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The Winooski River (formerly, Onion River) is a tributary of Lake Champlain, approximately 90 miles (145 km) long, in northern Vermont in the United States. Although not Vermont's longest river, it is one of the state's most significant, forming a major valley way from Lake Champlain through the Green Mountains towards (although not connecting in drainage to) the Connecticut River valley.
The river drains an area of the northern Green Mountains between Montpelier and Burlington. It rises in Washington County north of Montpelier, then flows southwest to Montpelier, passing through the city along the south side of downtown and the Vermont State House. From Montpelier it flows northwest into Chittenden County through Richmond, passing north of the city of Burlington. It enters the eastern side of Lake Champlain approximately 5 miles (8 km) northwest of downtown Burlington. The City of Winooski sits along the river approximately 8 miles (13 km) upstream from its mouth, on the northeastern edge of Burlington. The river was historically used for the transportation of timber in the logging heyday of Vermont during the 19th century. The valley of the river downstream from Montpelier is where both U.S. Highway 2 and Interstate 89 run between Montpelier and Burlington.
The river is used for a variety of purposes. It is popular for recreational uses such as canoeing, kayaking, fishing and, for the intrepid, swimming. Additionally, a number of hydroelectric dams are used for generating electricity.
The Winooski One Dam, located on the river in the City of Winooski, was the first high performance civil structure in the world imbedded with fiber optic sensors when it was built in 1992. For this reason, the dam was listed in the Guinness Book of World Records as the "world's most intelligent dam."
See also 
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- Local River Protection Group, Friends of the Winooski River
- Hydrologic data on Vermont rivers from the National Weather Service, including the Winooski River