Sea Level Rise in the DC Area

Aerial view looking south across the Anacostia River during the Potomac River flood, October 1942.

Navy Yard Historical Center through Library of Congress

17.7 Feet of Water Swamp DC

At the height of American involvement in World War II abroad, Washingtonians were fighting a battle of their own right here in DC. Over a three day span in October 1942, 5.4 inches of rain fell on the city. In addition to the local rainfall, two more torrential downpours descended upstream of DC in both the Blue Ridge Mountains and the Shenandoah Valley. The result was a 17.7-foot crest of water charging down the Potomac towards the city. Workers raced to erect a sandbag dike at 17th Street and Constitution Avenue, but at the height of the storm, a large part of the southwest of the city was flooded, including much of Navy Yard. Many were evacuated across the city, as water from swollen waterways flooded homes.

To this day, this extreme weather event on October 17, 1942 is the maximum water level the National Oceanic and Atmospheric Administration’s (NOAA) DC tide gauge has ever recorded. The NOAA tide gauge continues to monitor water levels in the city today, and with the modern threat of climate change-induced sea level rise, DC may experience similar extreme water levels again.

Why Care About Sea Level Rise?

Sea level rise is no longer a threat on the horizon. It is here now and affecting low-lying coastal areas across the United States and the globe. Climate change is the main cause of contemporary sea level rise, and in recent decades data from tidal gauges and satellite altimetry suggests that rates of rise are accelerating in most regions of the United States.

The parks in the National Capital Region Inventory and Monitoring Network (NCRN) include multiple low-lying tidal wetlands found along the banks of the Potomac and Anacostia Rivers. Accelerating sea level rise threatens the wetland ecosystems of these parks, and the nearby coastal communities. This resource brief summarizes current and projected rates of sea level rise, based on local water level data collected by the National Oceanic and Atmospheric Administration (NOAA).

Sea Level Rise by the Numbers

Scientists with NOAA have been recording data from tide gauges for decades. There are over 200 water level tide gauges deployed across the U.S. These gauges record water levels to provide information on tides at a local scale. NOAA collects tide and water level data at varying frequencies. Hourly and monthly verified water level data has been collected almost without interruption since 1924. In 1995, NOAA added verified six-minute water levels to their data collection.

Sea level rise (SLR) does not occur at a constant rate across the country, and by tracking water level at many sites, scientists are better able to understand how each location’s unique conditions cause variability in sea level.

Established in 1924, NOAA’s Washington DC tide gauge has provided almost a century of data that gives scientists accurate information on sea level trends, day-to-day tides, and much more. Washington’s station is located near the confluence of the Anacostia and Potomac Rivers, on the banks of the Washington Channel in Southwest DC (Figure 1).

Figure 1. The NOAA tide gauge station is located at the blue pinpoint.

The DC station monitors relative sea level. This is a measurement for sea level that compares sea level to a fixed reference point on the land. Relative sea level in Washington, DC has been increasing over the period of record at an annual rate of 3.43 millimeters (mm) per year, although accelerated sea level rise has been recorded in recent decades (Figure 2). Over the approximately 100 years on record, sea level has risen 344mm (13.5 inches). Figure 2 also shows a large amount of short-term variation in water level. What looks like “random noise” at first glance, is actually the product of well-documented patterns, including seasonal variation in sea level and the acceleration of sea level rise in recent decades.

Figure 2. Relative Sea Level Rise (Washington, DC)

National Oceanic and Atmospheric Administration

Sea level is not constant across an entire calendar year. Seasonal variability is caused by cyclical fluctuations in local coastal temperatures, salinity, wind speeds and direction, atmospheric pressure, and ocean currents. As shown Figure 3, average sea level is lower during the winter months and higher between the months of April and October. Sea level peaks in September and is lowest in January. This pattern could be of concern to NCRN parks because this peak overlaps with the Mid-Atlantic hurricane season, which runs from June to November. The damaging effects of extreme weather events, such as hurricanes, are likely to be made worse if they co-occur with periods of high water.

Figure 3. Average Seasonal Cycle of Sea Level (Washington, DC)

National Oceanic and Atmospheric Administration

Sea level rise isn't constant across longer temporal scales either. One way to bring this variation into focus is to summarize sea level trends for subsets of the period of record. Figure 4, for example, illustrates the relative sea level trends for different 50-year periods; the values associated with each vertical line are the results for each 50-year period (e.g., the 25 years before and after the corresponding year on the X-axis). For 50 year periods centered around the 1960’s and 70’s, sea level rise was slower (e.g., less than 3mm/yr, considerably less than the long-term average) than it has been in more recent 50-year periods, during which sea level rise was generally more than 3 mm/yr.

Figure 4. Variation of 50-Year Relative Sea Level Trend (Washington, D.C.)

National Oceanic and Atmospheric Administration

Resources Threatened by Rising Sea Levels

Accelerating sea level rise poses risks to low-lying areas of National Capital Area parks.+ In 2018, the National Park Service (NPS) published a report that used data from NOAA and the United Nations Intergovernmental Panel on Climate Change (IPCC) to project sea level rise and storm surges near coastal NPS units (Caffrey et al., 2018). While projected changes in relative sea level vary widely across the U.S., the report states that shorelines in the National Capital Area may experience the highest sea level rise of all NPS regions in the next century. Since the tidally-influenced NCA parks included in the study are within the Potomac Watershed, there was little variation in sea level rise projections between parks. Along with sea level rise, the report also projected higher storm surges from climatic weather events, which have potential to inundate areas along the Anacostia and Potomac. This coupled with the SLR projections is a striking finding, as it is likely to have detrimental impacts to the low-lying areas at parks such as George Washington Memorial Parkway (GWMP) and National Capital Parks - East (NACE).

On the Potomac River, Dyke Marsh (part of GWMP) is the largest remaining freshwater tidal wetland in the Washington DC area. Other smaller marshes are found in places such as Kenilworth Park, Kingman Lake, and Piscataway Park. Tidal marshes in the highly urbanized watersheds around DC are both ecologically and functionally important. They are home to flora and fauna that thrive on the fluctuation between wet and dry conditions that are characteristic to tidal marshlands. Communities adjacent to these low-lying parks rely on marshes for their ecosystem function, especially as natural buffers that protect inland areas from storm surges and erosion.

Due to accelerating sea level rise in the area, Dyke Marsh and these smaller marshes are becoming increasingly susceptible to flooding and erosion. If sediment accretion cannot keep up with the rate of sea level rise, the marshes could be converted into mudflats within this century. This would cause cascading declines that put these marshes at risk of disappearing.

As sea level continues to increase, marshes must build up sediment to keep pace with the rising water. If the sea level rise accelerates at a higher rate than can be sustained by sediment build-up, marshland will become inundated with too much water. Tidal marsh vegetation depends on the cycle of inundation at high tide and exposure to air at low tide. High rates of SLR would cause current vegetation communities to change or disappear as certain areas would have water continuously present, thus deteriorating the health of the marsh ecosystems.

Tracking Marsh Elevation

The gradual accumulation (accretion) of sediment in marshes is a vital process in the face of sea level rise. To track marsh elevation, the NCRN I&M uses a tool called a Sediment Elevation Table (SET) that measures the balance of accretion and erosion of the marsh surface. Tracking these gradual changes enables scientists to see whether marshes are building up sediments fast enough to keep pace with sea level rise. Otherwise, the marshes could drown. SET monitoring data helps resource managers make informed decisions about these resources and tells the story of these wetland habitats for the public.
A report covering marsh elevation data from the tidal freshwater marshes of the Anacostia and Potomac Rivers, forthcoming in 2022, will be posted in the report section of the NCRN I&M Air and Climate monitoring webpage. Preliminary results show that some marshes, like Kenilworth Marsh, are more resilient to climate change (in other words, accretion is keeping pace with sea level rise), whereas others, like Kingman Marsh, will be more challenging to manage in their current state as sea level rise continues.