California Code, Health and Safety Code - HSC § 40390

The board of supervisors of a county within a regional district may withdraw the county, or portion thereof, from the regional district to form a county district or to join the county, or portion thereof, with a unified district, the bay district, or another regional district upon the adoption of a resolution stating its intention to take such action.

The resolution so adopted shall be communicated to the clerks of the boards of supervisors of all counties comprising the regional district from which the county, or portion thereof, is to be withdrawn, that regional district board, and the state board.