Mystery lies in old millponds

Digging shows the forgotten ways in which people have changed Piedmont creeks

CorrespondentMarch 22, 2010 

  • Name: Karl W. Wegmann

    Age: 36

    Job: Assistant professor of geology, Department of Marine, Earth, and Atmospheric Sciences, N.C. State University

    Home: Raleigh

    Family: Married, two children

    Hobbies: Hiking, ski mountaineering, photography, canoeing

    Why do you do science? I am a naturally curious individual. I want to understand the "why" to how the natural world functions, and a career in the natural sciences is a path to this end.

  • Geologist Karl Wegmann's project at the Piedmont's old mill sites also includes biology and history. The team is looking at the life of plants that left their traces in the sediment. And a photographer is capturing the history.

    To read the Science Magazine article on Wegmann's work: cgi/content/abstract/319/5861/299

    For a map of old mills in Wake County:

    Watch a video of Wegmann and his work:

Standing on flat ground in an oxbow of Richland Creek in northwest Raleigh, Karl Wegmann scans a small valley surrounded by low hills. Earth carpeted by brown leaves, gray trunks and boughs of leafless trees emerge from the broad floodplain where the creek has carved a deep ravine.

But is that really floodplain? And did that canyon truly result from creeks turned to torrents by the stormwater runoff of modern development?

"People would think this is the floodplain," Wegmann says. "But it's not. It's a lake bottom."

Wegmann, a geologist at N.C. State University, is studying this creek system and other millponds in North Carolina to learn what nature intended for waterways before humans intervened to alter their courses.

His work could help keep important waterways from clogging with pollution. The Environmental Protection Agency calls urban stormwater runoff the most serious pollutant to estuaries, and one of the most significant sources of pollution in lakes, hurting drinking water sources and damaging wildlife.

To address runoff's effects on waterways, a stream restoration industry has emerged, trying to return streams in developed areas to their natural state. The problem is, people no longer agree on what that natural state was. That's why Wegmann - in waders, baseball hat and a bright orange vest - splashes around in Richland Creek on the border of William B. Umstead Park. He pokes the banks with a shovel, trying to answer the question: "What is a natural stream in, say, the Piedmont of North Carolina?"

Wegmann is standing upstream from what was once a grist mill - Cook's Mill, which shows up on Wake County maps as early as 1871 but probably operated for years before that.

According to the 1840 U.S. manufacturing census, more than 65,000 such water-powered mills peppered the creeks and streams of the Eastern United States. Early residents built mills on small creeks and streams to grind grain, while textile mills sprouted on nearly all the state's main waterways - the Catawba River, the Deep River, the Haw, the Tar, the Yadkin. Water was the oil that powered the Piedmont.

Each mill had a pond, and each pond filled with sediment - very rapidly, considering the kind of clear-cutting and farming the original settlers practiced upstream, releasing large amounts of sediments that built up in the ponds.

When the dams eventually burst or were dismantled, the creeks quickly carved their way back down through the sediment deposits, leaving the flat valley bottoms cut by deep river ravines with which we're all familiar. Wegmann and his colleagues call those millpond deposits "legacy sediments."

These sediments now interest scientists - and not just Wegmann, who quickly directs attention to a 2008 paper in Science magazine concluding that "most floodplains along mid-Atlantic streams are actually fill terraces, and historically incised channels are not natural archetypes for meandering streams."

That paper started the discussion, and there's nothing like consensus. But scientists including Wegmann are now researching these millponds for many reasons. One, Wegmann said, is to figure out. "What is a natural stream of the Piedmont?"

The problem is, Wegmann says, "we've so modified things that we don't even recognize that we've modified them. So that's one way this research could be useful: to change the way we think about stream restoration."

That natural stream would look a lot more like the terrain downstream from the forgotten mill than the terrain upstream, Wegmann said, looking at the flat expanse where he stands high above the creek.

Downstream, across Ebenezer Church Road, stand the picturesque ruins of the mill. Further along, the mill's retained water rejoined the Richland. The creek is rocky and burbling, running between banks a couple of feet above its surface - similar to the bank depth that may have appeared before European settlers arrived and started building dams.

"The hypothesis is that these were forested wetlands" before Europeans arrived, Wegmann says.

Wegmann envisions they were not just the pine and hardwood forests we presume they were, but mostly "sedges, ferns, water-tolerant trees," he says. Their flows commonly spread out into wetlands or multithread channels, occasionally dammed by beavers - not limited to the single-channel, gravel-bottom streams we see today and to which most stream restoration aspires.

Wegmann and his colleagues - students Kelly Johnson and Robert Lewis, plus geophysicist Del Bohnenstiehl, also of N.C. State - study the behavior of the sediments by checking water quality near the site of the old dam. They also check at the point furthest upstream - called the reach - of what would have been a pond of about 20 acres.

Johnson wades into the water, pouring water samples into a tall tube called a turbidimeter to measure suspended sediment. He compares samples taken upstream - before the Richland reaches the legacy sediments - with those taken at the dam site, after it's traveled through all the silt and clay.

These are baseline measurements that will be compared with those taken during times of high flow.

Gray Hauser, a sedimentation specialist with the state Department of Environment and Natural Resources, says Wegmann's research has applications down the road, "understanding and looking at how streams and their floodplain systems are supposed to work."

Hauser says waterways are not just affected by mills, but also by deforestation for agriculture.

Wegmann's work has dovetailed with that of photographer Doug Swords, who has been trying to document old mill sites before they disappear. Introduced to Wegmann by Umstead Park staff, their partnership has yielded an updated map of mills in Wake County and a variety of historical information Swords has donated to the county library.

"I have come to view these mills as servants," Swords says. "They worked for us when they were needed and were then discarded." He plans to update his map every year and still seeks more sites, using Wegmann's sedimentation profiles to help verify sites.

Wegmann also said he believes his work can provide a greater understanding of storm history - a science he calls paleotempestology.

"We're thinking of using these old millponds as archives of old storms," which leave behind deposits called sand lenses. During storms, streams wash down heavier sediments, which settle more quickly, leaving a record of major storms in sandy layers.

"This is an archive of Wake County," he says of the layers of sediment in the old pond. "These millponds may allow us to get a couple-hundred-year record of how our watershed is responding to settlement."

Biology shows up, too. Buried in sediments are well-preserved evidence of what used to be there. On Sycamore Creek in Umstead Park, Wegmann says, they found 300-year-old leaves "that still had color."

He shovels down into the base of the steep stream bank to show the gray sediment he said was probably deposited more than 500 years ago, which contains seeds and pollens from the plants that lived before the millpond sediments buried that earth.

"These millponds might be seed banks for pre-Colonial Piedmont ecosystems," he says.

So Wegmann keeps digging, searching for an entire ecosystem, buried for centuries, and hoping to raise it once again.

Scott Huler:

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