Six states make up the Ohio Valley: Ohio, Indiana, Illinois, West Virginia, Pennsylvania and Kentucky. It is the area surrounding the Ohio River, which is Mississippi River's largest tributary by volume.
The Ohio River is 981 miles long and is 1 mile wide at its widest point. It begins in Pittsburgh, Pa., where it is joined with the Allegheny and the Monongahela rivers. From there, the river flows along the southern borders of Ohio, Indiana and Illinois before emptying into the Mississippi River in Cairo, Ill.
Several major cities are located in the Ohio Valley. As of 2014, Pittsburgh is the largest metropolitan area located on the river. Louisville, Ky., is located on the Ohio River's widest and deepest point. The first city to construct a bridge across the Ohio River was Wheeling, W.Va.