Mainly located in New York, the Genesee is the remaining western branch of a preglacial system, flowing northward to Lake Ontario from its headwaters in northern Pennsylvania. Its rock layers tilt an average of 40 feet per mile, so the river flows across progressively older bedrock as it flows northward.
Further downstream it traverses an area known as The Grand Canyon of the East, where it falls three times through over 600 feet, passing through the gorges in New York’s Letchworth State Park.
The river often exposes older rocks at Letchworth and other canyons with three more waterfalls at Rochester cutting through the Niagara Escarpment to expose 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.
During the past million years there have been four glacial ages that covered the Rochester area with the southern edges of the Laurentide Ice Sheet. Those advances were major impactors in the formation geology and geography of the area.
Around 12,000 years ago, the area underwent massive changes, which included the rerouting of the Genesee River and other water bodies, while 10,000 years ago the most recent glacier caused compression of the earth by as much as 2,500 feet.
The eastern branch of the Genesee was completely diverted by extensive terminal moraines in Livingston County, diverting most of the upper section of the ancient river off Appalachian Plateau toward the Susquehanna River system. Currently only a small 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, flooding Irondequoit Bay to create the current bay. As the waters retreated, glacial debris caused the river to be rerouted to the west along its current path.
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 spelled Zinochsaa by early writers.
Historically, the river’s gorge formed a natural border between the lands of the Five Nations of the Iroquois to the east and the related tribes of the Erie people along the west side of the gorge.
Following the end of the Beaver Wars and the American Revolution, the lands in all of upstate New York into the Ohio Country were controlled by the Iroquois Confederation until 1779 when under orders from George Washington the Sullivan Expedition destroyed ov 40 Haudenosaunee villages to force the Seneca and allied nations out of the newly formed United States.
With most Iroquois having fled to Canada, most of New York state west of the Genesee River became part of the Holland Purchase.
From 1801 to 1846 the entire region was sold to individual owners from the Holland Land office in Batavia, New York. The river demarcates the “Genesee Country” of New York to the west and the Finger Lakes geographic region, and heartland of the Iroquois to the east.
Starting in 1836 the Genesee Valley Canal was begun. Construction of new sections extended upriver (southward) until 1880, when project was abandoned and the right of way was sold. The property became the roadbed for the Genesee Valley Canal Railroad, which eventually merged with the Pennsylvania Railroad.
Much of the canal and railroad right-of-way is now open to the public as the Genesee Valley Greenway.