Four of the five Great Lakes (Superior, Michigan, Huron, and Erie) drain into the Niagara River before emptying into Lake Ontario. These five Great Lakes make up almost one-fifth of the world's fresh water supply.

Yes, the water at Niagara Falls is entirely freshwater. It originates from the Great Lakes system, which is the largest surface freshwater system on Earth. Specifically, the water flows from Lake Erie into the Niagara River, over the Falls, and then into Lake Ontario. From there, it travels through the St. Lawrence River and eventually into the Atlantic Ocean. Because the water is "fossil water" left over from the last ice age, it is extremely clean, though it can appear brown or green due to the high volume of clay and suspended vegetative matter (tannins) stirred up by the immense pressure of the falling water. In 2026, the Niagara River remains a vital source of drinking water for over one million people in both the United States and Canada. While the rocks in the Niagara Gorge were formed in ancient saltwater seas millions of years ago (as evidenced by marine fossils found there), the water cascading over them today is 100% fresh.