For the purpose of securing the uninterrupted work of operating snag boats on the Upper Mississippi River, the Illinois River, from its mouth to Copperas Creek, and the Minnesota River and other tributaries of the Upper Mississippi River improved by the United States, the Secretary of the Army, upon the application of the Chief of Engineers, is authorized to draw his warrant or requisition from time to time upon the Secretary of the Treasury for such sums as may be necessary to do such work, not to exceed in the aggregate for each year the sum of $25,000.