Ohio, river, 981 mi (1,579 km) long, formed by the confluence of the Allegheny and Monongahela rivers in SW Pa., at Pittsburgh; it flows northwest, then generally southwest to enter the Mississippi River at Cairo, Ill. The Ohio's course follows a portion of the southern edge of the region covered by continental ice during the late Cenozoic era; glacial meltwater probably cut its original channel. The river is a major tributary of the Mississippi and supplies more water to it than does the Missouri River. The Ohio River basin covers c.204,000 sq mi (528,400 sq km); the chief tributaries are the Tennessee, Cumberland, Wabash, and Kentucky.

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