The Genesee River (/ˌdʒɛnɪˈsiː/ JEN-iss-EE) is a tributary of Lake Ontario flowing northward through the Twin Tiers of Pennsylvania and New York in the United States.

It begins in exposing the Allegheny Plateau's characteristic conglomerates: sandstones and shales in the rock columns of the Mississippian and Pennsylvanian subperiods.

As it passes through the gorges in New York's Letchworth State Park, the river also often exposes older rocks such as shales (some rich in hydrocarbons[5]), siltstones and some limestones of the Devonian period at Letchworth and, at other canyons with three more waterfalls[6][7][8] at Rochester cuts through the Niagara Escarpment, exposing limestones and shales of Silurian age in the rock column.

With cuttings in the geologic record showing so many early ages, the river area has a great variety of fossils for paleobiological and stratigraphic analysis.

The southern edges of the Laurentide Ice Sheet and those advances impacted the formation geology and geography of the area.

The pre-ice age eastern branch of the Genesee runs south of Mount Morris and was completely diverted by extensive terminal moraines in Livingston County with a key blocking dam just south of Dansville, so most of the upper section of the ancient river was diverted instead to fall the off Appalachian Plateau toward the Susquehanna River system (to an eventual destination well to the southeast).

In 1779, on the orders of George Washington, the Sullivan Expedition destroyed over 40 Haudenosaunee villages in and around the watershed to force the Seneca and allied nations out of the newly formed United States.

Subsequently, with most Iroquois having fled to Canada, the remnant tribal groups were in no position to further impede white settlers, so most of New York state west of the Genesee River became part of the Holland Purchase after the American Revolution.

In the 1797 Treaty of Big Tree, the Seneca tribes were granted six reservations along the river, among them Canawaugus, Little Beard's Town, Geneseo, Caneadea, Deyuitgau and Gardeau.

Although an important commercial route, the canal was plagued by frequent flood damage and the final leg down the Allegany River was never completed.