The board of supervisors of such district shall have full power and authority to build, construct, excavate and complete all or any works and improvements which may be needed to carry out, maintain and protect "the plan for reclamation." To accomplish that end such board of supervisors is hereby authorized and empowered to employ men and teams and to purchase machinery, employ men to operate the same and directly have charge of and construct the works and improvements, or by the use of other or more efficient means than provided for in the plans adopted. They may, in their discretion, let contracts for such works and improvements, either as a whole or in sections, and when such contract or contracts are to be let, they shall be advertised and let to the lowest and best bidder, who shall give a good and approved bond, in an amount equal to the contract price, with ample security, conditioned that the contractor will well and promptly carry out the contract for such work and improvements. The contract shall be in writing, and there shall be attached thereto and made a part thereof complete plans and specifications of the work to be done and the improvements to be made under such contract, which plans and specifications shall be prepared by the chief engineer, and before the work is commenced the contract shall be approved by the board of supervisors and signed by the president of the board and the contractor, and shall be executed in duplicate. No supervisor, secretary, treasurer, or engineer of any drainage district shall be awarded any such contract, or have any interest in the profits thereof, either directly or indirectly. The chief engineer shall be the superintendent of construction of all the works and improvements, and shall, whenever required, and at least once each year, make a full report to such board of all work done and improvements made, and make such suggestions and recommendations to the board as he may deem proper.

It shall be the duty of the board of supervisors to have the amount of work performed by any contractor estimated from time to time, as may be desirable, by the chief engineer, and the board shall draw its warrants in favor of the contractor for not more than eighty percent of the amount of work so required, reserving the remainder until it has been ascertained that the work has been completed according to contract, and is free from liens.