One of the most famous natural landmarks in North America, Niagara Falls is a relatively recent addition to our country's topography. As the Wisconsin Glacier plowed into the Upper Midwest, 60,000 years ago, it scooped out the broad basins of the Great Lakes. And, as the last Ice Sheet melted back into Canada, some 13,000 years ago, these basins began to fill with meltwater, expanding well beyond their current size. Eventually, governed by glacial deposits and a rebounding crust, the Lakes drained northeastward, through the St. Lawrence Seaway.
The Niagara River, which connects Lakes Erie and Ontario, meadered across soft, glacial sediments when it first formed; by 12,500 years ago, its channel encountered the edge of the hard, Lockport Dolomite, the uppermost layer of the Niagara Escarpment. This resistent bedrock, initially deposited in a Silurian sea (some 420 million years ago) abutted softer sediments to its north; these younger rocks were more easily eroded by the increasingly powerful river and the falls began to form. Since that time, Niagara Falls has gradually cut its way upstream, leaving a 7 mile canyon it its wake.
Today, the Niagara River drops 212 feet, from the upper rapids to the base of the Falls; more than 90% of its flow (which peaks at 100,000 cubic feet per second in the summer) cascades over Horseshoe Falls, which has a sheer drop of 170 feet and a width of 2600 feet. An equal flow is diverted through the Niagara Hydro-electric Power plants, first constructed in the 1890's; the diversion is increased at night to maximize power output. Less than 10% of the river flow tumbles over American Falls, which has a clear drop of 70 feet, broken by slump blocks of dolomite lower down the canyon wall.
The diversion of 50% or more of the Niagara River flow has greatly slowed the upstream movement of the Falls; prior to this engineering, the Falls were moving southward (upriver) about 5 feet per year. Despite human efforts to slow this process, the Falls will eventually cut back into Lake Erie, draining it down to a river channel.
