Cal. PCC § 20601

All contracts for construction of a work of improvement for the district in excess of three thousand five hundred dollars ($3,500) shall be let to the lowest responsible bidder, except that contracts under seven thousand five hundred dollars ($7,500) may be let pursuant to informal bidding procedures established by the board, and except that county waterworks districts may contract with each other, with the county, or, where the district is a subsidiary district of a city, with the city, for furnishing labor, materials, or supplies required for any improvement mentioned in this division upon such terms and conditions as their boards may elect. With the approval of the board of supervisors and the board of directors of the district, if any, or, where the district is a subsidiary district of a city, with the approval of the city council, and upon the terms and conditions they may determine, all contracts for construction of a work of improvement for the district, including those under three thousand five hundred dollars ($3,500), may be let by the county or city purchasing agent, as applicable.