Cal. Pub. Util. Code § 3108

If a vacancy on the board of directors occurs except by expiration of a term, the remaining members of the board, by a majority vote, shall fill the vacancy unless the bylaws provide for an election of directors by districts. If the bylaws provide for an election of directors by districts, the vacancy shall be filled by the election of a director from the district in which the vacancy occurs or the board may call a special meeting of the members in that district to fill the vacancy.