The simple answer is no. 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.

No, they do not "stop" the water at Niagara Falls on a regular basis, but the flow is significantly manipulated for power generation. Under an international treaty between the U.S. and Canada, the flow of water is maintained at a higher volume during "prime viewing hours" (typically 8:00 AM to 10:00 PM in peak season) to preserve the natural beauty for tourists. At night, a large portion of the water is diverted through massive intakes to hydroelectric power plants on both sides of the border. This reduces the "curtain" of the falls but never stops it entirely. Historically, the American Falls were "turned off" in 1969 by the U.S. Army Corps of Engineers using a cofferdam to study erosion, and there are occasional plans for temporary shutdowns for bridge repairs. However, for a standard visitor, the water flows 24/7, year-round, including during the winter. While the falls may look "frozen" due to ice build-up on the rocks, the massive volume of water continues to rush underneath the ice bridge.