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 considerably bigger than the Mississippi (Ohio at Cairo: 281,500 cu ft/s (7,960 m/s); Mississippi at Thebes: 208,200 cu ft/s (5,897 m/s)) and, thus, is hydrologically the main stream of the whole river system. 

The river flows through or along the border of six states, and its drainage basin includes parts of 15 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 ,  "Good River" or 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 17th century.
Where?
Cairo, Illinois