That, in order to further the declaration of policy and principles declared in sections 701a and 701b of this title, and to supplement the preliminary examinations and surveys which the Secretary of the Army has heretofore been, or is hereafter, authorized and directed to make of waterways with a view to the control of their floods, the Secretary of Agriculture be, and he is, authorized and directed to cause preliminary examinations and surveys to be made for run-off and water-flow retardation and soil-erosion prevention on the watersheds of said waterways, the costs thereof to be paid from appropriations heretofore or hereafter made for such purposes.