Cal. Water Code § 31142.36

If the public nuisance is not abated within the time specified by the district in the notice, the district may abate the public nuisance. Any entry upon private property by the district for this purpose shall be preceded by written notice to the record owner of the property stating the date and place of entry and that the purpose of entry is to abate the public nuisance. If the mailed notice is returned undelivered, the district shall post a copy of the notice at the proposed entry point of the property at least five days prior to entry. Thereafter, the district may take all actions necessary to abate the public nuisance.