Deglaciation and Formation of a Knickpoint: Niagara Falls

Location

Continent: North America
Country: United States and Canada
State/Province: New York and Ontario
City/Town: Niagara Falls
UTM coordinates and datum: 4771505 657028 17T

Setting

Climate Setting:
Tectonic setting:
Type:

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Niagara falls. Details

Great lakes profile. Details

Map of river escarpment. Details

Stratigraphic column. Details

Description

Niagara Falls is a stunning site. There are three separate waterfalls: the large Horseshoe Falls, the American Falls, and the Bridal Veil Falls. All three of the waterfalls begin at the same height; the three separate waterfalls are simply a result of two islands in the Niagara River. The Horseshoe Falls is the largest of the three waterfalls, standing about 52 m high and stretching about 792 m across. The average flow rate upstream of the waterfalls is 5,720 m3/s. The actual amount of water flowing over the falls is highly regulated. The 1950 Niagara Treaty between the U.S. and Canada regulates flow in the river to be 2,832 m³/s during tourist hours and 1,416 m³/s during the non-tourist hours and during the winter low tourist season. The rest of the flow is diverted to hydroelectric power facilities, generating 2.4 million kilowatts of electricity. The Falls attract about 20 million tourists a year, and legendary stories of people riding over the Falls in a barrel, or just in a life jacket, are part of American folklore. But what caused the formation of this natural wonder?.

Niagara Falls are part of the drainage system of the Great Lakes, and formation of the Great Lakes is closely linked to formation of the Falls. During the Wisconsin Glaciation, or the last ice age, a continental glacier extended south from what is now Canada into the Northern United States. The ice was up to 4 kilometers thick, and as it moved, it carved deep valleys and basins, including the depressions in which the Great Lakes are now contained. Deglaciation of the Great Lakes area started about 14,000 years ago. As the glaciers melted, freshwater filled in the large glacially carved basins, forming the Great Lakes. The Great Lakes were originally larger and did not reach their current drainage pattern - Lake Superior, to Lakes Michigan and Huron, to Lake Erie, to Lake Ontario, and then out to the Ocean through the St. Lawrence River — until somewhere between 5,000 to 3,000 years ago. Even though Lake Erie was already draining into Lake Ontario via the Niagara River before the current drainage pattern developed, flow through the Niagara River was much smaller until the Upper Great Lakes started to drain into Lake Erie. The Great Lakes are a natural wonder themselves, as they contain 20% of the freshwater on the surface of the Earth.

The actual waterfall, or knickpoint, started to form in the Niagara River when the Niagara Escarpment was exposed, some 12,000 years ago. The Niagara Escarpment is a cliff face, or cuesta, which runs westward from New York State through Ontario, Michigan, Wisconsin, and into Illinois. The top layer of rock in the Escarpment is the strong Lockport Dolomite, composed of limestone and dolostone. Below the Lockport Dolomite, are layers of shale and sandstone, with some smaller layers of limestone interbedded. These layers dip gently away from the Escarpment, but the Escarpment itself is a steep cliff face. The Escarpment was not formed by a tectonic event, rather millions of years of erosion produced the cliff. Wind, freezing and thawing, and running water act to weather and erode away more rapidly at the weak layers below the Lockport Dolomite. Eventually, the top layer is undermined by more rapid erosion below and large blocks break off the top layer, causing the Escarpment to retreat. This process has been occurring for millions of years, and deglaciation exposed the Escarpment that was already in place.

In many ways, retreat of a waterfall, or knickpoint, is similar to escarpment retreat. However, focused flow of a river causes weathering of weaker layers and knickpoint retreat to occur more rapidly than escarpment retreat. That is why Niagara Falls has retreated over 11 km upstream from the Niagara Escarpment. Retreat rates of Niagara Falls have varied through time, depending on the amount of flow through the Niagara River, which in turn depends on the drainage of the Upper Great Lakes. The Great Lakes play another important role in the evolution of the Falls. The lakes act as a sediment trap, so the Niagara River has very little sediment to deposit, and can not bury the Falls.

The fastest historical rates of retreat of the Falls were over 1 m/yr. Today, however, erosion rates are less than 0.3 m/yr. The reduction is mainly due to flow control measures. Human infrastructure surrounding the Falls depends on little to no erosion. Eventually, if left uncontrolled, the Falls would erode back all the way to Lake Erie.

Associated References:

• Philbrick, S. S., 1970, Horizontal Configuration and the Rate of Erosion of Niagara Falls: Geological Society of America Bulletin, v. 81, p. 3723-3732.
• Tinkler, K. J., Pengelly, J. W., Parkins, W. G., and Asselin, G., 1994, POSTGLACIAL RECESSION OF NIAGARA-FALLS IN RELATION TO THE GREAT-LAKES: Quaternary Research, v. 42, no. 1, p. 20-29.

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