35505

If 75 percent or more of the territory of one or more county sanitation districts is located within the boundaries of a California water district situated in one county, the board of supervisors of the county, or the governing body of the county sanitation district or districts, may, if the board of directors of the California water district gives its consent in writing, delegate the powers and functions of the governing bodies of the county sanitation districts to the board of directors of the California water district, which shall thereafter perform such powers and functions.