We love viewer questions, and today I am going to answer one I get asked a lot.

A viewer named Lynn asked, "Hey Ray! Why does it seem like the Allegheny River gets ice on it and the Mon doesn't, or is this just my imagination?"

While both the Monongahela River and Allegheny Rivers both can freeze in the winter, the Allegheny River will be the river that freezes first in Pittsburgh.

The biggest factor is the direction the rivers flow. The Monongahela flows from south to north. The Allegheny River flows from north to south. North is where you typically find colder temperatures and a lot more snowpack. This combination leads to colder temperatures. Even when you get winter warmups, the snow melt draining into the Allegheny River is colder than the rain runoff in the Monongahela. This gives the Allegheny a jump start on winning the race to freezing first.

Another factor is the composition of the rivers. There is more sediment flowing in the Mon. You may remember this from a previous Hey Ray where we discussed the line that shows up at the confluence of the rivers. The Monongahela has a siltier, sandstone bottom that gets stirred up a lot easier. The Allegheny River has bigger gravel and coarser sand, so it does not get stirred up as much. The extra sediment in the Monongahela slows the freezing process down.

So, no Lynn, it is not your imagination, the Mon does take longer to freeze than the Allegheny River.