The fresh water that plunges over Niagara Falls drains approximately 684,000 square kilometres ( 264,00 square miles) from four Great Lakes-Lakes Superior, Huron, Michigan and Erie.
Once it flows over the falls, it travels along the lower Niagara River another 21.7 km. (13- 1/2 miles) to Lake Ontario. From there, it travels down the St. Lawrence River to the Atlantic Ocean.
Another bit of trivia, One-fifth of the worlds surface fresh water flows down the Niagara River.