California Code, Public Contract Code - PCC § 21641

(a) All contracts for the construction of any unit of work, except as provided, estimated to cost in excess of twenty-five thousand dollars ($25,000) shall be let to the lowest responsible bidder in the manner provided in this article.

(b) The board shall advertise by three insertions in a daily newspaper of general circulation or two insertions in a weekly newspaper of general circulation published in the district inviting sealed proposals for the construction of the work before any contract shall be made, and may let by contract separately any part of the work. The board shall require the successful bidder to file with the board good and sufficient bonds to be approved by the board conditioned upon the faithful performance of the contract and upon the payment of the claims for labor and material. The board may reject any bid.

(c) If all proposals are rejected or no proposals are received pursuant to advertisement, or where the estimated cost of the work does not exceed twenty-five thousand dollars ($25,000), or where the work consists of channel protection, maintenance work, or emergency work, the board, by unanimous vote of all members present, may, without advertising for bids, have the work done by force account.In case of an emergency, if notice for bids to let contracts will not be given, the board shall comply with Chapter 2.5 (commencing with Section 22050). The district may purchase in the open market, without advertising for bids, materials and supplies for use in any work either under contract or by force account.

(d) The provisions of this section have no application to a contract entered into with the United States or to a contract authorized by a vote of the eligible voters of the district or a zone.