Long-awaited project to stem tide of erosion at DC area marsh
Reconstruction of promontory will be first step in restoring Dyke Marsh, its habitat, water-filtering function
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For hundreds who live within walking distance of the George Washington Memorial Parkway, the scenic road is more than a stoplight-less route from Mount Vernon to Washington, DC.
Its expansive views of the Potomac River, with trails winding through and alongside a cattail-filled Dyke Marsh, make it the daily stomping grounds of dog walkers, marathon runners and bicycle commuters. But the most observant among them might notice that the views have been changing — disappearing, actually.
For more than a half century, the marshy portion of this respite from city life has been shrinking at an ever-increasing rate. Today, Dyke Marsh loses about 1.5 acres per year as the shoreline retreats an average of 6–8 feet. If nothing is done, the marsh will be gone in 20 years.
Brent Steury had a sinking feeling that that might be the case when he first started working on the project a decade ago. As the natural resources program manager for the George Washington Memorial Parkway, which manages Dyke Marsh Wildlife Preserve, Steury started writing proposals to fund an environmental impact statement for rebuilding the marsh his first day on the job.
A decade later, the EIS is complete, the money is committed and the National Park Service is working with the U.S. Army Corps of Engineers to design a project that should start construction in the summer of 2017.
In another decade — or perhaps just five years — the largest remaining freshwater wetland in the DC area could be returned to its former, water-filtering glory, looking something like it did when George Washington first laid eyes on it.
When Washington surveyed the landscape in the mid-1700s, he deemed the wetland in Mount Vernon’s neighborhood a “fine improvable marsh,” according to a sign that quotes him along the marsh’s trail.
Even in its degraded state, the marsh is a sight for sore, city-worn eyes. On a recent mid-November morning, families walked their dogs along paths winding through late fall foliage. The tide was about midway through its twice-daily retreat from the shore, leaving behind evidence of the marsh’s slow changes. One portion of the trail showed traces of debris and sediment washed up during storms and high tides. And each year more trees and brush along the trail succumb to the encroaching waters.
Along both sides of a wooden boardwalk that stretches into the marsh, narrow-leaf cattails provide habitat for the only known breeding population of marsh wrens in the region, and around 270 other bird species. The boardwalk also offers views of National Harbor, the Woodrow Wilson Bridge and, to the south, undeveloped shorelines that George Washington might have taken in from his Mount Vernon home. Though it was built quite literally on shifting sands, the boardwalk — if not yet the marsh — was secured with steel and concrete footings after Hurricane Isabel.
While improving the marsh wasn’t the goal, the natural resource was put to what might have been called “better use” almost two centuries after Washington surveyed the wetland.
In the three decades leading up to 1970, commercial dredging for sand and pea gravel steadily chipped away at an 80-acre forested promontory to the south of the marsh. The dredging continued even after the parkway became a National Park Service holding and until the end of a 30-year lease for mining on the site.
It wasn’t until a 1974 law required the Park Service and the Corps to “implement restoration of the historical and ecological values of Dyke Marsh” that the painstaking process to do so began.
By then, the promontory that had protected Dyke Marsh from the powerful Potomac River and northbound storms for an estimated 2,000 years was gone. But the directive to protect Dyke Marsh without a barrier peninsula didn’t come with the funding to do so, Steury said.
It wasn’t even exactly clear what was causing the erosion when Steury came on board, so he called for a study by the U.S. Geological Survey. Flooding from the Potomac River was long thought to be the cause of the marsh’s precipitous decline, but the study showed it wasn’t.
Rather, the removal of the promontory — and dredging that changed the river bottom — was the trigger for degradation, exposing the tidal marsh to storms racing up the Potomac River with increasing frequency and power.
“Once that (promontory) was removed, these storms released their full force right on the marsh, tearing it up,” Steury said.
Some years were worse than others, he said, with Hurricane Isabel doing the most damage in 2003. Hurricane Sandy in 2012 hurt too, although the mitigation funds that came in its wake now are paving the way for the marsh’s restoration.
A $25 million grant from the Hurricane Sandy Mitigation Fund will provide “enough money to get a good start on” Dyke Marsh’s restoration, Steury said.
The work — or perhaps the prework — started this fall with geotechnical drilling that the Corps will use to determine how to best rebuild the protective promontory that once jutted into the Potomac just south of the marsh. That’s step one, Steury said, because “we need that protection” before any rebuilding of the marsh can begin.
Though previous generations might not have seen the marsh or its protective peninsula as more than sandpits, we now know them to be one of the Chesapeake Bay’s most important ecosystems. Tidal marshes like this one filter and clean the water while slowing its flow and providing habitat for hundreds of species.
That’s especially important in the Potomac River, “the drinking water source for almost everyone in the DC metro area,” Steury pointed out.
“There aren’t many tidal marshes left in the DC area anymore,” he said.
The nearest to the south would be Piscataway Park. In the District of Columbia, the Park Service has also worked to restore marshes on the Anacostia River near the man-made Kingman Island.
Once the 80-acre promontory is reconstructed, the marsh will be reconstructed one piece at a time using containment cells. The cells are basically underground boxes that will be driven into the river bottom with pilings and filled with clean sandy material to the target elevation of the historical marsh. The narrow leaf cattails that currently anchor the marsh will be planted in the new sections as well.
The fledgling plantings will be monitored to ward off invasive plants such as phragmites. Enclosures will also protect the new plants from snacking geese until they are established, a practice the Park Service has used when planting grasses in the Anacostia River.
The reconstruction phase of the project is expected to start in the summer of 2017 and last four years. The goal is to return Dyke Marsh to a stable state so it can sustain itself as it did for hundreds of years before the promontory was removed. Sea-level rise might have something to say about its “stability,” Steury acknowledged, though the project goals are factoring in a rise of 3 millimeters per year.
“Should things change or become worse, that changes the equation,” he said.
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