That any railroad or railway company having a right of way crossing any of the land so appropriated, shall not be deprived of an easement for the use of such right of way, but shall be permitted to retain the right it now has for the purpose of crossing or running along, upon or over such levee: Provided, however, That such use and occupancy shall not interfere with the widening of such stream and shall not become an obstruction to the flow of the water therein, and shall not interfere with the height or otherwise impair the usefulness of the levees to be constructed for the purpose of preventing overflow.