The Ohio River (Seneca: ohi:yó) (Shawnee: Pelewathiipi or Spelewathiipi ) 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.