Dr. Katharina (Katia) Engelhardt of the University of Maryland spoke to the Friends of Dyke Marsh about her research at the wetland and the prospects for its restoration. Dyke Marsh constitutes about 200 hectares of tidal freshwater marsh on the west bank of the Potomac River, just south of Alexandria, Va. and the Beltway. The marsh, as a wildlife preserve, is part of the George Washington Memorial Parkway lands, administered by the National Park Service. The NPS is considering acting on long-discussed restoration and management plans for Dyke Marsh.
Over the decades, parts of the marsh have been overtaken by the river. Some of the causes are global—worldwide sea level has been rising at the rate of 3.1mm/year and the Chesapeake Bay at twice that rate—and some are, we suspect, local. A bridge over Hunting Creek, just upstream of the marsh, as well as other development and urbanization, has changed siltation patterns, perhaps starving the marsh of sediment. Dr. Engelhardt stressed that very little good erosion data were available for the area, so we don’t really know how accretion nets out against sea level changes.
Dr. Engelhardt’s research turned up a couple of surprises. Though much of the ground lies in the range of elevations from 0.3m to 0.8m, there is much bumpiness in the terrain. Tidal channels, rather than following a hierarchical flow from lower to higher orders, instead form a complex web of cross-linked flows. Her focus on the botany was limited to the emergent herbaceous vegetation. Even though tidal freshwater wetlands tend to be species-poor, nevertheless she and her research students found representatives of eleven taxa, annual and perennial, including spatter dock, wild rice, and cattails. (Invasives tend to be more prevalent in relative upland of the woods, which was not the focus of her studies.)
Three restoration scenarios were outlined:
- Aggressive pumping and dredging to restore the marsh to its full extent from the first half of the twentieth century.
- Partial restoration, perhaps in a shallow area below Hog Island, which was designated as a demonstration area in a 1977 study, with the possibility of further efforts.
- Shoreline erosion control only.
Whatever we do, Dr. Engelhardt said, we must make sure that the effort is sustainable, that is, that future natural accretion is sufficient to maintain the marsh.
