Thinking Cool Thoughts
The Ohio River formed roughly 2.5 and 3 million years ago according to its article at Wikipedia. It was created during an Ice Age (cool) when glaciers (cool) dammed portions of north flowing rivers. The largest of these ancient rivers, the Teays River, formed many of the segments of the Ohio River today. 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 flood would have been very dramatic and exciting to see. The 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 rivers 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.