Last updated: April 5, 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 the rise in elevation of a marsh through sediment buildup (accretion) can’t keep up with the local rate of sea level rise (plus any fall in elevation from erosion, compaction, or subsidence), the marsh can eventually convert to a submerged mudflat.
In the National Capital Region (NCR), scientists monitor freshwater tidal marshes 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 2006 to 2019, the National Park Service’s Inventory and Monitoring (I&M) team tracked changes at Dyke Marsh in GWMP, the largest remaining freshwater tidal wetland in the National Capital Region. Twice a year, I&M scientists measured the elevation of the marsh surface at nine sites using surface elevation tables (SETs). The sites are roughly divided among three different habitats within Dyke Marsh: “river” sites along the Potomac River, “interior” sites inland from the river, and “creek” sites along a small creek on the western side of the marsh (Figure 1). 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 all nine SET monitoring sites at randomized locations to better represent the larger marsh area. The new sites will only monitor elevation, and not accretion.
How is Dyke Marsh Faring?
A 2023 report by Tredennick et al. analyzed marsh monitoring data from 2006 to 2019 and found that the elevations of the outer portions of the marsh—the river and interior sites—are keeping up with sea level rise while the inner portions—the creek sites—are not (Figure 2). Accretion rates show ample sediment inputs, which indicates that the creek sites are not keeping up with sea level rise in part because of shallow marsh subsidence. However, SET and accretion monitoring do not show the effects of shoreline erosion, which also heavily impacts the marsh. Of the nine SET sites established at Dyke Marsh in 2000, five creek and river sites disappeared due to loss of shoreline prior to major marsh restoration efforts described below. The SET results and the longevity of the plots themselves suggest that some areas of Dyke Marsh may be less resilient to sea level rise.
The situation may be even more dire than the report suggests. Tredennick et al.’s findings use the nearly 100-year average SLR rate of 3.43 mm/year, but this rate is increasing. Additionally, sea levels vary greatly within a calendar year. In DC, sea level peaks during the summer, which coincides with the Mid-Atlantic hurricane season. The combination of these patterns can create greater dangers to tidal wetlands like Dyke Marsh than accounted for in the report.
Fortunately, significant efforts to protect and restore Dyke Marsh are underway. The National Park Service (NPS), working with the U.S. Army Corps of Engineers, completed construction of a 1,500-foot stone breakwater on the south end of the marsh in 2020 to shield it against storms and erosion and promote sediment accretion (see Figure 1). In fall of 2022, NPS completed the addition of another 1,720 linear feet of rock sill north of the breakwater on the southeast edge of the wetland to provide even more buffer (also shown in Figure 1). The park plans to add more rock to raise the existing southern barrier in 2023.
Continued monitoring by the NPS I&M team will help reveal whether these projects are having the desired effect on marsh elevation. As the climate continues to warm and sea levels rise, these artificial buffers, along with future restoration and management efforts, will be needed to protect the health of Dyke Marsh and prevent this vital ecosystem from disappearing over the coming decades.
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.