Peculiar Pittsburgh

A Tale of Two Rivers: Why you can see the Monongahela and Allegheny merge into the Ohio

“[It] looked really, really dramatic because the Allegheny looked nice and clean and the Mon looked like melted chocolate ice cream.”

MJ Slaby / The Incline

There is no denying that Pittsburgh’s Allegheny and Mon rivers are distinct personalities; aquatic behemoths that merge to form a third, even larger waterway at the center of the city.

Lest you forget that, sometimes they remind us: just look at the dividing line between a very muddy Mon River and the much clearer Allegheny River beside it.

The Mon’s appearance drew at least one comparison to a Wonka-esque river of chocolate milk. But sweet tooths and Roald Dahl fans alike may be saddened to learn the cause behind this effect is far less magical and unusual — although no less interesting.

Lee Hendricks, a hydrologist with the National Weather Service in Pittsburgh, said this superficial split is due to a number of factors.

What lies beneath

First: The differing composition of the Allegheny and Mon’s riverbeds — the former being rockier and the latter being softer, siltier and more easily disturbed.

“The Allegheny River’s bottom is primarily rocky, and the river doesn’t pick up as much sediment,” Hendricks explained. On the other hand, he added, “The Mon River has a mostly smooth, silty bottom, and river turbulence brings it to surface.”

Brad Peroney, program development coordinator at the Carnegie Science Center in Pittsburgh, further explained that he’s also seen the roles and colors of the rivers reversed, with the Allegheny appearing muddier and the Mon remaining mostly clear.

‘Like melted chocolate ice cream’

The second factor is rainfall and runoff, which carries its own sediment and feeds that turbulence, Hendricks said. Just such a scenario occurred this weekend, with downpours in areas south of Pittsburgh that include Mon tributaries.

In some cases, river stages in the area hit the highest level in 17 years, Hendricks explained. The Cheat River, as seen below, and Marion County, West Virginia were among areas hit hard.

But even normal rainstorms can have a similar, albeit less-pronounced, effect where the rivers meet in Pittsburgh.

Peroney said the Carnegie Science Center’s windows overlook the confluence of the Allegheny and Mon, so while contrasting colors between the two rivers are a common sight for him — what occurred this week was particularly dramatic.

“This is something that happens pretty frequently here and pretty much anytime it rains we can see this happening,” Peroney said. “But [it] looked really, really dramatic because the Allegheny looked nice and clean and the Mon looked like melted chocolate ice cream.”

Peroney and Hendricks said it depends in large part on the weather upstream.

Hendricks explains: The Mon runs south to north from West Virginia. The Allegheny, north to south from northern Pennsylvania and southern New York. En route to Pittsburgh, the Allegheny crosses over granite and other igneous rocks, while the Mon runs through looser sandstone in its riverbed.

The Mon carries the silt and fine sand from that sandstone bottom with it, especially when its flow is intensified by rainfall and runoff. The Allegheny River’s bottom of gravel and coarser sand, meanwhile, isn’t stirred up or brought to the surface as easily.

Still, Peroney said, while all that sounds simple, it’s actually not. And the results are no less impressive to witness.

“There’s a lot going on whenever we see something simple like this,” he added.