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. The name "Ohio" comes from the Seneca word Ohi:yo', meaning "good river". 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." After the French and Indian War, Britain's trans-Appalachian Indian Reserve was divided by the river into colonial lands to the south and Native American lands to the north. In the late 18th century, the river became the southern boundary of the Northwest Territory. The river was in the early 19th century considered 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 Upland South. Where the river was narrow, it was crossed by thousands of slaves escaping to the North for freedom; many were 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 continental and humid subtropical climate areas. It is inhabited by fauna and flora of both climates. During winter, it regularly freezes over in Pittsburgh but rarely farther south towards Cincinnati and Louisville. Further down the river in places like Owensboro and Paducah, closer to its confluence with the Mississippi, the Ohio is ice-free year-round. Today, the Ohio River is one of the most polluted rivers in the United States.The Ohio has faced significant industrial pollution historically and has continued to be a dumping ground for major polluting companies. Low-income communities around the Ohio are disproportionately impacted by pollutants that cause human and environmental health problems.
