The simple answer is no BUT the water flowing over the American Falls and Canadian Horseshoe Falls is greatly reduced for power purposes.
After it gets dark each evening during the busy tourist season the water flowing over the falls is reduced to 50,000 cubic feet per second from the 100,000 cubic feet per second that flows over during daylight hours. This allows hydro plants on both sides of the border to draw more water for hydro generation.
During the night time, both the New York State Power Authority and Hydro One pump this extra water into gigantic reservoirs to be used at their call.
There is even more water diverted from going over the falls between November 1 and April 1. That’s when an additional 50,000 cubic feet per second is diverted allowing only one-quarter of the water that could go over Niagara Falls to do so. Authorities justify this diversion saying there are fewer visitors to the falls between these times. An argument could perhaps be made today that this isn’t quite the case.
Albeit, fewer visitors do come to Niagara Falls during the cooler season it has become a year-round destination then when these rates of diversion were set in the early 1950’s.