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How ancestral rivers carved up — and flattened — Pittsburgh

A white man smiles while pointing at a topographic map of the Pittsburgh region.
Jakob Lazzaro
90.5 WESA
Charles Jones, a professor in the University of Pittsburgh Department of Geology and Environmental Science, says flat land in Pittsburgh that's not right along a river is usually an ancient river bed. The largest example, in the East End, marks where the Allegheny and Monongahela once flowed.

Pittsburgh is a city of hills and valleys. Flat land is a rare commodity, usually limited to the areas hugging the curves of the Allegheny, Ohio and Monongahela rivers. But there’s one major exception — the East End.

If you look at the city using a topographic map, an arc of flat land is visible starting where Swissvale meets the Monongahela and curving up through Regent Square, Wilkinsburg, Homewood, Larimer and Point Breeze before reaching East Liberty and Shadyside.

A topographic map of the greater Pittsburgh area.

Here, the curve splits in three — one branch continues through Oakland to rejoin the Monongahela, another heads north into Highland Park and Stanton Heights before reaching the Allegheny, and the third heads through Friendship, Bloomfield and Central Lawrenceville before it also reaches the Allegheny.

Although it is now isolated hundreds of feet above the rivers, this flatness is actually an ancient riverbed marking where they once flowed, created millions of years ago by the wandering paths of the ancestral Allegheny and Monongahela.

River history

Currently, the Monongahela and Allegheny flow into the Ohio, which flows into the Mississippi and eventually into the Gulf of Mexico. But that hasn’t always been the case — millions of years ago, the Allegheny was actually split into three different rivers.

According to the book “Geology of Pittsburgh”, scientists hypothesize that the upper and middle portions flowed north into the ancestral Lake Erie drainage basin, while the lower Allegheny and Ohio rivers were tributaries of the Monongahela, which then also flowed north through Beaver, Pa. and Youngstown into the ancestral Lake Erie drainage basin. These rivers flowed in shallow valleys in a rolling plain, and would sometimes change course.

A map of Western Pennsylvania's rivers before glaciation.
J.A. Harper, "Of Ice and Waters Flowing: The formation of Pittsburgh’s Three Rivers," Pennsylvania Geology, Vol. 28, No. 3 of 4
Before glaciation, the Allegheny River was split into three separate sections. Each, along with the Ohio and the Monongahela, flowed north into the ancestral Lake Erie drainage basin.

“If you get a map and you look at the major rivers, the Monongahela River, the Youghiogheny River and the Allegheny River, you'll see they wind back and forth much like a meandering river,” Charles Jones, a professor in the University of Pittsburgh Department of Geology and Environmental Science, says.

Jones says a modern example of a meandering river is the lower Mississippi, which wanders back and forth across its floodplain, creating a loopy, squiggly course. Over time, sediment builds up on the insides of those long bends while erosion increases on the outsides. Eventually, when floods or high water levels come through, the river changes course. The water takes a more direct path, cutting off the previous loop and leaving oxbow lakes or flat (formerly riverbed) areas behind.

“And then the river will go back to developing new loops, and over and over again, the river just keeps developing loops, cutting them off, developing loops and cutting them off,” Jones said.

The ancient Allegheny, Monongahela and Ohio rivers acted similar to the modern Mississippi — meandering back and forth across what’s now the East End, and leaving behind that arc of flat land as a remnant of their course.

But today, things are different.

“We have rivers that are loopy, like meandering streams, but they're stuck down in deep valleys,” Jones said. “I like to call them fossil meandering rivers, because they're really not so active anymore.”

And we owe that change to the glaciers of the last ice age.

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The Pleistocene reshapes the landscape

The Pleistocene, or quaternary, glaciation began about 2.5 million years ago and saw numerous advances and retreats of massive ice sheets in North America. It’s certain that the two most recent — the Illinoian and Wisconsin glaciations — made it to Western Pennsylvania, but never all the way to Pittsburgh. Instead, the ice sheets blocked the northward flow of all of the rivers and created new, massive lakes at their base.

Eventually, the water in those lakes rose high enough to cut through the drainage divides and carve a new one flowing southwest parallel to the ice, combining the three Alleghenies into one river that flows into the Ohio, and then into the Mississippi, like it does today.

As the ice sheets advanced and retreated again and again, massive amounts of meltwater, silt sand and gravel would flow into the rivers, carving the river valleys hundreds of feet deeper and creating the Allegheny, Ohio and Monongahela as we see them today.

A map of the former river channels of the Monongahela, Allegheny and Youghiogheny rivers around Pittsburgh.
L. Heyman, "History of Pittsburgh’s rivers," in Wagner, et al (editors), 1970 Geology of the Pittsburgh Area, General Geology Report G59, Pennsylvania Geological Survey
A map of the former river channels of the Monongahela, Allegheny and Youghiogheny rivers around Pittsburgh.

“Even though these are fossil meandering rivers, occasionally, as they're cutting down deeper and deeper into the landscape, they will occasionally cut off a loop,” Jones said. “And so the flat areas around Pittsburgh that are not right next to a stream channel tend to be these abandoned meander loops.”

When the ice sheets retreated from Pennsylvania for the last time around 18,000 years ago, the rivers once again became valley-spanning conduits for massive amounts of meltwater, silt, sand and gravel.

“When all this water was flowing down, they carved wide valleys,” Jones said. “And then today the rivers are, you know, maybe a half or a third of the total width of the valley.”

But eventually, the glaciers were gone.

“As the flows began to wane, as you didn't get as much water flowing down, quite a bit of sediment was just sort of trapped,” Jones said. “Then, the river just snakes between the sediment sort of left over, and that's what the rivers are today.”

That final deluge of sediment became the underlying base of the flat lands that now run along the rivers — think Downtown, the Strip District, the South Side Flats and the flat portions of the North Side.

Human geography

Before Europeans arrived in Western Pennsylvania, most of the region was old growth forest and the old river beds had little, if any, impact on how people used the land.

Justin A. Meinert, the education and living history manager at the Fort Pitt Museum, says the greater Pittsburgh region was inhabited by what we call the Monongahela people. Their later village sites were built up on hills, away from water, but they disappeared from the archeological record around the 1630s, likely due to disease or war with the Seneca, who came down into Western Pennsylvania from New York.

By the 1720s, Delaware people were also moving into the area. Originally from New Jersey and Long Island, they were forced west by colonial expansion. Meinert says that typically, Delaware and Seneca villages would be built along a riverbank, but above the floodplain, while their cornfields would be down along the river in the fertile soil.

“Most of these village sites, they only last about 25 or 30 years. And then from then on, most of the resources are used up immediately in the area,” Meinert said. “The town is usually abandoned, and then they move somewhere else.”

After the American Revolution, Meinert says many people were granted land in Western Pennsylvania as a form of payment for serving in the Continental Army. Some sold their parcels, while others moved in and began farming them. Flatter land was more desirable for that purpose, so it’s likely the flatter portions of the East End became farmland.

By the late 1700s, Downtown Pittsburgh was an urban settlement, but the rest of the region was either farmland or still forested, depending on who owned the land. Pittsburgh city archivist Charles Succop says urban expansion from the Golden Triangle began in the 1830s.

“Our first annexation was called the Northern Liberties, which is part of what we know now as the Strip District,” Succop said.

A map of Pittsburgh in 1904.
1904 United States Geological Survey
Pittsburgh in 1904. At this time, urbanization in the East End clearly follows the curve of the ancient riverbed.

From there, Pittsburgh went on to annex many more municipalities to the east. In 1868, the city annexed the townships of Pitt, Peebles, Liberty, Collins and Oakland, and the Borough of Lawrenceville. This incorporated what’s now the East End into the city.

Subsequently, the agricultural East End was transformed — first into large mansions and estates for the wealthy, and then into newly built neighborhoods. But development came to the flatter areas first.

As an example, Succop says flatter Shadyside had been mostly developed by 1890, while Squirrel Hill North just across Fifth Avenue was still large mansions and farms. Additionally, Greenfield and the southern portions of Squirrel Hill remained rural until the early 1900s.

A black and white photograph of a dirt road going up a hillside.
Pittsburgh City Photographer Collection, 1901-2000
Historic Pittsburgh
The corner of Phillips and Murray Avenues in Squirrel Hill on October 21, 1908, looking east. A billboard on the left advertises a Shady Avenue plan with large lots from $1000.

Although the East End wasn’t flattened by hand, Succop says urbanization did change the geography of Pittsburgh in one big way. There used to be more ravines and runs, but many have been filled in.

One example is Schenley Plaza — it used to be a very large ravine stretching from the main Carnegie Library to Forbes Field, but it was filled in with dirt from the removal of Grant’s Hill downtown in 1911.

“They even had to build a bridge to cross the ravine right there, and that's all since been filled in,” Succop said. “And as far as I know, instead of deconstructing the bridge or blowing it up, they just filled it in. So the bridge still exists underground.”

Jakob Lazzaro is a digital producer at WESA and WYEP. He comes to Pittsburgh from South Bend, Ind., where he worked as the senior reporter and assignment editor at WVPE and had fun on-air hosting local All Things Considered two days a week, but he first got to know this area in 2018 as an intern at WESA (and is excited to be back). He graduated from Northwestern University in 2020 and has also previously reported for CalMatters and written NPR's Source of the Week email newsletter.