The county's primary geographic feature is Niagara Falls, the riverbed of which has eroded seven miles south over the past 12,000 years since the last Ice Age. The Niagara River and Niagara Falls, are in effect, the drainage ditch for four of the Great Lakes which constitute the world's largest supply of fresh water. The water flows north from Lake Erie, then through the Niagara River, goes over Niagara Falls, and then on to Lake Ontario and the St. Lawrence River, eventually emptying into the North Atlantic Ocean. Today, tourists and visitors to the Falls see a diminished flow of water over the Falls, since a portion of the flow has been diverted for hydroelectric power purposes. Both the American and Canadian side of the Niagara River have massive electrical power plants.