Peculiar Pittsburgh

The Allegheny River freezes faster than the Monongahela — but not just for the obvious reason

River ice was likely to grow this week at a rate of about three inches a day.

Three very cold rivers.

Three very cold rivers.

Colin Deppen / The Incline

It’s maybe telling that one of the first warnings issued by City of Pittsburgh officials ahead of this week’s brutal cold snap was a plea for members of the public to stay off of the city’s icy rivers.

Let’s call it unintended social commentary in the form of a PSA.

“We have, in the past, had individuals try to walk across the river,” Public Safety Director Wendell Hissrich said Monday. “We caution you, please don’t try.”

That ask was followed by a friendly reminder that the Mon River was less likely to freeze than the Allegheny as both waterways make their way toward their union at the start of the Ohio.

The obvious reason for this is that the Allegheny River flows north-south from colder regions (northern Pennsylvania and southern New York) and the Mon flows south-north from West Virginia.

But before you over-confidently broach this subject at a bar — or on a police boat after you’ve attempted to walk across a frozen Pittsburgh river — know there’s more to the story.

Fred McMullen, a meteorologist with the National Weather Service in Pittsburgh, said, yes, the Allegheny comes into Pittsburgh colder. But the greater likelihood that it gains ice cover also owes to its speed and its route.

“The [Allegheny] river seems to be slower, which allows ice to build up,” McMullen said. “It also has more bends in it, and that causes ice to pile up and slowly work its way around those bends.”

The Mon is also more likely to receive rainfall in winter, and rivers swell and run faster after a rain, which discourages ice formation, said Brad Peroney, program development coordinator with the Carnegie Science Center

“Because the Allegheny is more likely to have precipitation in the form of snow, it might be running more slowly in the winter than the Mon,” he continued.

The Mon, meanwhile, has more barge traffic that helps to break up ice as it forms, Peroney said.

“… My personal experience is that the Allegheny almost always freezes before the Mon,” he said, “which sometimes doesn’t freeze at all.”

In essence, the rivers have two distinct personalities owing to their distinct origins, something that’s also evident in the summer when they can take on entirely different colors.

That’s not to say the Mon can’t freeze — it can and has.

It’s just that ice tends to build up on the Allegheny more quickly, as proven this week. (To be clear, the surface of the Allegheny didn’t completely freeze in the last two days, but the process was certainly underway.)

And this time around, the window was tight. While the last two days saw sub-zero wind chills and record-breaking cold, we’ll be back in the 50s by Monday.

On Thursday, temperatures on the Mon were slightly above freezing, while water temperatures on the Allegheny were freezing or slightly below freezing, Peroney said.

With a mean temperature like we saw over the last two days, McMullen said, river ice grows at a rate of about three inches a day.

But this cold snap will be gone soon, along with your chance to recklessly defy public safety officials.

It’s probably for the best.

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