Innovations in Tidal Marsh Restoration

dikes. I t is not often that a project allows a company to experiment with innovative restoration techniques. Our firm, Biohabitats Inc., was fortunate enough to have such a chance on the Kenilworth Marsh, a 28-hectare (70-acre) freshwater tidal marsh along the borders of the Anacostia River near the National Arboretum in the District of Columbia. Part of the National Park System, the Kenilworth Marsh is adjacent to the Kenilworth Aquatic Gardens. In 1989, a coalition of federal and local agencies, led by the Washington Metropolitan Council of Governments (COG) chose the marsh for restoration as part of a long-term plan to restore water quality and habitat within the Anacostia River watershed. In 1989, COG retained Biohabitats, Inc. to perform the following activities: cbnduct a historical study of the marsh spanning the past 100 years; assess current conditions; and develop a plan to restore mid-marsh vegetation over 30 to 50 percent of the existing mudflats, an area totaling 12 hectares (30 acres). Preliminary research revealed that, at one time, portions of the Anacostia floodplain supported broad tidal marshes with emergent vegetation dominated by wild rice (Zizania aquatica) and other freshwater tidal marsh species typical of that area. Civil engineering operations, begun around the turn of the century and continuing for nearly 50 years, gradually reduced the floodplain wetlands to only a few severely degraded and highly altered patches, of which Kenilworth was one. In the area of Kenilworth Marsh, historic documents and a rare early aerial photograph revealed that emergent tidal wetlands, interspersed with wooded islands, had occupied the site as recently as 1927. These wetlands were dredged prior to 1948 to create a tidal lagoon connected to the Anacostia River. At about the same time, the river was dredged and lined with seawalls to control flooding and provide navigable water. During the past 40 years, spoils from the river dredging were put into the tidal lagoon, filling in open water habitat and smothering native riparian vegetation. Today the marsh consists mostly of open water habitat covered with water at high tide and exposed as mudflat at low tide. We found that the dredging and placement of spoils had steepened the grade in many areas, leaving the site with only two of the three vegetative zones typically found in freshwater tidal marshes along the Anacostia River. While the marsh provided high marsh and low marsh habitat, tidal wetland vegetation covered less than ten percent of the mid-marsh area. We learned that the elevation of the marsh substrate had stabilized at approximately 0.3 m (1 ft) above mean sea level (MSL). Comparing this elevation with the elevations of existing stands of low, mid, and high marsh vegetation, we found that the extensive mudflats were too low to support mid-marsh vegetation. We concluded that this was the primary factor limiting colonization by emergent midmarsh vegetation. This, then, became the basis for our restoration plan, which called for filling approximately 13 hectares (32.5 acres) of the marsh to achieve the proper elevations for establishment of emergent vegetation. In 1990 we designed a small-scale