This story was originally published in 2019.
The Monongahela and Allegheny rivers are a lot like twins.
They both flow through Pittsburgh, slicing the city into distinct neighborhoods, they’re both covered in bridges, and they both move commerce through the region.
During a cold spell in 2019, Good Question! listener Mark Brandfass observed a difference between the two waterways from his office downtown.
“I noticed that there were ice floes on the Allegheny, but none on the Mon,” Brandfass said. Large accumulations of ice have, indeed, formed on the surface of the Allegheny River along Pittsburgh’s North Shore. But over on the South Shore, the Monongahela’s surface is its normal, watery self, free from ice.
The quick answer, according to PennWest professor of biology, earth and environmental sciences Andy Turner, is that the rivers are different temperatures. The Monongahela tends to run a little warmer than the Allegheny because it’s flowing north from a watershed that’s located south of Pittsburgh. The Allegheny is chillier, Turner said, because it’s flowing south from a watershed located to the north.
“One of the neat things about water is it has a memory, if you will, it takes a lot of heat to warm water up and then it holds its heat for a long time,” Turner said. “So even though it’s cold around the region, the water still reflects the average air temperatures that we had throughout the late summer and fall and early winter.”
The reverse of this is also true: in the warmer air temperature months of summer, the river water temperatures still run cold.
Turner said water is weird with its “density anomaly."
"In winter, at water temps less than 40 degrees F, warmer water tends to sink to the bottom and cold water sits on top, in both lakes and rivers, allowing ice to form on top," Turner wrote. "In rivers, however, because of turbulent flow, that warm bottom water gets mixed back up to the surface, slowing the freezing process relative to lakes."
Plus, the Allegheny flows slightly slower. Impoundments, or reservoirs formed by dams, can also contribute to different water temperatures. Rivers with more impoundments tend to be warmer. But in the case of the Allegheny and Monongahela, Turner said it's not really an issue, as the two have nearly the same number of lock and dams.
The Monongahela River does have a slightly higher salt content, however, which can be attributed to the minerals already in the water, as well as runoff from industrial facilities and road salt.
“But it’s not enough to significantly affect the water freezing point,” he said.
When the two rivers meet at the Point, their temperatures will start to average out over several miles as they converge to form the Ohio River.
