California Code, Water Code - WAT § 30065

The inclusion in, or annexation or addition to, a county water district, of all or any part of the corporate area of any public corporation or public agency, shall not destroy the identity or legal existence or impair the powers of any such public corporation or public agency, notwithstanding the identity of purpose, or substantial identity of purpose, of such county water district.

No public corporation or public agency having identity of purpose or substantial identity of purpose shall be formed partly or entirely within a county water district existing under the provisions of this code without the consent of such county water district.  Nothing in this section shall be construed to prohibit any city from annexing territory within a county water district without the consent of such county water district.

All county water districts heretofore formed partly or entirely within another county water district previously existing under the provisions of this code, with or without the consent of such previously existing county water district and for a purpose not then actually exercised, or to provide a service not then provided by said previously existing county water district, are hereby declared to be valid and legally existing districts.