Bay Journal: Removing Legacy Sediment Reduces Runoff Pollution, Restores Habitat
By Donna Morelli, Chesapeake Bay Journal
When Joseph Sweeney purchased farmland in Lancaster County, PA, in 2001, its fairly level pastures were typical of a traditional local farm — so were the steep, failing banks that strangled the stream as it ran through the property.
“I could jump across any part of our stream when we first bought the property,” Sweeney said.
About 10 years later, after a major stream restoration project involving government agencies, university researchers and consultants, things had changed.
Approximately 22,000 tons of highly erodible sediment — which originated as build-up behind a former dam — were removed from the creek bed and floodplains.
The marshy wetlands that had existed there before the land was settled were uncovered and restored.
Streambanks were no longer small cliffs. And the stream was running a much wider, slower course.
Now, nearly 20 years later, Sweeney is heading up the Water Science Institute, a new nonprofit organization, in an effort to map locations in Pennsylvania where similar projects may reduce large amounts of sediment pollution in local streams and potentially, downstream in the Chesapeake Bay.
The institute received a $269,000 federal grant in September from the Pennsylvania office of the U.S. Natural Resource Conservation Service to expand its Lancaster County mapping project to six other counties in Southcentral Pennsylvania.
The institute has engaged Franklin & Marshall College, a private four-year school in Lancaster, as a partner in the work.
“We’re looking for areas with high rates of erosion — a hot spot — usually behind a dam or where a dam used to exist,” Sweeney said.
Fine soil built up behind dams is known as legacy sediment. It’s a particular problem in Pennsylvania, where mill dams were once ubiquitous — on some streams, they were built every two miles.
As European settlers and the communities that followed cleared land for timber and agriculture, they loosened soil that washed into streams and mill ponds and became trapped behind dams. Layers of sediment built up over hundreds of years.
When the dams are breeched or removed, the contained sediment is set free, filling in the former mill pond and all but smothering the creek. The creek then cuts a deeper, narrower channel through the accumulated sediment.
The end result is a mini canyon — some of which can be as deep as 20 feet. When stormwater surges into the stream, sediment is washed from the banks, smothering habitat for aquatic plants and animals.
Sediment is one of three pollutants — along with nitrogen and phosphorus — targeted for reduction under the “pollution diet,” or Total Maximum Daily Load, that the U.S. Environmental Protection Agency imposed on the Chesapeake Bay watershed in 2010.
Dorothy Merritts and Bob Walter, researchers at Franklin & Marshall College who are participating in the institute’s mapping project, said they believe that the legacy of those mill dams may be responsible for more of the sediment polluting the Bay than runoff from fields and urban areas.
Mapping stream banks that have suffered major erosion is the key to pinpointing areas where large amounts of sediment can be removed in a stream restoration project — rather than letting it be scoured downstream during a storm.
The mapping project combines high-tech desktop reconnaissance with in-field measurements. Computer data is gathered from planes using mapping technology known as Light Detection and Ranging (LiDAR) — similar to high-resolution Google Earth, but in three dimensions with the ability to measure objects, and the distance between them.
“I can look at the LiDAR and take all of the trees away from a stream and see where a dam now exists or where the water drops suddenly where one may have been removed,” Merritts said.
The researchers compare older, highly detailed LiDAR images of a stream corridor with new ones. By measuring the “missing” stream bank material, they can calculate how much erosion took place over time.
The accuracy of this method is enhanced, Merritts said, by data generated from a flyover in 2014 that documented the pronounced erosion caused by Superstorm Sandy more than a year earlier.
The new grant will allow the researchers to add a more finely tuned version of LiDAR that can measure tree height and even the biomass of vegetation and trees in an area. Other improvements include adding tax parcel data and existing conservation practices.
Those additions allow practitioners to identify sites that need restoration or have intact, healthy riparian and wetland areas.
As municipalities look to clean up local waterways and the Bay, this project could help pinpoint where expensive stream restorations can yield the greatest reduction of sediment.
“This is going to be a really important tool from a planning standpoint to be able to identify where these critical locations are,” said Mark Gutshall, vice president of LandStudies, an environmental design firm that specializes in reconnecting streams with their floodplains.
“How much legacy sediment contributes to Bay degradation over time is a question that hasn’t been answered yet,” said Matt Johnston, an analyst for the Chesapeake Bay Program.
While stream erosion is harmful for fish and aquatic plants and insects, the degree to which it originates from the legacy of past land disturbance, as opposed to contemporary sources, is not known.
However, the Bay Program does recognize stream bank restoration projects as an important tool for reducing sediment pollution.
“In a floodplain corridor, we give credit for stream restoration practices that would stabilize those areas,” Johnston said.
The first phase of the institute’s research is on Chiques Creek, a tributary to the Susquehanna River in Lancaster County.
Five hundred potential pollution-reduction projects have been identified for the creek that need to be prioritized by a partnership of nonprofit organizations, restoration professionals and municipalities.
The group is trying to calculate how much pollution they can prevent by installing practices that are designed to curb erosion or capture sediment, nitrogen and phosphorus before they reach the creek.
“[Data from the stream mapping project] can help us identify high-priority restoration projects,” said Matt Royer, director of the Penn State Center for Agriculture and Environment, which is leading the Chiques Creek initiative. “We can fine-tune our list of projects, and we have a much better chance at getting a high sediment load reduction.”
Sweeney, of the Water Science Institute, said the project will also examine the relative merits of stream restoration in terms of cost per pound of sediment removed. He thinks stream restoration, which requires expensive construction, may prove to be more cost-effective for reducing large amounts of sediment than other best management practices, or BMPs, such as building large vegetated ponds to capture sediment-laden runoff.
“We consider stream restoration to be the most cost-effective and efficient when combined with other BMPs,” he said.
It will take large, expensive projects to remove the amount of pollution needed to meet cleanup goals for the Bay, Sweeney said. “And at the end of the day, it’s about scale. We are not going to do this with shovels. We’re going to need bulldozers.”
(Photo: Before and after restoration at Bedford Springs Resort, LandStudies.)
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(Reprinted from the Chesapeake Bay Journal.)
[Posted: March 23, 2018]
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