Federal investigations and improvements of rivers and other waterways for flood control and allied purposes shall be under the jurisdiction of and shall be prosecuted by the Department of the Army under the direction of the Secretary of the Army and supervision of the Chief of Engineers, except as otherwise provided by Act of Congress; and in his reports upon examinations and surveys, the Secretary of the Army shall be guided as to flood-control measures by the principles set forth in section 701a of this title in the determination of the Federal interests involved: Provided, That the foregoing grant of authority shall not interfere with investigations and river improvements incident to reclamation projects that may now be in progress or may be hereafter undertaken by the Bureau of Reclamation of the Interior Department pursuant to any general or specific authorization of law.