Allegheny ăl´əgā˝nē, ăl´əgä˝nē [key], river, 325 mi (523 km) long, rising in N central Pa., and flowing NW into N.Y., then SW through Pa. to the Monongahela, with which it forms the Ohio River at Pittsburgh; drains 11,580 sq mi (29,992 sq km). Before the railroad era, the river was an important commercial route and is still used to transport coal and other bulky freight. Kinzua Dam (completed in 1965), a federal flood-control project on the river, forms a large lake; there are also dams on the river's tributaries. The Allegheny's basin has coal, oil, and natural gas.
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