The Niagara River is not a river at all, but a strait. It meanders 36 miles north from the eastern tip of Lake Erie to the west end of Lake Ontario, draining the overflow from four Great Lakes. Twenty-two miles downstream from Buffalo and Fort Erie the river is split by Goat Island into two channels. It is here that the rapids begin. The river-bed drops fifty-five feet in less than a half mile, and the water reaches speeds of up twenty-five miles per hour before reaching the edge of the Canadian Horseshoe, Bridal Veil and American Falls. From here it falls approximately 170 feet into the lower Niagara Gorge below the falls.

A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river.
A strait is a naturally formed, narrow, typically navigable waterway that connects two larger bodies of water. Most commonly it is a channel of water that lies between two land masses.