Michael Rahnis/Franklin and Marshall College
Geologists use high-resolution topographic data from airborne laser swath-mapping to gather information about a river's course. It can detect sedimentation and measure river elevations. Here, the elevation of the stream bed decreases from right (blue) to left (lavender-pink).
Geologists use high-resolution topographic data from airborne laser swath-mapping (LIDAR) to gather information about a river's course. It can detect sedimentation and measure river elevations. Here, the elevation of the stream bed decreases from right (blue) to left (lavender-pink) as the stream flows southward. The dam, still in place on Little Conestoga Creek, is at the point where blue changes to purple. Sediment has filled the pond behind the dam. Michael Rahnis/Franklin and Marshall College
Pa. Dept. of Environmental Protection/Courtesy Jeff Hartranft
Geologist Robert Walter says that deforestation contributed to the mud and sediment that has accumulated behind old dams. Early U.S. settlers deforested wide areas in the Northeast and Middle Atlantic States. This 1919 photo shows the Bender Mill dam near Millersville, Pa.
Geologist Robert Walter says that deforestation contributed to the mud and sediment that has accumulated behind old dams. Early U.S. settlers deforested wide areas in the Northeast. This 1919 photo shows the Bender Mill dam near Millersville, Pa. In recent decades, this stark landscape has been replaced by the regrowth of trees. The dam was built in the 1700s. It breached sometime before 1940, according to geologist Dorothy Merritts. Pa. Dept. of Environmental Protection/Courtesy Jeff Hartranft
Courtesy Robert Walter and Dorothy Merritts
Geologist Dorothy Merritts surveys river depth along Great Seneca Creek upstream of Watkins Mill, Md.
Geologist Dorothy Merritts surveying along Great Seneca Creek upstream of Watkins Mill, Md. All buff brown sediment under the grass canopy is historic mill pond sediment from the Watkins Mill dam, about 1 km downstream. Just under the water surface is a dark horizon that forms a ledge that protrudes out of the bank. The ledge is the original organic-rich wetland mud that formed the land surface at the time of European settlement and mill building, circa 1770 A.D. Courtesy Robert Walter and Dorothy Merritts
Courtesy Robert Walter and Dorothy Merritts
Tens of thousands of grist mills were built on streams in Pennsylvania and northeastern states in the early years of development in the United States. Here, Great Valley Mills on Valley Creek in Chester County, Pa., near Valley Forge.
Tens of thousands of grist mills were built on streams in Pennsylvania and other northeastern states in the early years of development in the United States, Here, Great Valley Mills sits on Valley Creek in Chester County, Pa., near Valley Forge. Courtesy Robert Walter and Dorothy Merritts
Billions of dollars have been spent in the United States on river restoration projects.
In many cases, the goal of these projects is a so-called "natural" river that curves broadly back and forth across a landscape.
But if a new paper in the journal Science is correct, there's not much that's natural about some of the curving rivers used as models for this restoration work.
The authors of the paper say that's because many eastern U.S. rivers weren't created by the forces of nature, but by Colonial farmers who built thousands of small dams across wetland areas in New England and the Middle Atlantic States. Geologists Robert Walter and Dorothy Merritts of Franklin and Marshall University in Lancaster, Pa., wrote the paper.
Walter says they first began to wonder five years ago whether winding creeks in the area were really all that natural. Merritts and some of her graduate students started pulling strange objects out of the bottoms of ancient-looking dirt embankments near the edges of the creeks.
"We found the stumps of giant trees that had been sawed down by European settlers," she says. "We found Indian artifacts and logged roads that the early settlers used to get across some of these marshy bottomlands."
Those finds made it look like the creeks had changed a lot over the past few hundred years. They also raised questions about whether the dirt embankments were really all that ancient.
These questions were answered by geologist Robert Walter, who is married to Merritts. He says tests on soil samples taken from the flood plain that surrounded the creeks showed that the riverbanks were hundreds, not thousands of years old.
Standing at the foot of a 20-foot tall embankment, he says, "basically everything you see above my ankles was deposited from 1730 to 1850 — 120 years."
In other words, the stream that had been here when European farmers first arrived is now buried underneath roughly 20 feet of mud. In retrospect, Walter says it's obvious where the mud came from.
He thinks it started washing down out of deforested areas and farm fields roughly 300 years ago. Then it started pooling up behind small dams the colonists built all over the region. More than 60,000 of these dams had been built by the end of the 1840s, Walter says, and, at one point, giant mill ponds formed behind all of them.
Eroded soils had filled most of these ponds to the brim by the start of the 1900s, Walter says. When that happened, the dams were abandoned and many were destroyed.
In the Science paper, Walter and Merritts argue that this is when many of the so-called "natural" winding rivers in the East took shape, as they cut deep channels through the leftover mud. They say that's why topographic photos of the region show that many flood plains look a little bit like giant staircases.
"Long flat stretches mark the locations of the filled-in mill ponds," Merritts says. "The sharp drops are where the dams were."
Merritts and Walter say their paper shows how quickly people can forget what natural landscapes used to look like.
David Montgomery, a geologist at the University of Washington, says the paper is also a cautionary tale for everyone involved in river restoration projects.
For example, says Montgomery, "if you are trying to restore rivers in a way that will benefit fish and other organisms — 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."
Basically, if you want to put Humpty Dumpty back together again, you need to know exactly what he looked like in the first place.
Merritts and Walter say it's likely that the landscape underneath a lot of winding rivers was dominated by broad wetlands full of tangled water channels. Walter says those broad wetlands are now all but nonexistent in the East. Whether they will be restored on a large scale is an open question, he says.