Niagara Falls is fed by the Great Lakes, which is the world’s largest surface freshwater system, containing about 18% of the world’s freshwater supply. Specifically, the water that plunges over Niagara comes from four of the Great Lakes: Lake Superior, Lake Michigan, Lake Huron, and Lake Erie. Once it travels over the Falls, it travels from the Niagara River, 21.7 kilometers (13 ½ miles) to Lake Ontario, and from there, it makes its way down the St. Lawrence River and eventually to the Atlantic Ocean. Interestingly, there is a drop in elevation between Lake Erie and Lake Ontario of 99 meters (325 feet), which is the reason why the Welland Canal exists. The canal includes 8 locks that gradually take ships up or down from one lake to the other, and without it, ships could not bypass the Niagara Falls and go freely through the Great Lakes and into the Ocean.