RiverQuestion of the Month

for May 2008

Compiled by Jennifer Robertson, M.Ed.

RiverQuest Education Specialist



Pittsburgh has three rivers, right?  Sometimes you hear about a mysterious fourth river that flows, unseen, beneath the city.

Is there really a fourth river in Pittsburgh?



No.  The alleged underground river is actually a sand and gravel aquifer with some river-like properties. Formally, this aquifer is a part of the Wisconsin Glacial Flow.

Prior to the last great ice age, deep river valleys were common in what is now western Pennsylvania.  During the Pleistocene Epoch (the geologic time period between 1.8-million and 10,000 years ago) glaciers moved into northwestern Pennsylvania altering the northerly flow of many of western Pennsylvania’s rivers and streams.  Glacial melting filled western Pennsylvania’s deep valleys to the brim with rushing water at the end of the Pleistocene Epoch.  The flow of water slowed after the ice was fully melted and the climate stabilized.  Sediments, primarily sand and gravel in this area, settled into the valley bottoms.  It is in these highly porous sediments that much groundwater is confined and flows as an aquifer.

The Wisconsin Glacial flow is one such aquifer, found in the Ohio and Allegheny River valleys.  Drinking water wells in Pittsburgh and surrounding areas have been tapping into this aquifer for over 140 years.  Water pumped from the aquifer is used to replace volume lost during normal operation of the fountain at Point State Park.  The sands and gravels of the aquifer are remnants of glacial activity, while the water flowing through them is recharged via the hydrologic cycle--the water cycle that moves water between the surface, subsurface and atmosphere through processes that include evaporation and precipitation.

The seemingly never ending supply of water, the high permeability of the sand and gravel glacial bed, and the channel created on its sides and bottom by non-porous rock gives the aquifer some river-like qualities. However, there is no openly flowing channel of water beneath the surface of Pittsburgh.



  • Fleeger, G.M. (1999). The geology of Pennsylvania’s groundwater (3rd ed.): Pennsylvania Geological Survey, 4th ser., Educational Series 3, 34 pp.



  • Pittsburgh in the morning mist, by educator Judy Eltschlager, taken from onboard RiverQuest's Discovery.

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