Whenever any bridge shall, in the opinion of the Secretary, at any time unreasonably obstruct such navigation, it shall be the duty of the Secretary, after notice to interested parties, to hold a hearing at which the bridge owner, those interested in water navigation thereunder or therethrough, those interested in either railroad or highway traffic thereover, and any other party or parties in interest shall have full opportunity to offer evidence and be heard as to whether any alteration of such bridge is needed, and if so what alterations are needed, having due regard to the necessity of free and unobstructed water navigation and to the necessities of the rail or highway traffic. If, upon such hearing, the Secretary determines that any alterations of such bridge are necessary in order to render navigation through or under it reasonably free, easy, and unobstructed, having due regard also for the necessities of rail or highway traffic thereover, he shall so find and shall issue and cause to be served upon interested parties an order requiring such alterations of such bridge as he finds to be reasonably necessary for the purposes of navigation.