33 U.S.C. § 701m
In any case where the total authorization for a project heretofore or hereafter authorized by Congress is not sufficient to complete plans that may have been made the Chief of Engineers is authorized in his discretion to plan and make expenditures on preparations for the project, such as the purchase of lands, easements, and rights-of-way; readjustments of roads, railroads, and other utilities; removal of towns, cemeteries, and dwellings from reservoir sites; and the construction of foundations. The Chief of Engineers is also authorized in his discretion to modify the plan for any dam or other work heretofore or hereafter authorized so that such dam or work will be smaller than originally planned with a view to completing a useful improvement within an authorization: Provided, That the smaller structure shall be located on the chosen site so that it will be feasible at some future time to enlarge the work in order to permit the full utilization of the site for all purposes of conservation such as flood control, navigation, reclamation, the development of hydroelectric power, and the abatement of pollution.
33 U.S.C. § 701m