The

Genesee River 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 provided the original power for the Rochester area's 19th century mills and still provides h ...

flows northward from its source in northern Pennsylvania to enter Lake Ontario at

Rochester Rochester may refer to: Places Australia * Rochester, Victoria Canada * Rochester, Alberta United Kingdom *Rochester, Kent ** City of Rochester-upon-Medway (1982–1998), district council area ** History of Rochester, Kent ** HM Prison ...

, New York. The present river valley has been modified extensively from preglacial river valleys. A lobe of the last glacier ( Wisconsonian) pushed southward almost to the

Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...

line, dramatically reshaping the drainage patterns of central and western New York.

Middle Genesee

The first image shows the evidence of the glacial rerouting for the middle section of the Genesee. The original river had two branches. The east branch runs southeast of Mount Morris. It was much larger than the west branch, flowing from near present-day

Canisteo, New York Canisteo () is a town in Steuben County, New York, United States. The population was 3,294 at the 2020 census. The name was taken from a former Native American village located there, and is Iroquoian in origin. The Town of Canisteo is in the ...

to empty into the preglacial Ontario River (now lake) north of Rochester. The headwaters of the east branch were blocked and diverted southward into the present Susquehanna River system by huge glacial terminal moraines just south of Dansville. The entire old valley was buried in that area. This left only the small Canaseraga Creek, to flow in the remnant of the large ancient east branch valley from Dansville to Mount Morris. The current upper Genesee (above Mount Morris) was only the west branch in preglacial times, a tributary river to the original. This branch, which entered the old valley was also dammed in the Nunda area by moraines. A lake was formed with

varve A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (va ...

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

sediments visible along the valley sides in many spots, but the lake drained as the river found a way to cut a new valley northeastward from Portageville a little west of the original valley, then drop back into the old valley at Mount Morris. This section of new valley is a spectacular gorge with three scenic

waterfall A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Waterfalls can be formed in severa ...

s, that is now the site of

Letchworth State Park Letchworth State Park is a New York State Park located in Livingston County and Wyoming County in the western part of the State of New York. The park is roughly long, following the course of the Genesee River as it flows north through a dee ...

. Portions of other preglacial valleys can be seen both east and west of the Genesee, occupied now by

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

s. The remnants of these valleys were buried by glacial drift. A

drumlin A drumlin, from the Irish word ''droimnín'' ("littlest ridge"), first recorded in 1833, in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated ...

field can be seen just north of Conesus and Hemlock Lakes.

Lower Genesee

In the lower part of the river, before entering Lake Ontario, the river appears to have been diverted once again from its ancient valley by the glacier. The original valley is thought to be that of Irondequoit Creek and Bay. The second image shows the most likely (buried) westward path originally followed by the river. With this blocked, the river cut a new channel, and created three waterfalls where it crossed limestone escarpments.

References

{{reflist Geology of New York (state) Genesee River Glaciology of the United States