California Code, Public Utilities Code - PUC § 29503

As an alternative method of annexation, a petition may be presented to the board of supervisors of a county proposed to be annexed, signed by qualified electors of the county, in number to at least ten (10) percent of the total vote cast at the last general election within the county.  The petition shall contain statements and declarations setting forth the reason for annexation.  Such petition may be on separate papers, but each paper shall contain the affidavit of the party who circulated it, certifying that each name signed thereto is the true signature of the person whose name it purports to be.  The clerk of the board of supervisors of such county shall compare the signatures with the affidavits of registration and certify to the board of supervisors the sufficiency or insufficiency of such petition.  If found and certified by the clerk as being sufficient, such petition shall have all the force and effect of a resolution adopted by the board of supervisors of such county.