The Ohio River (Seneca: Ohi:yo') is a 981-mile-long (1,579 km) river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing in a southwesterly direction from Pittsburgh, Pennsylvania, to its mouth on the Mississippi River in Cairo, Illinois. It is the third largest river by discharge volume in the United States and the largest tributary by volume of the Mississippi River. It is also the sixth oldest river on the North American continent. The 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 several states of the southeastern United States. It is the source of drinking water for five million people.
The river became a primary transportation route for pioneers during the westward expansion of the early U.S. The lower Ohio River just below Louisville was obstructed by rapids known as the Falls of the Ohio where the elevation falls 26 feet (7.9 m) in 2 miles (3.2 km) restricting larger commercial navigation, although in the 18th and early 19th century its three deepest channels could be traversed by a wide variety of craft then in use. In 1830, the Louisville and Portland Canal (now the McAlpine Locks and Dam) bypassed the rapids, allowing even larger commercial and modern navigation from the Forks of the Ohio at Pittsburgh to the Port of New Orleans at the mouth of the Mississippi on the Gulf of Mexico. Since the "canalization" of the river in 1929, the Ohio has not been a natural free-flowing river; today, it is divided into 21 discrete pools or reservoirs by 20 locks and dams for navigation and power generation.
