During the summer once the sun goes down, the water flowing over the Falls is reduced from the daylight flow of 100,000 cubic feet per second to 50,000 - allowing hydro plants on both sides of the border to draw more water for hydro generation. Both the New York State Power Authority and Hydro One pump this extra water into gigantic reservoirs to be used at their call.
Once the cold weather sets in (November to April) even more water is diverted from going over the Falls. An additional 50,000 cubic feet per second is diverted for power generation allowing only one-quarter of the water that could go over Niagara Falls to do so. In the early 1950s when this was decided, authorities justified the diversion by saying there are fewer visitors to the Falls during this time. However, today an argument could be made that this isn’t quite the case.