Jody Victor : The Ohio River is young from a geologic standpoint. The river formed on a peicemeal basis beginning between 2.5 and 3 million years ago. The earliest Ice Ages occurred at this time and dammed portions of north flowing rivers. The Teays River was the largest of these rivers, and the modern Ohio River flows within segments of the ancient Teays. The ancient rivers were rearranged or consumed by glaciers and lakes.
The upper Ohio River formed when one of the glacial lakes overflowed into a south flowing tributary of the Teays River. Prior to that event, the north flowing Steubenville River (no Longer in existence) ended between New Martinsville and Paden City, West Virginia. Likewise, the south flowing Marietta River (no longer in existence) ended between the cities. The overflowing lake carved through the separating hill and connected the rivers. The resulting floodwaters enlarged the small Marietta valley to a size more typical of a large river. The new large river subsequently drained glacial lakes and melting glaciers at the end of several Ice Ages. The valley grew with each major Ice Age.
Many small river were altered or abandoned after the upper Ohio River formed. Valleys of some abandoned rivers can still be seen on satellite and aerial images of the hills of Ohio and West Virginia between Marietta, Ohio, and Huntington, West Virginia. As testimony to the major changes that occurred, the valleys are actually found on hilltops.
The middle Ohio River formed in a manner similar to formation of the upper Ohio River. A north-flowing river was temporarily dammed southwest of present-day Louisville, Kentucky, creating a large lake until the dam burst. A new route was carved to the Mississippi River, and eventually the upper and middle sections combined to form what is essentially the modern Ohio River.