Science Fair Project Encyclopedia
The Ohio River is a principal tributary of the Mississippi River, 1,579 km (981 mi) long in the eastern United States. Of great significance in the history of North America dating from the time of the Native Americans, the river was a primary transportation route during the westward expansion of the early U.S. It flows through or along the border of six states, and its watershed encompasses 14 states, including many of the states of the southeastern U.S. through its largest tributary, the Tennessee. During the eighteenth century it was the southern boundary of the Northwest Territory, thus serving as the border between free and slave territory.
The river is formed by the confluence of the Allegheny and Monongahela rivers in downtown Pittsburgh. From Pittsburgh, it flows to the northwest through western Pennsylvania, before making an abrupt, almost 180 degree, turn to the south-southwest at the West Virginia state line where it then forms the border between West Virginia and Ohio. The river then follows a roughly southwestern and then western course between Kentucky and Ohio, Indiana, and Illinois until it joins the Mississippi from the east at Cairo, Illinois. At its mouth, the Ohio actually is wider than the Mississippi. 
Major tributaries of the river, indicated by the location of their mouth, include:
- Allegheny River — Pennsylvania
- Monongahela River — Pennsylvania
- Beaver River— Pennsylvania
- Little Muskingum River — Ohio
- Duck River — Ohio
- Muskingum River — Ohio
- Little Kanawha River — West Virginia
- Hocking River — Ohio
- Kanawha River — West Virginia
- Guyandotte River — West Virginia
- Big Sandy River — Kentucky-West Virginia border
- Scioto River — Ohio
- Little Miami River — Ohio
- Licking River — Kentucky
- Great Miami River — Ohio-Indiana border
- Kentucky River — Kentucky
- Green River — Kentucky
- Wabash River — Indiana-Illinois border
- Cumberland River — Kentucky
- Tennessee River — Kentucky
The Ohio's watershed covers 490,603 square kilometers, including the eastern-most regions of the Mississippi Basin. States drained by the Ohio include:
- Illinois (the southeast corner of the state),
- Indiana (all but the northern area of the state),
- Ohio (the southern half of the state),
- New York (a small area of the southern border along the headwaters of the Allegheny River),
- Pennsylvania (a corridor from the southwestern corner to north central border),
- Maryland (a small corridor along the Youghiogheny River on the state's western border),
- West Virginia (all but the eastern border of the state),
- Kentucky (all but a tiny part in the extreme west of the state drained directly by the Mississippi River),
- Tennessee (all but a small part in the extreme west of the state drained directly by the Mississippi River),
- Virginia (the western border of the state),
- North Carolina (the western border of the state),
- Georgia (the northwest corner of the state),
- Alabama (the northern fringe of the state), and
- Mississippi (the northeast corner of the state).
See  for a map and information on the Ohio's watershed.
The Ohio River was formed by glacial meltwater from the last stage of this ice age, the Wisconsin glaciation. During the glacial retreat, the river was temporarily dammed just southwest of Louisville, Kentucky, creating a large lake until the dam burst. The Ohio River largely supplanted the former Teays River drainage system, which was disrupted by the glaciers. Today, the river still follows a significant portion of the old Teays River course in southernmost Ohio.
Since it was considered by pre-Columbian inhabitants of eastern North America to be part of a single river continuing on through the lower Mississippi, it is perhaps an understatement to characterize the Ohio as a mere tributary of the Mississippi. The river is 981 miles (1579 km) long and carries the largest volume of water of any upper tributary of the Mississippi. In fact, the Ohio typically carries a much greater volume of water than the upper Mississippi.
Louisville, Kentucky was founded at the only major natural navigational barrier on the river, the Falls of the Ohio . These were a series of rapids where the river flowed over hard, fossil-rich beds of limestone. The first locks on the river were built at Louisville to circumnavigate the falls. Today, this is the site of McAlpine Locks and Dam .
Because the Ohio River flowed westwardly, it became the convenient means of westward movement by pioneers from western Pennsylvania to St. Louis, Missouri, where some continued on up the Missouri River, some up the Mississippi River, and some further west over land routes. In these early days, in the early 19th century, pirates set up shop at Cave-in-Rock in southern Illinois, waylaid settlers on their way down the river, killed them, stole their goods, and scuttled their boats.
Because of its significant role as the southern border of Ohio, Indiana, and Illinois, the Ohio River is historically famous as the border dividing free states and slave states. As depicted in several novels by Harriet Beecher Stowe and Toni Morrison, the Ohio River was the barrier which, by crossing by boat or 'on ice floes', slaves were freed. Today, the Ohio River generally separates Midwestern and Great Lakes states from Southern border states.
Interestingly, by an accident of history, the charter for Virginia went not to the middle of the Ohio River, but to its far shore so the entire river was included. Wherever the river serves as a boundary between states—Illinois, Indiana and Ohio on the north, and Kentucky and West Virginia on the south, the river essentially belongs to the two states on the south that were divided from Virginia. Kentucky brought suit against Indiana in the early 1980s because of the building of the Marble Hill nuclear power plant in Indiana, which would have discharged its waste water into the river. The U.S. Supreme Court ruled that Kentucky's jurisdiction (and, implicitly, that of West Virginia) extended only to the low water mark of 1793, important because the river has been extensively dammed for navigation, so that the present river bank is north of the old low water mark.
In the early 1980s, the Falls of the Ohio National Wildlife Conservation Area was established at Louisville, Kentucky.
Cities along the Ohio
Besides Pittsburgh and Cairo, other cities along the Ohio include:
- Steubenville, Marietta, Belpre, Pomeroy, Gallipolis, Ironton, Portsmouth, and Cincinnati in Ohio
- Weirton, Wheeling, Parkersburg, and Huntington in West Virginia
- Ashland, Newport, Covington, Louisville, Owensboro, Henderson, and Paducah in Kentucky
- Madison, Jeffersonville, Clarksville, New Albany, Tell City, Evansville, and Mount Vernon in Indiana.
- Metropolis in Illinois
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