river that is the drainage outlet for the four upper Great Lakes (Superior, Michigan, Huron, and Erie (Erie, Lake)), having an aggregate basin area of some 260,000 square miles (673,000 square km). Flowing in a northerly direction from Lake Erie to Lake Ontario (Ontario, Lake), a distance of about 35 miles (56 km), the Niagara constitutes part of the boundary between the United States and Canada, separating New York state from the province of Ontario. The relatively high, steady flow and steep gradient (about 326 feet 【99 m】) of the river combine to make it one of the best sources of hydroelectric power in North America. About halfway along the river's course lies Niagara Falls, one of the major scenic spectacles of the continent. U.S. and Canadian cities of the same name stand on either bank of the river.

From its head at Lake Erie, the river flows through a single channel for about 5 miles (8 km). It is then divided into two channels by Strawberry and Grand islands, the eastern, or U.S., channel running for about 15 miles (24 km), the western, or Canadian, for about 12 miles. At the foot of Grand Island the two merge again about 3 miles (5 km) above Niagara Falls. From Lake Erie to the upper rapids the river descends about 10 feet (3 m), whereas in the short rapids it falls 50 feet (15 m) before pouring over the falls.

Below the falls and extending for 7 miles (11 km) is the Niagara Gorge. The stretch of 2.25 miles (3.6 km) from Horseshoe Falls is known as the Maid of the Mist Pool. It has a descent of only 5 feet (1.5 m) and is navigable by excursion boats. Beyond this, the gorge descends another 93 feet (28 m), flowing northwestward first through the narrow Whirlpool Rapids for 1 mile (1.6 km) to the Whirlpool. There the gorge makes a 90° bend to the northeast for 2 miles (3 km) and turns north for another 1.5 miles (2.5 km) to the foot of the Niagara Escarpment at Lewiston, New York. In its final 7 miles, the river flows across a lake plain to Lake Ontario.

The river came into existence late in the Pleistocene Epoch, more than 10,000 years ago, when the margin of a great continental ice sheet melted back and exposed the escarpment of Niagaran dolomite rock of Silurian origin, allowing the discharge from the Lake Erie Basin to pour over it. Recession of the falls created the Niagara Gorge, the age of which, when calculated by dividing its length by the average rate of recession of the falls in recent time, is about 7,000 years. Other considerations led some geologists to estimate an age as great as 25,000 years. Determinations of the age of the last glacial ice advance in the area suggest, however, that the Niagara River is about 12,000 years old.

The river is navigable from Lake Erie to the upper rapids. Waterborne traffic along the Niagara passes through the upper single channel and the U.S. channel and enters the New York State Barge Canal (New York State Canal System) at Tonawanda, New York. That canal, with a minimum depth of 12 feet (4 m), connects with the Hudson River and has branches that connect with Lake Champlain and Lake Ontario. The Black Rock Canal, from Buffalo Harbor to a point a few miles down the Niagara River, extends the navigation period locally through a greater part of the winter, when the river itself becomes jammed with Lake Erie ice. The principal shipping between Lakes Erie and Ontario, however, passes through the Welland Canal, an important link in the Great Lakes-St. Lawrence Seaway.

Canada and the United States agreed, in a treaty signed in 1950, to reserve sufficient amounts of water for flow over Niagara Falls to preserve their scenic value. All water in excess of a certain amount, estimated to average about 130,000 cubic feet (3,600 cubic metres) per second, was made available for diversion for power generation (hydroelectric power), to be divided equally between the United States and Canada. This power is developed by public-authority power-plant installations on both sides of the gorge. A major impetus for the diversion of water into Lake Superior via the Long Lake-Ogoki River watersheds was the additional hydroelectric power that the water would eventually bring to the United States and Canada at Niagara Falls. Two large power plants situated at the end of the gorge, one near Lewiston, N.Y., and one at Queenston, Ont., receive water diverted from the river above the falls and carried to them by tunnels and canals. The electrical power is used by nearby electrochemical industries and is also sent to various cities for other uses.