Last updated: March 31, 2023
By Nicholas Tait, NCRN I&M Science Communication Intern
Rising Seas, Receding Marshes
Sea level rise (SLR), once a future concern, is now a problem of the present. Its impact is felt most acutely in low-lying marshlands across the United States. The plants that hold together tidal marshes are adapted to a certain range of water levels, known as their growth range. As sea level rises, flooding and erosion can reduce the area of the marsh within the growth range, ultimately leading to a loss of the wetland plant species that hold the marsh in place. If marsh elevation isn't rising quickly enough through sediment buildup (accretion) to keep up with the local rate of sea level rise (and any elevation fall from erosion, compaction, or subsidence), the marsh can eventually convert to a submerged mudflat.
In the National Capital Region (NCR), freshwater tidal marshes occur in several park areas, including Kenilworth Marsh and Kingman Lake in National Capital Parks-East (NACE) and Dyke Marsh Wildlife Preserve within George Washington Memorial Parkway (GWMP). These ecosystems are ecologically and functionally important within NCR’s highly urbanized watersheds. They serve as a home for native wildlife that thrive on the ebb and flow between wet and dry conditions, including the least bittern and marsh wren. They also act as natural buffers for inland areas against storm surges and erosion. NCR’s tidal wetlands are crucial to the region, and due to climate change and sea level rise, they may have challenges ahead.
Monitoring Marsh Health
From 2002 to 2019, the National Capital Region, Inventory & Monitoring Network (NCRN I&M) tracked the elevation of Kingman Lake and Kenilworth Marsh in NACE. Typically, twice a year, I&M scientists measured the elevation of the marsh surface at ten sites—five at Kingman, five at Kenilworth—using surface elevation tables (SETs). The team also measured accretion rates at each site using a horizon marker, usually an artificial layer of white feldspar clay placed on the marsh surface and later cored to determine how much sediment had accumulated on top of it. The resulting accretion data shows surface buildup or erosion. Compared to accretion, elevation is a better representation of overall marsh status, and when directly compared to sea level rise rates. Elevation accounts for vertical erosion and buildup, as well as compaction and shallow subsidence.
I&M paused marsh elevation monitoring after 2019 and is in the process of replacing SET monitoring sites at randomized locations to better represent the larger marsh area. I&M scientists replaced the five sites at Kenilworth with six new ones in 2021 and sites at Kingman Lake are due to be replaced soon. The new sites will only monitor elevation, and not accretion.
How are Kingman Lake and Kenilworth Marsh Faring?
A 2023 report by Tredennick et al. analyzed marsh monitoring data from 2002-2019 and found that the elevation at Kenilworth Marsh is keeping up with sea level rise while Kingman Lake is not (Figure 1). Even though accretion rates at both marshes show ample sediment inputs, at Kingman the sediment building up is offset by shallow subsidence. These results suggest that the marsh at Kingman Lake may be less resilient to sea level rise.
Kingman’s history may play a role in its future. In 1920, following centuries of sediment build up in the Anacostia River from upstream development and erosion, the US Army Corps of Engineers dredged the river to create both Kingman Island and Kingman Lake. Since then, challenges with continued sediment from upstream erosion, dumping of untreated sewage, and other forces have shaped Kingman.
In 1999, with increased recognition of the important role of the area’s remaining wetlands, efforts began to restore and protect the tidal wetlands at Kingman Lake. Federal agencies and the District of Columbia replanted acres of marsh grasses whose root fibers would help stabilize the wetland against erosion. Unfortunately, large populations of resident (non-migrating) Canada geese consumed nearly all early vegetation restoration. As a result, the park now works with partners to protect against overgrazing by maintaining healthy numbers of geese in the park.
Kenilworth Marsh was once a permitted dump site for the District. It had been dredged by the Army Corps of Engineers to create a lake and several islands home to freshwater emergent wetlands. In 1992 and 1993, the marsh underwent major restoration when the Army Corps added dredged material from the Anacostia to build up the marsh mudflats. This project resulted in twelve hectares (nearly 30 acres) of tidal wetland, numerous channels cut to restore tidal flow, and over 350,000 native species planted.
Monitoring and management of Kenilworth Marsh, including SET monitoring (restarted in 2022), vegetation monitoring, and ongoing invasive plant control (primarily targeting invasive purple loosestrife and phragmites), is part of how NPS is protecting the health of this vital ecosystem for the future.
Tredennick, A. T., M. Tabak, J. Lombardi, and L. Starcevich. 2023. Rates of surface change and accretion in coastal National Park Service marshes in the northeast U.S.: Trend analyses for the Northeast Coastal and Barrier Network, Northeast Temperate Network, and National Capital Region Network parks. Western EcoSystems Technology, Inc. Laramie, Wyoming.