Cal. Pub. Contract Code § 21091
All contracts for any improvement or unit of work, except as otherwise provided in this act, estimated to cost in excess of fifteen thousand dollars ($15,000) shall be let to the lowest responsible bidder in the manner provided in this article. The board of supervisors of the district shall advertise by three insertions in a daily newspaper of general circulation or two insertions in a weekly newspaper of general circulation printed and published in the district inviting sealed proposals for the construction of the improvement or work before any contract is made, and may let by contract separately any part of the work or improvement. 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 their claims for labor and material. The bonds shall contain the terms and conditions set forth in Sections 10223 and 10224, and Section 9554 of the Civil Code and be subject to those sections. The board shall also have the right to reject any bid. If all proposals are rejected or no proposals are received pursuant to advertisement, or where the estimated cost of the work does not exceed fifteen thousand dollars ($15,000), or the work consists of channel protection, maintenance work, or emergency work, the board of supervisors 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 improvement or unit of work either under contract or by force account.
Ca. Pub. Cont. Code § 21091