Kenilworth Marsh and Kingman Lake: Which Marsh Is Keeping Up With Sea Level Rise

Inventory and Monitoring scientists measuring marsh elevation at Kenilworth Marsh with a surface elevation table (SET). Credit: NPS / Oettel.

Rising seas, receding marshes

Sea level rise is no longer only a future problem, and it is felt most in low-lying marshes. Tidal marsh plants are adapted to a particular range of water levels, their growth range. As the sea rises, flooding and erosion can shrink the part of the marsh within that range, and the plants that hold it together die out. Unless the marsh surface builds up through sediment (accretion) fast enough to match local sea level rise — and any loss of height from erosion, compaction or subsidence — the marsh can eventually become a submerged mudflat.

In the National Capital Region, freshwater tidal marshes include Kenilworth Marsh and Kingman Lake in National Capital Parks–East and Dyke Marsh Wildlife Preserve on the George Washington Memorial Parkway. In a heavily urbanised region they are home to wildlife that thrive on the ebb and flow of wet and dry, such as the least bittern and marsh wren, and they buffer land from storm surges and erosion.

How the marshes were measured

From 2002 to 2019, the National Capital Region Inventory and Monitoring Network tracked elevation at Kingman Lake and Kenilworth Marsh, usually twice a year, at ten sites — five in each.

• Elevation, with surface elevation tables. Elevation is the better measure of a marsh's overall state, and the one to compare with sea level rise, because it reflects erosion and build-up as well as compaction and shallow subsidence.
• Accretion, with horizon markers — usually a layer of white feldspar clay laid on the surface and later cored to see how much sediment has built up on it.

Monitoring paused after 2019 while the sites are moved to random locations that better represent the whole marsh: six new sites replaced Kenilworth's five in 2021, and Kingman's are due to follow. The new sites measure elevation only.

Figure 1. Estimated elevation trends at Kingman Lake and Kenilworth Marsh; the vertical line is the average annual rate of sea level rise. Credit: National Park Service.

The results

A 2023 analysis of the 2002–2019 data (Tredennick and others) found that Kenilworth Marsh is keeping up with sea level rise and Kingman Lake is not. Both receive plenty of sediment, but at Kingman the build-up is offset by shallow subsidence — suggesting Kingman may be less resilient to rising seas.

Two histories

Kingman Lake. After centuries of sediment washed into the Anacostia River by upstream development and erosion, the U.S. Army Corps of Engineers dredged the river in 1920, creating Kingman Island and Kingman Lake. Continued sediment from upstream, untreated sewage and other forces have shaped it since. From 1999, as the value of the area's remaining wetlands was recognised, federal agencies and the District of Columbia replanted acres of marsh grasses, whose roots help hold wetland against erosion — but large flocks of resident Canada geese ate nearly all the early plantings. The park now works with partners to keep goose numbers at a healthy level and prevent overgrazing.

Kenilworth Marsh was once a permitted dump site for the District, dredged by the Army Corps to create a lake and several islands of freshwater wetland. In 1992 and 1993 the Corps restored it, adding dredged Anacostia sediment to build up the mudflats. The work produced twelve hectares (nearly 30 acres) of tidal wetland, many channels cut to restore tidal flow, and more than 350,000 native plants.

Elevation monitoring at Kenilworth restarted in 2022, alongside vegetation monitoring and continuing control of invasive plants, mainly purple loosestrife and phragmites.

Sources

• Nicholas Tait (National Capital Region Inventory and Monitoring science communication intern), "Long-Term Monitoring Reveals Challenges and Resilience at Kenilworth Marsh and Kingman Lake," National Park Service. https://www.nps.gov/articles/000/long-term-monitoring-reveals-changes-and-resilience-at-kenilworth-marsh-and-kingman-lake.htm
• The analysis: 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. Western EcoSystems Technology, Inc., Laramie, Wyoming.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.