Except as otherwise provided in Chapters 1 and 2 of this part, the Reclamation Board, in conjunction with the War Department, shall execute the plans and projects referred to in Section 12657 and exercise all powers granted to it in Part 4, Division 5, of this code. The Reclamation Board may make such modifications and amendments to the plans as may be necessary to execute them for the purposes stated in Chapters 1 and 2 of this part.