How Deep Are Pittsburgh’s Three Rivers? Fort Pitt Bridge’s Secret Room Holds The Answer

Jun 7, 2018

Pittsburgh is known for its rivers. But many residents, like 90.5 WESA listener Judith Hoover, aren't sure where the bottom of each of the three lie. 

Hoover has a view of the Allegheny River from her North Side home, and during days of heavy flooding this past year, she and her husband watched the water level rise.

"And so it was because of the flooding that we wondered how deep the water is, for all the rivers," Hoover said.

Wayne Grgurich, a listener from Lawrenceville and an avid paddler, keeps an eye on the newspaper's regular listings of river stages before he goes out on the water.

"I'm curious who measures them and how they're measured," he asked.

Experts who study bodies of water say "depth" isn't exactly the correct term, because the bottom of the river is fixed but the level can fluctuate. Instead, they use the terms "river stage" and "gauge height." Matthew Kramar, a meteorologist with the National Weather Service, said to calculate the river stage, it's important to know the volume at any given stretch.

"The amount of water that a segment of the river can hold is dependent on the width, height and length of the river segment," Kramar said.

Imagine a four-lane highway. When it merges down to two lanes, traffic will back up and slow down, because the same number of cars are trying to fit into a smaller space. The river works the same way. As it narrows, water flowing at the same speed will back up and the river stage will increase.

The other measurement term, gauge height, has to do with two permanent tools affixed to the Fort Pitt Brige.

The two gauges running down the support beam of the Fort Pitt Bridge.
Credit Kathleen J. Davis / 90.5 WESA

Jamie McCoy, a hydrologic surveilance program chief with the U.S. Geological Survey, said the first gauge is called the bubbler system. It's a rusty pipe that runs down the side of a support beam into the water, and sits about a foot off the river bottom. It forces compressed air down the pipe, and released about 60 bubbles per minute. 

"The sensor in there can tell the higher the pressure on the end of that orifice line, we know the pressure going up and the river is going up," McCoy said. "If that pressure starts to decrease, the river is going down."

McCoy said this system is usually very accurate, within two hundredths of a foot. However, there's a backup plan in case the bubbler fails called a non-contact sensor.

"And what that does is use a radar pulse," McCoy said. "Every 15 minutes it shoots a pulse, bounces off the river and back, and sends the elevation to a computer inside."

This is part of our Good Question! series where we investigate what you've always wondered about Pittsburgh, its people and its culture.

The river stage in Pittsburgh is on average between 16 and 17 feet, according to McCoy, and that level stays consistent from the Point, to several miles on both the Allegheny and Monongahela rivers, and down the Ohio to the Emsworth Locks and Dams at Neville Island.

When the stage rises to 18 feet, the Mon Wharf parking lot downtown starts to flood. At 22 feet, the 10th Street Bypass on the Allegheny floods. At 25 feet, the Parkway East that hugs the Mon Wharf fills with water, earning it the nickname "the bathtub."

The gauge height reads at 17.21 feet on the bubbler system.
Credit Kathleen J. Davis / 90.5 WESA

There are controls to keep the river stage consistent most of the time. It's maintained most days between 16 and 17 feet thanks to a system of locks and dams, which exists to help boats navigate the rivers.

On the USGS website, river stage data is updated about every 15 minutes for both gauges. There's a little windowless room in one of the Fort Pitt Bridge support beams that houses the tech and computers that ping the gauge height data.   

McCoy said the USGS will probably have real-time river stage data available online within the next few years. That way riverside residents, avid kayakers and curious Pittsburghers can check on the status of the rivers that make Pittsburgh the city of rivers.