Niagara Falls, a colossal natural landmark shared by the United States and Canada, stands as a spectacular example of geological processes and hydrological power. Located on the Niagara River, which connects Lake Erie and Lake Ontario, the falls are not a single cascade but a collection of three: the Horseshoe Falls on the Canadian side, and the American Falls and Bridal Veil Falls on the U.S. side. Its existence is owed to a specific combination of ancient rock layers and recent glacial history.
How Glacial Retreat Created the Falls
The initial formation of the falls is a relatively recent geological event, dating back about 12,000 years to the end of the last major ice age, known as the Wisconsin Glaciation. Massive sheets of ice covered much of the continent, carving out the basins that would become the Great Lakes. As the climate warmed and the glaciers retreated northward, immense amounts of meltwater were released.
This deluge of water found an outlet by flowing over a steep geological feature called the Niagara Escarpment. The meltwater carved the Niagara River channel, beginning the process of waterfall creation near the escarpment’s edge. The initial cataract was significantly taller than the falls today, dropping into Glacial Lake Iroquois, a precursor to Lake Ontario. Since that time, the falls have been slowly eroding their way upstream, moving approximately 11 kilometers (7 miles) to their current position.
The Caprock Mechanism: Why the Falls Persist
The continued existence of the falls is a direct result of the unique rock structure of the Niagara Escarpment, often referred to as the caprock mechanism. This structure consists of a hard, resilient layer of rock overlying softer, more easily eroded layers. The uppermost layer, the caprock, is primarily composed of erosion-resistant dolomite and limestone from the Lockport Formation.
Beneath this tough caprock lies the softer Rochester Formation, which is mostly made of shale. The turbulent action of the falling water at the base of the falls, combined with spray and groundwater seepage, constantly attacks this vulnerable shale layer. This process causes the soft rock to wear away much faster than the hard caprock above it, creating a deep recess or notch behind the waterfall’s curtain. As the undercutting of the shale progresses, the massive, unsupported slab of caprock eventually breaks off in large blocks due to gravity, maintaining the sheer vertical cliff face. This cycle of undercutting and collapse drives the falls’ upstream migration.
Water Volume, Erosion, and Modern Flow Control
The Niagara River is an integral part of the Great Lakes system, carrying the outflow from four of the five lakes: Superior, Michigan, Huron, and Erie. This immense drainage basin ensures an extraordinarily high volume of flow, averaging about 2,400 cubic meters of water per second. This sheer volume of water created the powerful erosive force that historically caused the falls to recede upstream at a rapid rate, sometimes estimated at one to 1.5 meters per year.
However, the current flow is heavily modulated by the Niagara Treaty, a 1950 agreement between the United States and Canada. This agreement regulates the division of water between scenic display and hydroelectric power generation. Up to 75% of the Niagara River’s total flow is diverted via massive tunnels upstream of the falls to hydroelectric power plants. This diversion significantly reduces the water volume flowing over the cataracts, particularly at night and during the winter non-tourist season. The reduction in water flow has slowed the falls’ recession rate to less than 0.3 meters per year, effectively preserving the location. The water that does go over the falls carries a substantial amount of rock flour—finely ground rock particles—which contributes to the distinctive greenish hue of the water.