Living at a Crossroads - The Restoration of Dyke Marsh Wildlife Preserve
By Dr. Katia Engelhardt, Research Associate
Professor, University of Maryland Center for Environmental Science, Appalachian
Laboratory.
The significance of tidal
freshwater marshes
Tidal freshwater marshes were once extensive along the Coastal Plain rivers of
the mid-Atlantic region of the United States. However, after centuries of
intense coastal development, tidal freshwater marshes have been reduced to
scattered remnants that can no longer provide the extent of ecosystem services
characteristic of widespread, healthy marsh ecosystems. This has far-reaching
consequences for the sustainability of wildlife populations and estuarine
ecosystems. Nonetheless, even remnant marshes provide numerous benefits and
services: resident and migratory wildlife habitat, refuge for endangered
species, spawning and nursery grounds for anadromous fish, attenuation of tidal
energy, shoreline stabilization, flood control, water quality enhancement,
carbon storage, aesthetic enjoyment, and recreational activities. Consequently,
the maintenance and enhancement of the remaining tidal freshwater marshes within
the mid-Atlantic regions as well as world-wide are imperative both socially and
ecologically.
Tidal freshwater marshes are located at the boundary between tidal and non-tidal
riverine wetlands, and they are at risk to be eliminated or severely reduced in
extent by sea level rise because there are few suitable stream habitats
available for species and marsh migration. Tidal freshwater marshes have more
rare and endangered species than other wetland types and are thus most
susceptible to dramatic changes in patterns of species diversity. They differ
from other tidal marshes in patterns of nutrient assimilation. Changes to the
unique structure and function of tidal freshwater marshes are bound to impact
food web dynamics and patterns of nutrient cycling in estuaries. It is
therefore imperative that tidal freshwater marshes are protected and restored to
the extent that they can adjust to changes in environmental conditions, such as
sea level rise concomitant with other environmental stressors.
The significance of Dyke
Marsh Wildlife Preserve
Dyke Marsh Wildlife Preserve is an extremely important tidal marsh, offering
invaluable ecosystem services, serving as a fantastic educational classroom, and
providing a wide variety of recreational opportunities. Dyke Marsh is one of the
largest tidal freshwater marshes located along the Potomac River in the
Washington, D.C. area and is viewed as a national treasure because of its
proximity to the nation’s capital and a large urban/suburban population, its
history, and its current potential for provision of ecological services,
recreational values, and educational opportunities. In addition, the marsh is
located along a major travel corridor (G. W. Memorial Parkway) to one of the
most popular tourist destinations in the D.C. area (Mount Vernon).
Sedimentation and sea level
rise
It is estimated that the formation of Dyke Marsh began 5,000 to 7,000 years ago,
although recent coring and dating by the US Geological Survey suggests that the
marsh may be significantly younger. Hunting Creek enters the Potomac River
immediately upriver of Dyke Marsh. The velocity of the stream current is
decreased as the stream enters the slower moving Potomac River. As a result,
deposition occurs. Twice daily tides carry additional sediments from the
Potomac estuary to the area. The constant deposition of sediments ultimately
results in marsh surface elevations suitable for marsh vegetation. This
deposition is usually sufficient to keep pace with sea level change, where
deeper water tends to enhance sediment deposition and shallower water tends to
decrease deposition, resulting in the relatively flat marsh platform
characteristic of tidal marshes and apparent at Dyke Marsh.
Recent research shows that the rate of marsh accretion (i.e., the rate of
mineral and organic accumulation of sediment) is everywhere equal to the rate of
sea level rise when sea level rise is steady and moderate. In this case, water
depth and biological productivity remain constant through time. An increase in
sea level rise, a reduction in sediment supply, and disturbances to marsh
vegetation can destabilize the marsh system. In such cases, marshes can erode
rapidly and may be irreversibly lost. Projected increases in sea level rise at
Dyke Marsh and the region are a) minimal increase of 3 mm/y, the expected
increase for the Chesapeake Bay region determined using past records of sea
level rise; b) moderate increase in sea level of 6mm/y, which is a median
scenario of the Intergovernmental Panel on Climate Change report published in
2001; and c) large increase of 11mm/y, the maximum projected sea level rise
according to current scenarios, which may underestimate sea level rise by some
accounts. Given these potentially large sea level rise scenarios, it is
imperative that disturbances to marsh vegetation are minimized and that sediment
supply is maintained, if not enhanced in areas that have seen hydrologic
alterations and changes in sediment delivery.
 |
| |
DMWP in 1937 and 1994
(images at left) highlighting the impacts of dredging, the hardening of
terrestrial surfaces and the severe alteration of Hunting Creek (entering
the Potomac River at the top left of the images). The image on the right
shows the placement of a potential restoration area proposed in a 1977
Environmental Assessment. |
|
Dyke Marsh at a crossroads
Dyke Marsh consisted of 650 acres in the 1930s when Smoot,
Sand and Gravel Corporation (SSGC) acquired ownership. By 1940, SSGC began
dredging open water areas for sand and gravel found between 16 and 40 feet
deep. In 1959, an agreement was made between the U.S. government and SSGC to
stop dredging in specific areas, yet dredging the marsh continued with increased
environmental destruction from barges and a haul road. These activities stopped
in 1972. In 1976, the National Park Service took over direct management
responsibilities of the remaining 485 acres of wetland, of which close to 200
acres had been dredged. Since then, determining whether it is feasible to undo
the damage, or at least minimize ongoing and future degradation, has been an
important consideration for Dyke Marsh management. The current Environmental
Assessment is an extremely important process which will end in decisions with
far-reaching consequences for Dyke Marsh.
Dyke Marsh is currently at an important crossroads. After many years of abuse,
continued development of the Potomac River shoreline and watershed, and
increased boat traffic on the Potomac River, some marsh shorelines are eroding.
Adding the threat of accelerated sea level rise (currently 3mm per year in the
area, but could accelerate to 6mm or more), the worry is real whether the marsh
will continue to exist in its present state for much longer without human
intervention. Still, the current marsh is in excellent health, with vegetation
and wildlife communities that are diverse and productive. If Dyke Marsh were a
human patient, we might ask whether the patient needs major surgery, a minor
procedure, or simply some regular monitoring for vital signs with possible
intervention in the future. We might also ask if the patient is even sick or
whether we are watching the natural process of aging, which includes changes in
form and function.
The case for restoration
A case for restoration can easily be made. Approximately one third of the marsh
surface as it existed only 100 years ago was lost to dredging activities. As
area is lost, so are species and the capacity to provide ecosystem services.
Therefore, an argument can be made from an ethical as well as an ecological
perspective that marsh area needs to be restored, at least in part, to regain
what was previously lost owing to human negligence.
Dredging left behind deep holes and an altered
hydrology. Combined with sea level rise, changes in sediment delivery and
increased boat traffic on the Potomac River, sediment deposition and erosion has
likely been altered at Dyke Marsh. Thus, even though we still have incomplete
information on the rate and extent of marsh surface loss (and potentially gain
in other places), one could argue that, at a minimum, hydrology needs to be
restored and shorelines stabilized.
The case against restoration
Global warming is a contentious issue. Some people flat out deny global warming
is happening; some say it is a natural process so no change in human behavior is
necessary; and some argue that humans are accelerating global warming and that a
decrease in the carbon foot print is absolutely necessary. Obviously, the same
data can be interpreted many different ways. The same arguments can be made for
the management and restoration of Dyke Marsh. Not withstanding the loss of marsh
area to dredging, it is still unclear whether and how much marsh is concurrently
lost and whether this loss is a natural process or accelerated by human
activity. Marshes are dynamic systems that constantly lose and gain ground
depending on environmental conditions, in particular storm events and sea level.
Managing for a static system would be Sisyphean. Hence, one could make the
argument to let the marsh be and to allow it to migrate, perhaps, closer to
Hunting Creek.
The current marsh is in good health. Any human intervention will increase the
chance that the existing marsh will be negatively affected. For example, it is
inevitable that new marsh areas will support populations of exotic plants, which
then become sources of seeds for the existing marsh. Canada geese may be
attracted to new mud flats and then learn to feed in existing portions as well.
Engineered structures may add stability to new and existing shorelines but may
change the hydrology to the extent that sediment delivery to the existing marsh
is altered. A restoration will therefore require careful consideration of
alternatives and close collaboration between managers, scientists, and
engineers.
Conclusion
Dyke Marsh Wildlife Preserve is an important and irreplaceable ecosystem, and
management of the marsh is currently at an important, yet difficult, crossroads.
Doing nothing may result in the degradation and migration of the marsh and
potential loss of the entire system. Restoration of previously lost areas may
greatly enhance the ecosystem, but may also have significant unintended
consequences, potentially requiring constant intervention and endless tinkering.
A sound, science-based restoration plan is therefore key for minimizing
potential adverse impacts.
Editor’s Note: The National
Park Service has determined that restoration of Dyke Marsh is feasible and
desirable. NPS is currently preparing a wetland restoration and long-term
management plan/environmental impact statement which will present several
alternatives for public consideration.
Back
to top