Niagara Falls

Long ago, thick ice covered this land.

When the ice melted, the water filled big lakes. A river flowed from one lake to another. Where it fell over a cliff, Niagara Falls began.

About 18,000 years ago, during the last Ice Age, sheets of ice two to three kilometers thick covered this part of North America. As the ice moved, it dug out the basins of the Great Lakes. When it melted, it filled them with water.

About 12,500 years ago the land around Niagara was free of ice, and meltwater began to flow from Lake Erie to Lake Ontario. Where it poured over a steep slope called the escarpment, at Queenston and Lewiston, the first Niagara Falls began. Since then the falls have worn their way back about 11 kilometers (seven miles) upstream, leaving the deep Niagara Gorge behind them.

Niagara Parks describes the river and the whole Great Lakes Basin as a legacy of the last Ice Age: 18,000 years ago southern Ontario lay under ice two to three kilometers thick, which gouged out the lake basins and then filled them with meltwater. The Niagara Peninsula became ice-free about 12,500 years ago, and of five original spillways from Lake Erie to Lake Ontario, one survived: the first falls, at the escarpment at Queenston-Lewiston.

About 10,500 years ago, ice re-advances and isostatic rebound rerouted the meltwater through northern Ontario; for some 5,000 years Lake Erie was half its present size, the river carried about 10 percent of its present flow, and the reduced falls stalled near the Niagara Glen. Full flow returned about 5,500 years ago. Since its birth the falls have receded about seven miles, carving the 11 km Niagara Gorge through rock layers laid down as sediments in tropical, saltwater seas some 400 to 440 million years ago.

The falls are slowly moving backward up the river.

Water, spray and ice wear away the rock. Sometimes big pieces of rock break off and fall. Today the falls move much more slowly than before.

At the top of the falls is a layer of hard rock, dolostone and limestone. Under it are softer rocks called shales and sandstones. Frost from the spray, the spray itself and fallen boulders grinding against the soft shale all wear the rock away, and from time to time big pieces of rock fall. Bit by bit, the edge of the falls moves back.

Niagara Parks estimates that for at least the last 560 years the falls moved back about 1 to 1.5 meters a year. Today much of the river's water is taken away to make electricity and the flow is controlled, so the falls wear back far more slowly: about 30 centimeters (1 foot) a year by Niagara Parks' estimate, or 8 to 15 centimeters (3 to 6 inches) by New York State Parks'.

Erosion at Niagara acts on a resistant cap of Lockport Group dolostones and limestones over less resistant shales and sandstones such as the Rochester Shale, as New York State Parks describes the escarpment. Niagara Parks lists frost action from the spray, the dissolving action of the spray and abrasion of the softer shales by fallen limestone boulders, plus cavitation at the base, where collapsing bubbles send shock waves into the rock. It adds that the crest retreats relatively rapidly when it develops a notch, until a new stable position is reached.

Niagara Parks estimates recession over at least the last 560 years at 1 to 1.5 m a year; with flow control and hydroelectric diversion it puts the current rate at about 1 foot a year, possibly falling to 1 foot per 10 years, while noting that the current rate is unclear. New York State Parks gives 3 to 6 feet a year in the past and 3 to 6 inches a year now. Niagara Parks reports that scientists speculate the American Falls, which carried less than 7 percent of the flow even before diversion, could dry up in perhaps 2,000 years, while the Horseshoe Falls may notch back about four more miles over 15,000 years until it reaches soft Salina shale.

The river near the falls is very fast and very deep.

Never go into the water. Stay on the marked paths. Take only pictures, and leave rocks and plants where they are.

Niagara Parks reminds visitors that the river is beautiful but fast, deep and very dangerous, and swimming is not allowed. Even calm-looking spots near the shore can have a strong undertow, an invisible current under the surface. The Whirlpool Rapids in the gorge move at more than 40 kilometers per hour.

Stay on marked trails, which keep a safe distance from the water; hiking on closed or unmarked trails is dangerous. Wear sturdy shoes, carry water, and remember that every step down into the gorge is a step you must climb back up. Leave no trace: take only pictures, leave only footsteps, and leave rocks and plants where you found them.

Niagara Parks' trail etiquette asks visitors to stay on sanctioned, marked trails, which are a safe distance from the water, and warns that hiking on closed or unmarked trails can result in serious injury; in 2021 it said the Niagara Parks Police were patrolling the Glen and the Whirlpool Trail to enforce park regulations. Swimming is not permitted: the Whirlpool Rapids are rated Class 6, move at over 40 km/h, and even calm-looking shorelines have a strong undertow.

The Glen trails are rated moderate and the Whirlpool Trail difficult. Visitors are asked to wear supportive footwear, drink about one liter of water for every two hours of hiking, know their limits and avoid the jagged, uneven shoreline. Leave-no-trace rules ask people not to touch cultural and historic structures or artifacts and to leave rocks, plants and other natural objects as they found them.