Pittsburgh’s rivers are freezing —- but that does not mean they’re safe to walk on,
To those looking down from the bridges Thursday, it might have seemed as if the Penguins game could have been played on the Allegheny River instead of in PPG Paints Arena.
But even though the rivers look frozen — and fascinating from above — river ice in Pittsburgh isn’t at an unprecedented level, even given the wave of bone-chilling temperatures Pittsburgh has endured this week.
As of early Thursday afternoon, the ice on the Allegheny River near the Pittsburgh Zoo & PPG Aquarium was 2 inches thick, covering about 10 percent of the water, according to data from the National Weather Service’s Sharpsburg lock and dam. The Ohio River near Emsworth was fully covered in 1-inch-thick ice, and the Monongahela was 40 percent frozen in 4 inches of ice near Braddock. The National Weather Service in Pittsburgh tweeted: “With the cold weather ice is building on the Mon river it’s already on the Allegheny!”
For the rivers to ice consistently, it takes an average daily air temperature of 22 degrees or lower and strong, gusty winds. Although it hasn’t been windy enough, the recent 10-day stretch of frigid weather brought water
temperatures down from about 40 degrees in midDecember to below the freezing mark at 32 degrees, said hydrologist Lee Hendricks of the National Weather Service.
“What we’re seeing right now on the rivers is, generally, what’s called ‘pan ice,’” Mr. Hendricks said. “It’s flat ice. It’s fairly stable and not really moving anywhere. To get that ice to move, you generally have to have a river rise of 3 feet or more to break ice up enough where it will start moving.”
It starts to get risky for barges on the rivers when ice exceeds 4 inches in thickness, as it has near Braddock. That much ice can damage their hulls and strand them on top of it, which happened three years ago just south of the Montgomery lock and dam. In 2007, the U.S. Coast Guard had to halt the movement of all barges carrying hazardous cargo because of thick ice on the Allegheny near Highland Park.
This puts the weather service on constant alert and in frequent contact with the Coast Guard and Army Corps of Engineers, which together monitor the conditions to ensure river traffic continues to flow. They also monitor the rotting of the ice when air temperature rises or when there’s rainfall, either of which could cause flooding. That happened on Jan. 20, 1996, when rainfall broke loose all of the ice on the Allegheny, creating ice jams and bringing the river to 34.6 feet in height — the seventh-highest reading in Pittsburgh history. On that day, barges plowed down the river and banged into bridges.
The rivers could accumulate another 2 to 3 inches of ice between now and Sunday morning as temperatures drop to 8 degrees Friday, with a windchill advisory in effect through 3 p.m. Saturday. The ice will begin to rot as temperatures climb above freezing early next week, Mr. Hendricks said.
But while the ice is still there, the National Weather Service wants Pittsburghers to heed one important warning.
“Just because it looks like the ice is thick enough to walk on, don’t do it,” Mr. Hendricks said. “It’s a bad thing. Every winter when we have a lot of ice, someone does it somewhere, and it rarely turns out good.”