The Ohio River is a major river in the midwestern United States and the largest tributary of the Mississippi River by volume. Formed at Pittsburgh by the confluence of the Allegheny River and Monongahela River, it flows about 981 miles (1,579 km) southwest to its mouth at Cairo, Illinois, where it joins the Mississippi. Its drainage basin extends across parts of 14 states, and the river itself flows through or along the borders of six states, including Ohio, West Virginia, Kentucky, Indiana, and Illinois.
The Ohio has long been an important transportation route. During westward expansion in the early United States, it served as a principal highway of commerce and settlement. Navigation was historically limited by the Falls of the Ohio near Louisville, but the construction of the Louisville and Portland Canal in the 19th century improved passage for larger vessels. Since canalization in 1929, the river has been managed by a series of locks and dams and is no longer free-flowing throughout its course.
The river’s name comes from a Seneca word meaning “good river.” It also held major political significance as a boundary between colonial and Native American lands and later as part of the divide between free and slave territory. The Ohio remains a vital waterway, but it has also faced significant industrial and agricultural pollution.