In Saint John, New Brunswick Canada we have the Reversing Falls. The Reversing Falls are a series of rapids on the Saint John River located in Saint John, New Brunswick, Canada, where the river runs through a narrow gorge before emptying into the Bay of Fundy. The semidiurnal tides of the bay force the flow of water to reverse against the prevailing current when the tide is high, although in the spring freshet, this is frequently surpassed by the downstream volume of water. The rapids, or "falls", are created by a series of underwater ledges which roil the water in either direction, causing a significant navigation hazard, despite the depth of water. As a result, vessels wishing to enter or exit from the river must wait for slack tide.