File:Ohio River at Proctorville, Ohio and Barboursville, West Virginia (14311768707).jpg

The Ohio River, which streams westward from Pittsburgh, Pennsylvania, to Cairo, Illinois, is the largest tributary, by volume, of the Mississippi River in the United States. 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.

The 981-mile (1,579 km) river flows through or along the border of six states, and its drainage basin includes parts of 14 states. Through its largest tributary, the Tennessee River, the basin includes many of the states of the southeastern U.S. It is the source of drinking water for three million people.

It is named in Iroquoian or Seneca: Ohi:yó, lit. "Good River" or Shawnee: Pelewathiipi and Spelewathiipi. The river had great significance in the history of the Native Americans, as numerous civilizations formed along its valley. For thousands of years, Native Americans used the river as a major transportation and trading route. Its waters connected communities. In the five centuries before European conquest, 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. 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.

In 1669, René-Robert Cavelier, Sieur de La Salle led a French expedition to the Ohio River, becoming the first Europeans to see it. After European-American settlement, the river served 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. 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."

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 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.

en.wikipedia.org/wiki/Ohio_River