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The Allegheny River (historically, especially in New York state, also spelled Allegany River) is a tributary of the Ohio River, approximately 325 mi (523 km) long, in the U.S. states of New York and Pennsylvania. It drains a rural dissected plateau of 11,580 sq mi (29,992 km²) in the northern Allegheny Plateau, providing the northeasternmost drainage in the watershed of the Mississippi River. Its tributaries reach to within 8 mi (13 km) of the shore of Lake Erie in southwestern New York. The valley of the river has been one of the most productive areas of energy extraction in U.S. history, with extensive deposits of coal, petroleum, and natural gas.
It rises in north central Pennsylvania, in central Potter County, Pennsylvania, approximately 10 mi (16 km) south of the New York border. It flows west past Coudersport then turns north into western New York State, looping westward across southern Cattaraugus County for approximately 30 mi (48 km), past St. Bonaventure University and Salamanca and forming the northern boundary of Allegheny State Park before re-entering northwestern Pennsylvania approximately 20 mi (32 km) southeast of Jamestown, New York.
It flows in a broad zigzag course generally southward across western Pennsylvania, first flowing southwest past Warren, Oil City, and Franklin, forming much of the northwestern boundary of Allegheny National Forest. South of Franklin it turns southeast across Clarion County in a meandering course, then turns again southwest across Armstrong County, flowing past Kittanning. It enters the Pittsburgh suburbs from the northeast and joins the Monongahela River in downtown Pittsburgh to form the Ohio.
It is joined from the south by Potato Creek in McKean County, Pennsylvania near its source. At Olean, New York it is joined form the north by Olean Creek . After re-entering Pennsylvania, it is joined by Kinzua Creek from the east 10 mi (16 km) upstream from Warren. At Warren it is joined from the north by Conewango Creek . At Franklin, it is joined by French Creek. In eastern Clarion County, it is joined from the east by the Clarion River, one of its principal tributaries. Southeast of Kittanning it is joined from the east by Crooked Creek , then from the east by the Kiskiminetas River, another of its principal tributaries at Freeport.
In the 16th century control of the river valley passed back-and-forth between Algonguin-speaking Shawnee and the Iroquois. By the time of the arrival of the French in the early 18th century, the Shawnee were once again in control and formed an alliance with the French against the incursion of British settlement across the Allegheny Mountains. The conflict over the expansion of British settlement into the Allegheny Valley and the surrounding Ohio Country was a primary cause of the French and Indian War in the 1750s. During the war, the village of Kittaning the principal Shawnee settlement on the river, was completely destroyed by British reprisal raids from central Pennsylvania.
Nevertheless, the British, after gaining control of the area in the 1763 Treaty of Paris, kept the area closed to white settlement, in part to repair and maintain relations with the Native Americans. The pressure to open the river valley and the surrounding area to settlement is considered by historians to be one of the root causes of the American Revolutionary War in the following decade.
During the 19th century, the river became a principal means of navigation in the upper Ohio valley, especially for the transport of coal. Although the building of the railroads lessened the importance of the river somewhat, the lower river (navigable as far as East Brady, Pennsylvania through locks) has continued to serve as route of commercial transportation until the present day. In 1875, the first U.S. petroleum was drilled north of the river at Titusville.
In 1965 the completion of the federally-sponsored Kinzua Dam for flood-control in northwestern Pennsylvania east of Warren created the long Allegheny Reservoir , part of which is included in the Allegheny National Recreation Area . The dam flooded parts of lands deeded "forever" to the Seneca tribe and to Cornplanter and his descendents.
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