river mainly in New York state, U.S. The Genesee flows generally north from its headwaters in Pennsylvania, crosses the New York State Canal System (New York), and bisects Rochester to enter Lake Ontario (Ontario, Lake) after a course of 158 miles (254 km). At Portageville, midway along its course, the river flows into a 17-mile- (27-km-) long postglacial, winding, rocky gorge with sides rising as high as 600 feet (180 metres) above its banks. Called the “Grand Canyon of the East,” this gorge, cut into sedimentary rocks of the Appalachian Upland, is the focal point of Letchworth State Park, which is noted for its scenic beauty and is the site of a well-known Native American and pioneer museum. At the southern end of the park, the Genesee cascades over three waterfalls. There is a flood-control dam at Mount Morris, where the river leaves the gorge to enter a wide, preglacial, moraine-filled plain. At Rochester it enters another gorge, which is 7 miles (11 km) long and contains three waterfalls that supply hydroelectric power. The name Genesee is derived from an Iroquoian term meaning “beautiful valley”; the Senecas called it Casconchiagon (“River of Many Falls”).