The fresh water that plunges over Niagara takes around 685,000 gallons (2.6 million litres) of water from four great lakes: Lake Superior, Lake Michigan Lake Huron and Lake Erie - in fact, ? of the world's fresh water is found in these four great lakes.

Niagara Falls is fed by the Niagara River, which serves as a massive drain for the four upper Great Lakes: Superior, Michigan, Huron, and Erie. Specifically, the water flows directly from Lake Erie into the Niagara River, heading north toward Lake Ontario. Because the Great Lakes represent about 20% of the world's surface freshwater, the volume of water feeding the falls is immense. During peak daytime hours in 2026, over 6 million cubic feet (approx. 168,000 cubic meters) of water go over the crest line every minute. This flow is naturally regulated by the massive drainage basin of the Great Lakes, but it is also human-controlled; a treaty between the U.S. and Canada allows for water to be diverted into hydroelectric power plants, particularly at night and during the winter, to preserve the "aesthetic" flow during tourist hours.