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ANDREA SEABROOK, host:

Time now for Science Out of the Box. This week: how the urge to return a landscape to its pristine state can be misguided.

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Billions of dollars have gone to restoring degraded American rivers to what's supposed to be their natural form. Often, the goal is to recreate a river that curves back and forth, winding beautifully. But according to a new study, the idea that a meandering river is natural maybe flawed.

NPR's John Nielsen met the authors of the new study near a winding creek in rural Pennsylvania.

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Mr. ROBERT WALTER (Geologist): We're along the banks of the Little Conestoga Creek, which flows into the main Conestoga. Now, we're going to walk along the right bank of the stream.

JOHN NIELSEN: Geologist Robert Walter used to think the Little Conestoga was about as natural as the creek can get. It's beautiful, it's tree lined and it wind past tall soil embankment that look like they've been around for thousands of years. In short, this is just the kind of stream that river restoration experts like to use as a model in this part of the East.

But then, about five years ago, geologist Dorothy Merritts, who is married to Robert Walter, started finding strange objects. There are even some of the tall banks that lined its stream.

For example, stumps from giant swamp trees cut by European settlers.

Professor DOROTHY MERRITTS (Earth and Environment, Franklin & Marshall College; Geologist): We also find log roads that early settlers built to get across these marshy bottomlands. They would cut down some of the younger trees' tall, straight trunks and lay them out in a row and lash them all together.

NIELSEN: Those finds made it look like Little Conestoga Creek had changed a lot over the past few hundred years. To find out more, Robert Walter took a bunch of soil samples and then ran tests that were designed to tell him whether the riverbanks were really as ancient as they looked.

He says it turns out that there is nothing at all ancient about big dirt embankments like the one we are now facing.

Prof. WALTERS: Everything that you see basically from my ankles to about 20 feet above it was deposited between 1730 and 1850, about 120 years.

NIELSEN: In other words, whatever the stream used to look like is now buried under 20 feet of mud. And in retrospect, Walter thinks it's obvious where all this mud came from. Three hundred years ago, they started washing down off of deforested hillsides and farm fields, he says. Then, it started pooling up behind small dams colonists had built to cover local grist mills.

He found one of those early dams nearby. A winding creek flowed over the top of it then fell 10 feet into some rocks.

Prof. WALTERS: What they did initially was probably filled a timber-crib structure and maybe fill that with sediment. And then as time went on, they reinforced it with limestone, the Conestoga limestone.

NIELSEN: Walter says more than 60,000 of these dams were turning Eastern Rivers into mill ponds by the end of the 1840s. But by the beginning of the 1900s, most of the ponds behind the dams were full of mud, and the dams themselves were abandoned or destroyed.

In the journal Science, Walter and Dorothy Merritts argued that this is when a lot of supposedly natural winding rivers actually got their start, by cutting deep channels down through the leftover mud.

Walter and Merritts teach geology at Franklin and Marshall College in Lancaster, Pennsylvania. They say the story of what happened at Little Conestoga Creek shows how quickly people can forget what natural landscapes used to look like.

Other river experts, like David Montgomery of the University of Washington, say the study is also a cautionary tale for everyone involved in the art of river restoration. He says the basic message of this paper is that if you want to put Humpty Dumpty back together again, you need to know exactly what he looked like in the first place.

Prof. DAVID MONTGOMERY (Geomorphology, University of Washington): If you are trying to restore rivers to benefit fish and other organisms in those rivers, if you're not just trying to make them look pretty — the best model for that is the system in which they evolved and thrived.

NIELSEN: Back in Pennsylvania, Robert Walter says the landscape under the Little Conestoga was probably full of swampy streams that split off into lots of little twisted channels. He says wetlands like this were probably found all over the East Coast at one time. Excavating them will not be easy.

John Nielsen, NPR News, Washington.

SEABROOK: Now, this election update. Hillary Clinton has won the Democratic caucuses in Nevada, besting Barack Obama by six percentage points. On the Republican side, Mitt Romney won an overwhelming victory in Nevada. The polls in the GOP primary in South Carolina have closed. John McCain and Mike Huckabee appear locked in a close race.

You can see results from South Carolina as they become available at npr.org/elections. While you're there, read up on what's at stake next week in South Carolina, and the upcoming presidential contest with our interactive election map.

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