Ohio River, the chief eastern tributary of the Mississippi River, and one of the longest rivers in the United States. Including its longest headstream, the Allegheny River, it is 1,306 miles (2,102 km) long; the Ohio proper is 981 miles (1,579 km) long. The Ohio is formed at the junction of the Allegheny and Monongahela rivers at Pittsburgh, Pennsylvania, and flows generally southwestward, entering the Mississippi at Cairo, Illinois. The river is the southern border of Ohio, Indiana, and Illinois, and the northern border of West Virginia and Kentucky.

The Ohio's chief tributaries are the Wabash, Miami, Scioto, and Muskingum on the north; and the Tennessee, Cumberland, Green, Kentucky, Licking, and Kanawha on the south. The Ohio and its tributaries form a vast system of navigable waterways. The river's basin covers much of seven states and small sections of seven other states.

The Ohio is 1,100 feet (335 m) above sea level at Pittsburgh and 280 feet (85 m) above sea level at Cairo, a drop of less than one foot per mile (20 centimeters per kilometer). Because of this relatively small gradient (degree of slope), the valley is easily flooded by excessive spring rains or melting snow, and disastrous floods have occurred, the most recent in 1959, 1964, and 1997. The threat of floods has been decreased by an extensive system of dams and locks, which has also improved the river for navigation.