Pittsburgh Boasts A Trio Of Tributaries, But Are There Really Three Separate Rivers?

Apr 24, 2017

Next to steel and Super Bowl championships, Pittsburgh is synonymous with three rivers. In the summer, the Three Rivers Arts Festival dominates downtown and the moniker is part of a number of companies in the region -- not to mention there used to be a stadium that bore the name.

But does the city technically have three distinct rivers?

Lorraine Caplan, of Squirrel Hill, and Ken Jakub, of McCandless, wondered, too. Both asked if the Allegheny and Monongahela rivers truly formed a new waterway with the Ohio.

Clarion University Aquatic Ecologist Andrew Turner, an expert in limnology, the study of bodies of fresh water, said technically, no, there are not three separate rivers.

“That’s just a way of breaking things up to make it easier to organize,” Turner said. “That’s an attempt to sort of put a human structure on a natural process.”

Turner said changing the name of a river is usually related to a region’s history or the culture surrounding the waterway. The Ohio, for example, means “Great River” in Seneca and Iroquoian.

Boats from the Gateway Clipper fleet float at Pittsburgh's confluence. Pittsburgh has always been known for its three rivers, but some water experts say geologically, there are two.
Credit Keith Srakocic / AP

But if rivers were named based on limnologic factors alone, Turner said downstream waterways would more likely bear the name of their largest tributary. 

At the confluence of the Mississippi and the Ohio Rivers in Cairo, Ill., the Ohio is 281,500 cubic feet per second, while the Mississippi just upstream in Thebes, Ill. is 208,200 cu ft/s. Following the Ohio upstream to Pittsburgh, the Allegheny is about 19,750 cu ft/s, while the Monongahela is only 8,433 cu ft/s.

By that measure, the Mississippi River could actually be called the Allegheny.

Turner said he recognizes that the method can be confusing, considering it’s rare to name rivers this way, but it’s fun to think about. He said the U.S. Geological Survey, which names and documents rivers, won’t be changing the name of the Mississippi any time soon.

“They heavily weigh the traditional, cultural names of rivers in making those choices,” Turner said.

This map from the U.S. Geological Survey shows different regions of the hydrological unit code, another method of categorizing rivers and other waterways. Pittsburgh's rivers are in the Ohio drainage basin.
Credit U.S. Geological Survey

Another method used by limnologists is the hydrological unit code, or, HUC, system, in which rivers are assigned codes based on regions, watersheds and tributaries. The Upper Allegheny River’s HUC is 05010001, while the Upper Monongahela’s is 05020003.

Turner said that while the HUC system makes a lot of sense, it “hasn’t really caught on outside of scientific circles.”

Rivers could also be named by following their lengthiest tributary upstream. 

With that example, the Madison River in Yellowstone National Park, Wyo., at 183 miles, would be the headwaters of the Mississippi, and therefore warrant its namesake.

So barring any striking geological evidence, Turner said there will always be three rivers in Pittsburgh.

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