Restorers banking on reconnecting streams to their flood plains
When Mark Gutshall wants a glimpse of what mid-Atlantic streams looked like before European settlers arrived, he only needs to look outside his office window.
Along a 900-foot stretch of Lititz Run, his firm removed between 3 and 4 feet of “legacy sediment” that smothered the historical flood plain in the centuries after the region’s first farmers began tilling the soil.
The steep, unstable banks that once held the stream in a fairly straight path through the property was replaced with a broad flood plain just inches above the groundwater, through which the stream moves in broad, slow, meanders.
When it rains and the water level rises, the water quickly overflows onto the flood plain where it is retained, reducing damaging peak flows downstream. “This is the kind of look that our streams used to have in Pennsylvania,” said Gutshall, president of LandStudies, an engineering firm based in Lititz that specializes in stream restoration.
Such projects, often called “flood plain reconnections,” seek to mimic pre-settlement conditions by exposing the long-buried wetland flood plains which once dominated valley floors. The flood plain soil, filled with marsh grasses, is typically only about a half-meter thick, and rests above an older layer of gravel through which groundwater flows. “These plants have their feet wet all the time,” Gutshall noted.
More remarkably, the native plants didn’t have to be planted, Gutshall said. When the old wetland soils were exposed, the smothered seeds from sedges and other vegetation burst forth, rather than highly invasive nonnative species such as purple loosestrife and multiflora rose. “There’s not one invasive plant in here,” he said.
The technique is viewed by many as an effective means to deal with stream valleys laden with sediment that has accumulated for centuries. But the project outside his office cost $120,000 to restore 900 feet of stream and a portion of its flood plain. Other projects have cost hundreds of thousands of dollars to restore streams and their adjoining flood plains.
“It’s a practice that certainly warrants a significant look,” said Harry Campbell, a scientist with the Chesapeake Bay Foundation and a member of a state Department of Environmental Protection workgroup that is examining ways to mitigate legacy sediment impacts. “It is probably more beneficial ecologically than traditional stream bank restoration, where armoring is often an option, because you are creating wetlands and flood plains and you have biodiversity associated with the plant species and things of that nature,” he said.
But with limited public funds available to meet Bay cleanup goals, Campbell said a widespread application of groundwater reconnection projects is likely limited unless other means are found to pay for them. “Even if the science bears itself out to being a very good best management practice, when you look at the costs, you need to prioritize where you apply this because we simply don’t have the money to do it,” he said.
Gutshall said some costs of future projects might be offset through trading. Pennsylvania recently established a program that allows people to sell “credits” generated by removing sediment, nitrogen and phosphorus.
Right now, only the nutrients in the soil actually removed from the site are considered tradeable. But beyond the initial removal of sediment and any nutrients it contains, the restored flood plain can also remove pollutants in the future: Whenever water overflows the stream channel, the flood plain can trap sediment, while plants growing on it can absorb nutrients.
Scientists are now trying to estimate the amount of nutrients that such practices can remove. Eventually—maybe later this year—that could lead to such projects getting more nutrient removal credit.
Further, Gutshall said value can be added if such restoration projects were also able to get credit for their ability to temporarily retain stormwater runoff and create wetlands.
In addition, he said, restored streams and flood plain wetlands provide valuable wildlife habitat. As piles of sediment are removed and streams and their flood plains are reconnected with cool groundwater supplies, they can even support trout.
It’s also possible that a market may be developed for selling the nutrient-rich sediment that was excavated as topsoil. If all of those values are added up, Gutshall said, “this thing could potentially be generating revenue for somebody.”
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