The original Niagara Falls were near the site of present day 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 Horseshoe Falls to the west from American and Bridal Veil Falls to the east. Although erosion and recession have been slowed in this century by engineering, 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 retard this event as long as possible.

The Falls drop about 170 feet (52 m), although the American Falls have a clear drop of only 70 feet (21 m) before reaching a jumble of fallen rocks which were deposited by a massive rock slide in 1954. The larger Canadian 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 ft³/s (5,720 m³/s).1,2 During the summer months, when maximum diversion of water for hydroelectric power occurs, 100,000 ft³/s (2,832 m³/s) of water actually traverses the Falls, some 90% of which goes over Horseshoe Falls. This volume is further halved at night, when most of the diversion to hydroelectric facilities occurs.