i found this article about our "4th river" and thought i'd share...










Fear not, 'fourth river' unaffected by tunneling



Monday, June 16, 2008
By Mark Roth, Pittsburgh Post-Gazette
Here are the facts:

A) There is a fourth river under Pittsburgh.

B ) It's not what you think it is.





The "river" is actually a layer of sand and gravel, saturated with water, that was laid down 12,000 years ago at the end of the last Ice Age, says John Harper, a geologist with the state Department of Conservation and Natural Resources.

This glacial plain, also known as an aquifer, is 30 to 85 feet thick, Mr. Harper said, and lies beneath the Golden Triangle and below the riverbeds of the lower Allegheny and Monongahela rivers and the upper Ohio River.

Questions about the so-called fourth river have come up persistently since the Port Authority began drilling two transit tunnels from the North Side to Downtown, beneath the Allegheny River.

Will the tunnels run into the fourth river?

One of the aquifer's special functions is to provide the water for the Point State Park fountain, through an artesian well that is sunk into that soppy layer of earth.

Will the tunnels endanger the water supply to the fountain?

The answers: Yes, and no.

Winston Simmonds, Port Authority operations manager, said the tunnels will intersect the glacial layer on both the North Shore and Downtown, but beneath the Allegheny itself, the tunnels will go through denser clay stone and silt stone.

In fact, the first tunnel already has bisected the "fourth river" layer on the North Side and is now nearing the Downtown bank of the Allegheny, 25 feet or so below the riverbed.

"We performed our own geotechnical study," Port Authority spokesman David Whipkey said, "and found out that the aquifer in question ... does not affect us. We have not been impacted by the aquifer whatsoever, and do not anticipate any future impact."

On the Downtown side of the project, the tunnels will pierce the glacial layer well away from the area from which the Point State Park fountain gets its water.

About 10,000 B.C., when the last glaciers in the region melted and receded northward, water flowed southward to create the current Allegheny and Ohio river valleys, Mr. Harper explained. Going along for the ride were sand and gravel that had been trapped in the glaciers, and that is what formed the glacial plain.

Over time, layers of sediment built up on top of the sand and gravel, forming the riverbeds and the Golden Triangle.

Today, water circulates constantly between the rivers and the aquifer. Most of the water flowing in the rivers at the Point, Mr. Harper said, has probably come from upstream, but a portion of it is being discharged into the rivers from the aquifer.

In a typical forest stream, those proportional sources are reversed, said hydrogeologist Daniel Bain of the University of Pittsburgh. Those babbling brooks often get only 5 percent of their water from the surface, and 95 percent from underlying groundwater.

The pressure in the aquifer is strong enough to feed the Point fountain without a pump, although powerful pumps are used at the surface to shoot the water 150 feet into the air.

As the Port Authority drills the twin 2,240-foot tunnels to extend light-rail service to the North Shore, it is making the cylindrical holes slightly bigger than the tunnel walls, and then using cement grout to fill the space between the earth and the outside of the circular walls. That not only helps stabilize the tunnels, but provides an extra barrier against any moisture seepage from the surrounding earth.

And the fourth river is nothing to worry about, Mr. Simmonds said, because, well, it isn't a river. There is no free-flowing stream that you would stumble across if you went spelunking below the Triangle.

That throws a wet blanket, so to speak, on one other fourth river rumor.

After a World War II-era B-25 bomber crashed into the Monongahela River near the Homestead Grays Bridge in 1956, rumors began circulating that the reason the plane had never been found was that it ended up in the fourth river.

It now appears the only way that could have happened is if the crew flew through a time warp and landed here 12,000 years ago.

Mark Roth can be reached at mroth@post-gazette.com or at 412-263-1130.
First published on June 16, 2008 at 12:00 am
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