From April through October, during tourist season, the water flows at a rate of 100,000 cubic feet per second. Canadian and American governments made an agreement that the flow would stay at this rate throughout the tourist season. The Falls have an estimated 12 million visitors each year, and the agreement between the Ontario Hydro and the New York Power Authority allows the flow to remain at that rate while allowing them to draw a reduced amount of water so that it does not interfere with the amount of water going over the Falls.
In order to satisfy the needs of all parties, at dusk each night, during the tourist season, the flow rate is reduced to 50,000 cubic feet per second. This allows the Hydro Authority and New York State Power Authority to fill up their massive reservoirs. During the daytime when the flow is increased over the Falls, the Power Authorities draw water from their reservoirs to make up for any deficit.
From November to April the rate of flow is reduced to 50,000 cubic feet per second, 24 hours a day. The Hydro plants draw the remainder for hydro generation. This reduction in water flow is why the Maid of the Mist has to be out of the water by October 31st of each year.
The control dam is owned jointly by Canada and the USA. By agreement, it is operated by Ontario Hydro. It was located on the Canadian side due to the fact that the river bed slants toward the Canadian side. Each side has specific water drawing times, these are strictly regulated. When the American side is drawing water, the gates on the dam are closed causing the water to be forced to the water intakes on the American side. The dam also assists in drawing the water to the intakes on the Canadian side when they are drawing water.
The Niagara River flows at a rate of 212,000 cubic feet per second. Even on the hottest summer day, an estimated 100,000 cubic feet of water per second is being drawn for hydro generation.