Major river, eastern central U.S. Formed by the confluence of the Allegheny and Monongahela rivers, it flows northwest out of Pennsylvania, and west and southwest to form the state boundaries of OhioWest Virginia, Ohio-Kentucky, Indiana-Kentucky, and Illinois-Kentucky.
After a course of 975 mi (1,569 km), it empties into the Mississippi River at Cairo, Ill. It is navigable and has supported commerce since the earliest settlements. The river's strategic importance was recognized by the 1750s. After the French and Indian War , the English controlled the territory around the river.