The Ohio River is formed by the union of the Allegheny and Monongahela rivers at Pittsburgh, Pennsylvania. It flows northwest out of Pennsylvania and then turns southwest to join the Mississippi River in Illinois. One of the main tributaries of the Mississippi, it is 981 miles (1,579 kilometers) long.

Physical features

The Ohio River forms the state boundaries between Ohio and West Virginia; Ohio and Kentucky; Indiana and Kentucky; and Illinois and Kentucky. Its longest tributary is the Tennessee River, which begins in the Appalachian Mountains.

The Ohio River has a fairly regular flow, but it has caused many destructive floods. Following major floods at Johnstown, Pennsylvania, in 1889, and Portsmouth, Ohio, in 1937, the federal government built a series of flood-control dams. These helped keep the river at a depth of 9 feet (3 meters).

Economy

The Ohio serves large industrial centers such as Pittsburgh; Cincinnati, Ohio; and Louisville, Kentucky. The river is used to transport coal, oil, steel, and many manufactured goods.

History

The French explorer La Salle is said to be the first European to see the Ohio River. He reached it in 1669. In the 1750s the river played a strategic role as the English and the French struggled to control exploration of the interior part of the continent.

By 1763 the English had control of the river. Twenty years later the Ohio country was acquired by the United States. The region was opened to settlers after the Northwest Territory was organized in 1787. Most of the settlers entered the region along the Ohio.