How The Mighty Monongahela Lost Its Crown

This used to be the Monongahela River (photo by Kate St. John)

Two million years ago the Monongahela was a mighty river.  Instead of being a short tributary of the Ohio and draining to the Gulf of Mexico, it flowed north to Lake Erie and drained into the Atlantic.  This stretch of the Ohio River in Pittsburgh was not the Ohio at all. It was the Monongahela.

Here’s how the mighty Mon River lost its crown and the reason why the Ohio turns south at Beaver, Pennsylvania.

Before the Pleistocene era, the Monongahela River drained 75% of today’s Ohio, Allegheny and Monongahela watersheds as it flowed north from West Virginia to Lake Erie (roughly the red arrow path below). 

Back in those days the Ohio River was just a tributary whose northernmost point was in Pennsylvania where it joined the Mon.  The Beaver and Shenango Rivers did not exist as they do today.  Their valleys carried the Monongahela north. 

Approximate flow of historic Monongahela River (derived from river maps at

But then the climate changed. The Great Ice Age began.

Glaciers blocked the Monongahela’s northward flow so the river backed up and formed Lake Monongahela.  The pale dashed lines show the paths of our rivers today bending away from the prehistoric glacier.  

Lake Monongahela (image from Wikimedia Commons)

Eventually Lake Monongahela rose so high that it breached the lowest barrier in the Ohio valley near present day New Martinsville, WV (see orange arrow).

The Ohio started flowing “backwards.” It cut the Ohio River valley deeper, orphaned the northern Monongahela and reversed its flow, creating the Shenango and Beaver Rivers.

All of this was helped by the huge volume of water joining the Mon from the re-formulated Allegheny River watershed.  The Upper and Middle Allegheny river systems used to flow north to Lake Erie too, but were also blocked by glaciers. Their proglacial lakes overflowed and joined the Lower Allegheny River flowing into the Ohio watershed.

And so the Monongahela River became a lowly tributary of the Ohio.

Climate change is big stuff.  When it gets cold it changes major rivers.  When it gets hot … Well, we’ll find out.

p.s. I don’t completely know the Monongahela River’s historic route to Lake Erie. If you do, let me know by leaving a comment below.

(photo by Kate St. John. Red-arrow map derived from OH & PA river maps at, map of Lake Monongahela from Wikimedia Commons; click on the captions to see the originals)