Cal. Water Code § 34575

If at any time after the formation of a district the boundaries of the principal county are changed so as to exclude therefrom all of the land of the district situated in that county, the records and documents of the district in the possession of the board of supervisors of that county, together with a certified copy of the proceedings had by the district under jurisdiction of that board of supervisors shall be transferred and filed with the clerk of the board of supervisors of the county in which the greater portion of the land of the district is located.