The Secretary of the Army is authorized to allot not to exceed $7,500,000 from any appropriations heretofore or hereafter made for any one fiscal year for flood control, for removing accumulated snags and other debris, and clearing and straightening the channel in navigable streams and tributaries thereof, when in the opinion of the Chief of Engineers such work is advisable in the interest of flood control: Provided, That not more than $500,000 shall be expended for this purpose for any single tributary from the appropriations for any one fiscal year.