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.
The Ohio is prone to spring flooding, and extensive flood control and protection devices have been constructed along the river and its tributaries. These devices also improve the river's navigability; a 9-ft (2.7-m) channel is maintained along its entire length. A system of modern locks and dams, constructed since 1955 to replace older structures, speeds the transit of barges and leisure craft. A canal (first opened in 1830) at Louisville bypasses the Falls of the Ohio, a 2 1/4-mi (3.6-km)-long series of rapids having a 24-ft (7-m) drop.
The Ohio River basin is one of the most populated and industrialized regions of the United States. Oil and steel account for most of the cargoes moved on the river. The principal river ports are Cincinnati, Louisville, and Pittsburgh. Eight states (Ill., Ind., Ky., N.Y., Ohio, Pa., Va., and W.Va.) affected by the river's industrial pollution ratified (1948) the Ohio River Valley Sanitation Compact. Some results of their cleanup efforts have become discernible, and the river now supports marinas and recreational facilities.
The French explorer La Salle reportedly reached the Ohio River in 1669, but there was no significant interest in the valley until the French and the British began to struggle for control of the river in the 1750s. An early settlement was established at the forks of the Ohio (modern Pittsburgh) by the Ohio Company of Virginia in 1749, but it was captured by the French in 1754, and the unfinished Fort Prince George was renamed Fort Duquesne; it was recaptured by the British and renamed Fort Pitt in 1758. At the end of the French and Indian Wars, Britain gained control of the river by the treaty of 1763, but settlement of the area was prohibited. Britain ceded the region to the United States at the end of the Revolutionary War (1783), and it was opened to settlement by the Ordinance of 1787, which established the Northwest Territory.
Until the opening of the Erie Canal in 1825, the Ohio River was the main route to the newly opened West and the principal means of market transportation of the region's growing farm output. Traffic declined on the river after the railroads were built in the mid-1800s, although it revived after World War II. Comparatively little traffic remains on the Ohio, despite the new locks, dams, and channel improvements, which were all meant to spur economic activity on the river.
Relief: 2300 m Basin area: 529,000 km 2 Mean discharge: 8733 m 3 /s Mean annual precipitation: 104 cm Mean air temperature:...
The Ohio River was the great boundary between slave and free territory in the trans-Appalachian west and was known as the “Great River” to the Native