Cal. Pub. Contract Code § 21151
All contracts for any improvement or unit of work when the cost according to the estimate of the engineer will exceed twenty-five thousand dollars ($25,000), with or without the furnishing of materials or supplies, shall be let to the lowest responsible bidder or bidders in the manner provided in this article. Construction of improvements or works shall not be staged to avoid the bidding process. The board shall first determine whether the contract shall be let as a single unit for the whole of the work, or shall be divided into severable parts, or both, according to the best interests of the district. The board shall call for bids and advertise the call by inviting sealed proposals for the construction or performance of the improvement or work before any contract is made. The board shall invite the bids by publishing a notice of the call for bids pursuant to Section 6062 or 6066 of the Government Code in a newspaper of general circulation in the county. The call for bids shall state whether the work is to be performed as a unit or shall be divided into severable specific parts, or both, as stated in the call. The board may let the work by single contract or it may divide the work into severable parts by separate contracts, as stated in the call, according to the best interests of the district. The board shall require the successful bidder or bidders 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 to contain the terms and conditions set forth in Title 3 (commencing with Section 9000) of Part 6 of Division 4 of the Civil Code and to be subject to the provisions of that title. The board shall also have the right to reject any bid. In the event the engineer's estimate is twenty-five thousand dollars ($25,000) or less, or in the event no proposals are received pursuant to advertisement therefor, or in the event the work consists of channel protection or maintenance work, or emergency work, the board of supervisors may, without advertising for bids, have the work done by day labor under the direction of the board, by contract, or by a combination of the two. 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 acquire in the open market without advertising for bids, materials, equipment, and supplies for use in any work or for any other purpose.
Ca. Pub. Cont. Code § 21151