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The Allegheny River ( AL-ig-AY-nee) is a 325-mile-long (523 km) headwater stream of the Ohio River that is located in western Pennsylvania and New York in the United States. It runs from its headwaters just below the middle of Pennsylvania's northern border, northwesterly into New York, then in a zigzag southwesterly across the border and through Western Pennsylvania to join the Monongahela River at the Forks of the Ohio at Point State Park in Downtown Pittsburgh, Pennsylvania.
The Allegheny River is, by volume, the main headstream of both the Ohio and Mississippi Rivers. Historically, the Allegheny was considered to be the upper Ohio River by both Native Americans and European settlers.
This shallow river has been made navigable upstream from Pittsburgh to East Brady by a series of locks and dams that were constructed during the early 20th century. A 24-mile-long portion of the upper river in Warren and McKean counties of Pennsylvania and Cattaraugus County in New York forms the Allegheny Reservoir, which was created by the erection of the Kinzua Dam in 1965 for flood control.
The name of the river is derived from one of a number of Delaware/Unami phrases that are homophones of the English name, with varying translations.