Ohio River

The Ohio River (Seneca: ohi:yó) is the largest tributary, by volume, of the Mississippi River. At the confluence, the Ohio is even bigger than the Mississippi (Ohio at Cairo: 281,500 cu ft/s (7,960 m3/s); Mississippi at Thebes: 208,200 cu ft/s (5,897 m3/s)) and, thus, is hydrologically the main stream of the whole river system, including the Allegheny River further upstream. It is approximately 981 miles (1,579 km) long and is located in the Eastern United States.

The river had great significance in the history of the Native Americans, as numerous civilizations formed along its valley. In the five centuries prior to European contact, the Mississippian culture built numerous regional chiefdoms and major earthwork mounds in the Ohio Valley, such as Angel Mounds near Evansville, Indiana, as well as in the Mississippi Valley and the Southeast. For thousands of years, Native Americans, like the European explorers and settlers who followed them, used the river as a major transportation and trading route. Its waters connected communities. The Osage, Omaha, Ponca and Kaw lived in the Ohio Valley, but under pressure from the Iroquois to the northeast, migrated west of the Mississippi River to Missouri, Arkansas, and Oklahoma in the 1600s.

René-Robert Cavelier, Sieur de La Salle led an expedition to the Ohio River in 1669. His French party were the first Europeans to see the river. After European-American settlement, the river served at times as a border between present-day Kentucky and Indian Territories. It was a primary transportation route for pioneers during the westward expansion of the early U.S. The Ohio flows through or along the border of six states, and its drainage basin includes all, or part of, 14 states. Through its largest tributary, the Tennessee River, the basin includes many of the states of the southeastern U.S.

During the 19th century, the river was the southern boundary of the Northwest Territory. It is sometimes considered as the western extension of the Mason–Dixon Line that divided Pennsylvania from Maryland, and thus part of the border between free and slave territory, and between the Northern and Southern United States or Upper South. Where the river was narrow, it was the way to freedom for thousands of slaves escaping to the North, many helped by free blacks and whites of the Underground Railroad resistance movement.

The Ohio River is a climatic transition area, as its water runs along the periphery of the humid subtropical and humid continental climate areas. It is inhabited by fauna and flora of both climates. In winter it regularly freezes over at Pittsburgh but rarely so as it travels further south toward Cincinnati and Louisville. At Paducah, Kentucky in the south, near the Ohio’s confluence with the Mississippi, it is ice-free year round. Paducah was founded there because it is the northernmost ice-free reach of the Ohio. In his Notes on the State of Virginia published in 1781–82, Thomas Jefferson stated: "The Ohio is the most beautiful river on earth. Its current gentle, waters clear, and bosom smooth and unbroken by rocks and rapids, a single instance only excepted."

• The Allegheny River (left) and Monongahela River (right) join to form the Ohio River at Pittsburgh, Pennsylvania—the largest metropolitan area on the river.

• Wheeling, West Virginia, a major city on the river, was the first city to have a bridge across the river.

• Louisville, Kentucky is situated at both the widest and deepest level of the Ohio River.

• A barge hauling coal in the Louisville and Portland Canal, the only man-made portion of the Ohio River.

• Cincinnati, Ohio is a well-known city along the Ohio River, historically known for its riverboats. The Tall Stacks festival celebrates the Cincinnati riverboats and the Ohio River every three or four years.

• Evansville, Indiana, the third-largest city in Indiana and the fourth-largest city on the river.