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The Unadilla River in New York State flows 71 miles (114 km) from south of Utica to the village of Sidney, where it flows into the Susquehanna River, which eventually empties into the Chesapeake Bay, an arm of the Atlantic Ocean.
Most of the length of the Unadilla forms the western border of Otsego County and the eastern borders of Chenango and Madison Counties. This border made up a significant portion of the Fort Stanwix Treaty Line of 1768.
The Unadilla has two branches which join at Unadilla Forks on the Otsego-Madison County border.
- The main branch of the Unadilla starts at Unadilla Lake, at Miller's Mills in southwestern Herkimer County.
- The West Branch of the Unadilla starts a few miles north of Bridgewater in southeast Oneida County. This branch was known as Eghwagy Creek during the early 18th century.
The Unadilla's tributaries, from north to south, include:
- Campbell Brook: from the east, near the Skaneateles Turnpike north of Leonardsville
- Button Creek: from the west, between Leonardsville and West Edmeston
- Beaver Creek: from the west, between South Brookfield and Columbus Quarter
- Tallette Creek: from the west, near Columbus Quarter
- Showler Brook: from the west, north of New Berlin
- Wharton Creek: from the east, at New Berlin
- Butternut Creek: from the east, near Mount Upton
- Guilford Creek: from the west, north of Sidney
Alternate names and spellings 
Unadilla has had various spellings and alternate names, particularly in the 18th century:
- Che-on-a-dill-ha: Oneida dialect
- Teyonadelhouogh: Gideon Hawley, Journey to Broome County, Oquaga (Onaheghgwage), 1753
- Tiona-derha: Guy Johnson's 1768 map
- Tienaderha: Guy Johnson's 1771 map and Southeir's 1779 map
- Tianaderaha: Surveyor-General, Simeon De Witt's map c.1790
- Tianaderha: Fort Stanwix Treaty of 1768
- Tunadilla: Common spelling 18th century correspondence
- Susquehannock: Vaughan's 18th century Chorographical Map