Niagara Falls is divided into the Horseshoe Falls and the American Falls . The Horseshoe Falls drop about 173 feet (53 m). The American Falls drop about 70 feet (21 m) before reaching a jumble of fallen rocks that were deposited by a massive rock slide in 1954. The larger Horseshoe Falls are about 2,600 feet (792 m) wide, while the American Falls are 1,060 feet (323 m) wide. The volume of water approaching the falls during peak flow season is 202,000 cubic feet per second (5,720 m³/s).

View of Niagara Falls , showing parts of Canada and the United States

By comparison Africa's spectacular Victoria Falls has over 15 million cubic feet (424,750 m³) of water falling over its crest line each minute during the peak of the wet season (250,000 cu ft/7,079 m³ per second). Since the flow is a direct function of the Lake Erie water elevation, it typically peaks in late spring or early summer. During the summer months, 100,000 cubic feet per second (2,832 m³/s) of water actually traverses the Falls, some 90% of which goes over the Horseshoe Falls, while the balance is diverted to hydroelectric facilities. This is accomplished by employing a weir with movable gates upstream from the Horseshoe Falls . The Falls flow is further halved at night, and during the low tourist season in the winter, remains a flat 50,000 cubic feet per second (1,416 m³/s). Water diversion is regulated by the 1950 Niagara Treaty and is administered by the International Niagara Board of Control (IJC).

Viewpoints on the American shore generally are astride or behind the falls. The falls face directly toward the Canadian shore. Thus, the most complete views of Niagara Falls are available from the Canadian shoreline.

The features that became the Niagara Falls were created by the Wisconsin glaciation, about 10,000 years ago. The same forces also created the North American Great Lakes and the Niagara River . All were dug by a continental ice sheet that drove through the area like a giant bulldozer, deepening some river channels to make lakes and damming others with debris. Scientists believe that there is an old valley, buried by glacial drift, at the approximate location of the present Welland Canal .

When the ice melted, the upper Great Lakes emptied into the Niagara River , which followed the rearranged topography across the Niagara Escarpment. In time, the river cut a gorge through the north facing cliff or cuesta.

The unusual rock formations did not erode evenly because of the interactions of three major rock formations.[citation needed] One rock formation was composed of erosion-resistant limestone and Lockport dolostone. That hard layer of stone eroded more slowly than underlying materials. The aerial photo clearly shows the hard caprock, the Lockport Formation (Middle Silurian), which underlies the rapids above the falls and approximately the upper third of the high gorge wall.

Immediately below the hard-rock formation, comprising about two thirds of the cliff, lay a weaker, softer, sloping Rochester Formation (Lower Silurian). The formation was composed mainly of shale, though it has some thin limestone layers. It also contains ancient fossils. In time, the river eroded the soft layer that supported the hard layers, undercutting the hard cap rock. Eventually the process carved out the falls.

Submerged in the river in the lower valley, hidden from view, is the Queenston Formation (Upper Ordovician), which is composed of shales and fine sandstones. All three formations were laid down in an ancient sea, and their differences of character derive from changing conditions within that sea.

The original Niagara Falls were near the sites of present-day Queenston , Ontario , and Lewiston , New York , but erosion of their crest has caused the waterfalls to retreat several miles southward. Just upstream from the Falls' current location, Goat Island splits the course of the Niagara River , resulting in the separation of the Canadian Horseshoe Falls to the west from the American and Bridal Veil Falls to the east. Although engineering has slowed erosion and recession in this century, the falls will eventually recede far enough to drain most of Lake Erie , the bottom of which is higher than the bottom of the falls. Engineers are working to reduce the rate of erosion to postpone this event as long as possible.