33 U.S.C. § 543
Employment of civil engineers on western and northwestern rivers
The Chief of Engineers may, with the approval of the Secretary of the Army, employ such civil engineers, not exceeding five in number, for the purpose of executing the surveys and improvements of western and northwestern rivers, ordered by Congress, as may be necessary to the proper and diligent prosecution of the same; and the persons so employed may be allowed a reasonable compensation for their services, not to exceed the sum of $3,000 a year.