Any person or persons, corporations, municipal or private, who desire to improve any navigable river, or any part thereof, at their or its own expense and risk may do so upon the approval of the plans and specifications of said proposed improvement by the Secretary of the Army and Chief of Engineers of the Army. The plan of said improvement must conform with the general plan of the Government improvements, must not impede navigation, and no toll shall be imposed on account thereof, and said improvement shall at all times be under the control and supervision of the Secretary of the Army and Chief of Engineers.