The board of supervisors of the district may build, construct, excavate and complete any and all works and improvements which may be needed to carry out, maintain and protect the Plan of Reclamation. To accomplish that end the board of supervisors may employ men and teams and purchase machinery, employ men to operate the same and directly have charge of and construct the works and improvements in such manner as, or by the use of other or more efficient means than, is provided for in the plans adopted. It may, in their discretion, let the contract for such works and improvements either as a whole or in sections and when such contract or contracts are let they shall be advertised and let to the lowest and best bidder who shall give a good and approved bond with ample security, conditioned that he will well and promptly carry out the contract for such work and improvements.