The Chemung River is a tributary of the Susquehanna River, approximately 46.4 miles long, in south central New York and northern Pennsylvania in the United States. It drains a mountainous region of the northern Allegheny Plateau in the Southern Tier of New York. The Chemung River is formed near Painted Post in Steuben County, just west of Corning by the confluence of the Tioga River and Cohocton rivers. It flows generally east-southeast through Corning, Big Flats, Elmira, and Waverly. It crosses into northern Pennsylvania before joining the Susquehanna River approximately 2 miles (3 km) south of Sayre.
The name of the river comes from an Lenape word meaning "at the horn" composed of the root chemu 'horn' and the suffix -ng meaning 'at/on'. Another possible etymology is "big horn", possibly dating from the discovery of large mammoth tusks in the river bed. Most of the valley is cut into Devonian age shale, sandstone, and limestone. The hilltops are rounded by glaciation. The tributaries, particularly the Cohocton River, have captured some of the former Genesee River drainage, due to terminal moraines that filled some valley areas and diverted streams.
The area near the river's source was referred to as Concanoga, or the land of three rivers, by the Seneca who lived in the area. In colonial times the river valley was a major trade route through the hill country of western New York, first for the Iroquois and other Native Americans, and later for the European settlers. In 1779 during the Revolutionary War, American troops of the Sullivan Expedition defeated a combined force of Iroquois, Tories and British at the Battle of Newtown along the river southeast of Elmira. The victory opened the way for Sullivan to systematically destroy Iroquois villages and settlements throughout their homeland of central and western New York.
The construction of the Chemung Canal which was completed in 1833 between the Chemung and the southern end of Seneca Lake allowed the shipment of Pennsylvania anthracite coal, lumber and agricultural products to the Erie Canal system, leading to the growth of Elmira as a regional center of manufacturing. The canals were rendered obsolete by the coming of the railroads in the late 1840s and 1850s.