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 where Lake Erie is today and then to 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 the Lake Erie area (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.
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.
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 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.
UPDATE: See the comments! And here’s a map of the ancient Erigan River drainage from Ohio DNR.
(photo by Kate St. John. Red-arrow map derived from OH & PA river maps at geology.com, map of Lake Monongahela from Wikimedia Commons, annotated map of Erigan River via CVNP; click on the captions to see the originals)