Colo. Rev. Stat. § 37-48-128
When it is determined to let the work by contract, contracts in amounts in excess of ten thousand dollars shall be advertised after notice by publication calling for bids, and the board may reject any or all bids or may let the contract to the lowest responsible bidder who gives a good and approved bond with ample security, conditioned on the carrying out of the contract. Such contract shall be in writing and shall be accompanied by or shall refer to plans and specifications for the work to be done prepared by the chief engineer. Said contract shall be approved by the board of directors and signed by the president of the district and by the contractor and shall be executed in duplicate; but, in case of sudden emergency when it is necessary in order to protect the district, the advertising of contracts may be waived upon the unanimous consent of the board of directors; but the provisions of this section shall not apply if it is determined by the board of directors that the work be done on force account.
C.R.S. § 37-48-128