Niagara Falls Can Be Turned Off And On!!
Niagara Falls, a set of three waterfalls lying across the United States-Canadian border, can indeed be turned off and on! Sometimes, for construction and repair work around the falls for bridges and oil rigs or for conducting geological survey of the rock face of the falls, the falls are switched off. This is done by building coffer dams (made of welded steel) across the falls to reduce the water flow to the tiniest trickle possible.
In fact, every night, the hydro control dam (jointly owned by Canada/USA and operated by the Ontario Hydro) diverts water to reservoirs for hydro-generation, reducing the water flow rate over the Falls to a mere 50,000 cubic feet per second and re-diverting it back to the Falls during the day.