The fresh water that plunges over Niagara drains approximately 684,000 square kilometres (261,000 square feet) of water from four great lakes: Lake Superior, Lake Michigan Lake Huron and Lake Erie - in fact, one-fifth of the world’s fresh water is found in these four great lakes. Once it flows over the Falls it travels from the Niagara River, 21.7 kilometres (13 ½ miles) to Lake Ontario. From there, it makes its way down the St. Lawrence River and eventually to the Atlantic Ocean.
Interestingly enough, there is a drop in elevation between Lake Erie and Lake Ontario of 99 metres (325 feet). That’s the reason why you find the Welland Canal here. The canal includes 8 locks that gradually takes the ships up or down from one lake to the other, and without it ships could not by-pass the Niagara Falls and go freely through the Great Lakes and into the Ocean.