The owners of sawmills on the Mississippi River and the Saint Croix River in the States of Wisconsin and Minnesota are authorized and empowered under the direction of the Secretary of the Army, to construct piers or cribs in front of their mill property on the banks of the river, for the protection of their mills and rafts against damage by floods and ice: , That the piers or cribs so constructed shall not interfere with or obstruct the navigation of the river. And in case any pier or crib constructed under authority of this section shall at any time, and for any cause, be found to obstruct the navigation of the river, the Government expressly reserves the right to remove or direct the removal of it, at the cost and expense of the owners thereof.Provided, however