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Seasons at Dyke Marsh © Paula Sullivan

About the Marsh

Dyke Marsh is a freshwater tidal wetland consisting of 485 acres just south of Alexandria, Virginia, and north of historic Mt. Vernon. Located on the west bank of the Potomac River some 95 miles from the Chesapeake Bay, Dyke Marsh is owned by the U. S. Federal Government and is managed by the National Park Service as a part of the George Washington Memorial Parkway. It is an area of open water, cattail marsh, wetland shrubs and plants, and deciduous swamp forest.

In the early 1800’s earthen walls were built around the perimeter of the marsh in order to create more "fast land," land not inundated by high tides. The "dyked" area was then used to graze livestock and to grow crops. Over the years, the marsh has developed into a wide expanse of marsh vegetation, and tidal silts have provided a footing for marsh plants and the floodplain tree species of sycamores, maples and poplars. Today, the area is unique in that it represents the largest remaining piece of freshwater tidal wetlands in the Washington Metropolitan area. It is best known for its resident and migratory bird populations, the weekly bird walks that take place year-long, and for the extensive breeding bird survey conducted every summer.

Animals of the Marsh

Dyke Marsh provides habitat which supports a diverse collection of animals. Evidence of beaver activity is visible along the haul road and beavers, along with muskrats, can most often be seen in the early evening swimming in the marsh. Other mammal species observed include gray squirrels, cottontail rabbits, shrews, field mice, river otters, red fox, little brown bats, and whitetail deer. Marsh-dwelling fish include carp, bullhead, chain pickerel, shad, striped bass, and shiners. Reptiles such as snapping turtles and northern water snakes, and amphibians such as frogs, are also common.

Plant Species


Turk's Cap Lily (Lilium Superbum) Photo by Ed Eder

To date, more than 360 species of plants have been recorded in Dyke Marsh. The dominant species in the marsh itself is the narrow-leaved cattail, which typically develops its characteristic flower spike by June. Other species associated with the tidal marsh include: arrowhead, arrow arum, pickerelweed, sweetflag, spatter dock, and northern wild rice. A result of human disturbance in the marsh has been the introduction of several exotic, or non-native, plant species. Exotic species are very opportunistic, growing in disturbed areas and oftentimes outcompeting beneficial native species. Portions of the floodplain forest are being overgrown with several exotic vines: porcelainberry, Japanese honeysuckle, and Asian bittersweet. In the marsh, yellow iris, common reed and purple loosestrife are slowly encroaching on the native species.

The Dyke Marsh Centipede and Garbology of the Potomac River

On cool, overcast April 8, 2008, students of Georgetown University’s Forest Ecology 355 visited Dyke Marsh and Jones Point to study the biodiversity of the area and the impact of garbage on the environment.

Mr. Danny Vance (my senior-thesis student), Mrs. Carol Day (Director, Georgetown University Health Education Services), Ms. Michelle Williams (American University student and film producer) and her two-person film crew also participated. Michelle is producing an environmental film (her M.F.A. thesis) which will be shown at FODM’s November 19 meeting.

Before our visit, we watched the 2006 film On the Edge: The Potomac River Dyke Marsh. So, of course, soon after our arrival at DMWP, we searched for movie star Chip Johnston. We found him and exchanged greetings. Then we enjoyed the vista of the northern part of DMWP from the large dock. I hopped, made the dock move a bit, and explained to the class, “Remember when we discussed the floating mat of bogs? This dock is moving somewhat like such a mat. From here you can see the main habitats of the Preserve — the open marsh, floodplain forest and the swamp forest. Look south. That’s the Haul Road peninsula. We’ll be walking down the Haul Road to its end and enjoy the view of the Preserve from the boardwalk.” Shadowed by Michelle and her crew, the class, Carol, Danny, Friends of Dyke Marsh’s Robert Smith and I walked down Haul Road, learning about birds, bugs, plants and other biota. The class enjoyed interacting with Robert who offered interesting tidbits about DMWP.


A handsome larvae of the Smeared Dagger Moth(Acronicta oblinita, J. E. Smith), aka the Smartweed Caterpillar, is common in the Eastern U.S. This specimen is more reddish than most individuals and may be newly molted. Photo by Ed Eder.

We all were a little hesitant to leave the scenic boardwalk area to speed walk to our van to go to our next stop on time. As I sped back, I heard teaching assistant Katie Shaeffer call me, “What is the big bug?” I walked back to the site, and saw the chelicerae (poison fangs) of the rare-and-elusive giant Dyke Marsh Centipede (Scolopendra dykemarshiensis Linnaeus 1759)*. “Wow, what great eagle-eyes you have, Katie,” I exclaimed. The film crew moved in for a close-up shot, and the class marveled at the arthropod’s size and striking coloration. We visited Jones Point to study garbage and our unbridled consumer and throw-away society — major threats to forests, marshes and other habitats. People cleaned up DMWP’s garbage a few days earlier, so National Park Service Ranger Miguel Roberson advised us to go to the Point for our garbological investigation. As Robert led us along the trail to the Point, the contrast between the highly disturbed area near the new Wilson Bridge and the healthful peace of DMWP struck us. The Bridge area was nerve-wrackingly noisy from traffic, smelled strongly of acetone from a substance applied to the bridge and had a small “pea-soup” pond, eutrophically choked by cyanobacteria and other biota. Carol thought that we should not breathe the unhealthy Bridge odor. I tried to hold my breath to protect myself against the chemical assault. We also saw the rolling garbage dunes along the Potomac River in the area. Each student and Carol, who were protected by brown gardening gloves, filled a bag with garbage that we took back to our lab for study. During the last 15 minutes of our visit, we sat at a picnic table while Carol talked about environmental and human health and asked each of us to comment on our visit to DMWP and Jones Point. On Earth Day 2008, Katie gave us a creative PowerPoint presentation of her statistical garbage analysis in alluring earth tones and textures. After her report, we viewed the somewhat startling video, "The Story of Stuff on YouTube."

*No one has really found the actual Scolopendra dykemarshiensis yet, but we are not giving up and are continuing the search.

Dr. Edd Barrows - Georgetown University Laboratory of Entomology and Biodiversity

Learn More About DMWP’s Complex Biodiversity


	Dr. Edd Barrows leads students in study along Haul Road in Dyke Marsh. Photo by Robert Smith.

An excellent website for learning about the many natural resources of Dyke Marsh Wildlife Preserve (DMWP) is the Biodiversity Database of the Washington, D.C., Area (BDWA), produced by Professor Dan Kjar of Elmira College and Professor Edd Barrows of Georgetown University’s Laboratory of Entomology and Biodiversity. The BDWA has arthropod lists and photographs of many DMWP organisms, provides helpful links and lists some publications related to the Preserve.

To access DMWP information, visit the Biodiversity Database website.

Dr. Barrows wrote in the spring 2008 Marsh Wren, the newsletter of the Friends of Dyke Marsh, that there are thousands of “key players” in the wetland’s ecosystem, yet to be studied and recorded.

About invertebrates (including arthropods), BDWA quotes Professor Edward O. Wilson: "The truth is that we need invertebrates but they don’t need us. If human beings were to disappear tomorrow, the world would go on . . . . But if invertebrates were to disappear; I doubt that the human species could last more than a few months. Most of the fishes, amphibians, birds and mammals would crash to extinction about that same time. Next . . . the bulk of the flowering plants . . . and the world would return to the state of a billion years ago. . . ."

Nothing New About Skinks

Observing reptiles and amphibians in the field is always challenging. The conditions have to be “just right” – warm enough (they are cold-blooded) to be outside, cool enough to justify the dangers of sunning, good chances of finding something to eat, etc. Usually, you will see nothing.

On a warm, sunny day in spring, I saw my first lizards in Dyke Marsh despite walking the trail often for six years. The first was eight inches long with a four inch body. It was a dark tan brown in color with very pale stripes. The head was orange. A second, smaller lizard was 4 inches long, black with bright white stripes and a bright blue tail. Both lizards had short legs, long tails and bullet-shaped heads. They appeared to slither over the ground, but could easily climb vertical wooden surfaces.

The ground-dwelling Five-lined Skinks (Eumeces fasciatus) range throughout the eastern half of North America and are most abundant on the coastal plain. As skinks mature, their stripes fade and their tails lose the bright blue coloration. The heads of mature males turn reddish orange during the breeding season.

Our area has isolated enclaves of several different types of lizards. Five-Lined Skinks are well known to be native to this area. But seeing them for yourself is a different matter. Despite visiting the same spot over a dozen times since, I have seen no lizards. This experience tells us something about field observation. A place we think we know extremely well can still surprise us.

-- Doug Wilson

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