Why is the Allegheny still frozen over but the Monongahela is not?

Calling all hydrologists, amateur and professional. Why is the Allegheny still frozen over but the Monongahela is not?

I was wondering if it could be connected to the fact that one has a mud bottom and the other has a pebble bottom. Does the amount of pollution in the Mon also contribute to a lower freezing point? I would guess the Mon also has more boat traffic.