The Genesee River ( ) is a tributary of Lake Ontario flowing northward through the Twin Tiers of Pennsylvania and New York in the United States. The river contains several waterfalls in New York at Letchworth State Park and Rochester.
The river was historically used as a border between the lands of the Seneca to the east and the Erie and Wenro to the west. Later, the river provided the original power for the Rochester area's 19th century mills, and still provides hydroelectric power for downtown Rochester. Flooding occurred periodically in the river valley prior to the construction of the Mount Morris Dam in the 1950s.
Geology
The Genesee is the remaining western branch of a preglacial system, with rock layers tilted an average of 40 feet (12 m) per mile, so the river flows across progressively older bedrock as it flows northward. It begins in exposing the Allegheny Plateau's characteristic conglomerates: sandstones and shales in the of the Mississippian and Pennsylvanian subperiods. Thereafter, farther downstream as it traverses the area known as The Grand Canyon of the East, where it falls (three times) through over 600 feet (180 m). 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), siltstones and some limestones of the Devonian period at Letchworth and, at other canyons with three more waterfalls 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.
During the past million years, four glacial ages covered the Rochester area. The southern edges of the Laurentide Ice Sheet and those advances impacted the formation geology and geography of the area. The most recent glacier that left evidence here was about 10,000 years ago and it caused compression of the earth by as much as 2,500 feet (760 m). About 12,000 years ago, the area underwent massive changes, which included the rerouting of the Genesee and other water bodies. 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).
Only a small creek (Irondequoit Creek) flows in what is left of this large paleogeologic valley. The area of the lower river was also affected. Since the earth rebounded from the melting glaciers more rapidly in Canada than in New York, water from Lake Ontario was spilled over New York due to its lower elevation. During this time, the original outlet of the Genesee River, Irondequoit Bay, was flooded out, creating the current bay. As these waters later retreated, glacial debris caused the river to be rerouted to the west along its current path.
History
The Seneca nation traditionally lived between the Genesee River and Canandaigua Lake. The region was surveyed by Thomas Davies in 1766. The High Falls was then also known as the Great Seneca Falls, and the Genesee River was also spelled Zinochsaa by early writers.