About the Marsh

The Dyke Marsh Wildlife Preserve is a 485-acre freshwater, tidal wetland complex just south of Alexandria, Virginia, in Fairfax County, Virginia, on the western shoreline of the Potomac River around 95 miles from the Chesapeake Bay. Congress added the preserve to the national park system in 1959 "so that fish and wildlife development and their preservation as wetland wildlife habitat shall be paramount." (Public Law 86-41)

Dyke Marsh is one of the largest, most significant temperate, climax, narrow-leafed cattail marshes in the national park system, a unit of the George Washington Memorial Parkway managed by the National Park Service. It has tidal wetland, swamp forest, upland forest and open water. The southern part of the marsh is at least 2,200 years old.

In the 18th and early 19th century, the marsh was called "Hell Hole Swamp." In the early 1800s, people built earthen walls around the perimeter of part of the marsh to create more "fast land," land not inundated by high tides. The "dyked" area was then used to graze livestock and to grow crops. The walls failed.

From 1940 to 1972, Smoot Sand and Gravel dredged and hauled away almost half of the marsh, destabilizing the entire system. See "The Accelerating Erosion of Dyke Marsh" and our restoration page.

Today, Dyke Marsh is one of the largest remaining, freshwater, tidal wetlands in the Washington metropolitan area, but the wetlands have diminished to under 50 acres.

Dyke Marsh is home to many species that can only survive in wetlands. Wetlands are by their nature a water-laden environment with an enormous diversity of life forms. Wetlands provide ecological services for free. For example, they mitigate flooding, buffer storms, filter pollutants and provide habitat.

Dyke Marsh is habitat for 300 known species of plants, 6,000 arthropods, 38 fish, 34 mammals, 16 reptiles, 14 amphibians, over 270 species of birds and at least 20,000 species of insects.

Under the sponsorship of the Friends of Dyke Marsh, ecologist and author David W. Johnston, Ph.D., (1926-2015) consolidated and summarized many studies and reports addressing Dyke Marsh and published a paper in the Virginia Journal of Science (Volume 51, Number 4, Winter 2000). This compilation is an invaluable historical record and source of data about Dyke Marsh’s ecology, flora and fauna and human interaction.

The Virginia Journal of Science has made it available online here. You can access it on our website at Johnston Study. FODM has a limited number of reprints. Email us to determine availability and briefly explain your interest in the document.

Dr. Johnston was a member of FODM’s Board of Directors. He served in the U.S. Navy, earned his Ph.D. from the University of California at Berkeley, taught ornithology and conducted research at Wake Forest University, the University of Virginia and the University of Florida. He was Ecology Director at the National Science Foundation and worked at the National Academy of Sciences. He published widely on ornithology and ecology.

Dyke Marsh supports a diversity of animals, including gray squirrels, cottontail rabbits, shrews, field mice, river otters, red fox, little brown bats and whitetail deer. Evidence of beaver activity is often visible along the Haul Road and boardwalk. Beavers and muskrats can be seen swimming in the marsh in the early evening. 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. The table at right lists all the mammals observed in Dyke Marsh over more than 30 years as documented in the FODM sponsored study The Dyke Marsh Preserve Ecosystem by David W. Johnston.

Virginia Oppossum (Didelphis virginiana)
Northern Short-tailed Shrew (Blarina brevicauda)
Eastern Mole (Scalopus aquaticus)
Star-nosed Mole (Condylura cristata)
Little Brown Myotis (Myotis lucifugus)
Northern Red Bat (Lasiurus borealis)
Hoary Bat (Lasiurus cinereus)
Silver-haired Bat (Lasionycteris noctivagans)
Eastern Pipistrelle (Pipistrellus subflavus)
Big Brown Bat (Eptesicus fuscus)
Evening Bat (Nycticeius humeralis)
Eastern Cottontail (Sylvilagus floridanus)
Eastern Chipmunk (Tamias striatus)
Woodchuck (Marmota monax)
Eastern Gray Squirrel (Sciurus carolinensis)
Southern Flying Squirrel (Glaucomys volans)
American Beaver (Castor canadensis)
Marsh Rice Rat (Oryzomys palustris)
Eastern Harvest Mouse (Reithrodontomys humulis)
White-footed Mouse (Peromyscus leucopus)
Deer Mouse (Peromyscus maniculatus)
Eastern Woodrat (Neotoma floridana)
House Mouse (Mus musculus)
Norway Rat (Rattus norvegicus)
Black Rat (Rattus rattus)
Meadow Vole (Microtus pennsylvanicus)
Common Muskrat (Ondatra zibethicus)
Red Fox (Vulpes vulpes)
Common Gray Fox (Urocyon cinereoargenteus)
Common Raccoon (Procyon lotor)
Mink (Mustela vison)
Striped Skunk (Mephitis mephitis)
Northern River Otter (Lutra canadensis)
White-tailed Deer (Odocoileus virginianus)

FODMer Laura Sebastianelli is monitoring beavers in the western part of Dyke Marsh, behind River Towers. On May 1, she spotted two families, two adults and a kit and two adults and two kits nursing. At one point, she saw five adults and three kits on top of the lodge (see photo) and at least one beaver swimming around. Thank you, Laura. This is a very special, little-visited part of Dyke Marsh.

FODM member Deborah Hammer led a group of 35 on a frog and bat walk in the western part (photo, left) of the Dyke Marsh Wildlife Preserve on the evening of May 1. Deborah is a bat educator and rehabilitator. She lives nearby and has observed frogs and bats in this part of Dyke Marsh for the last 13 years. She is concerned that she is not hearing many spring peepers or seeing as many bats as she used to. “The sky used to be filled with them,” she commented.

Deborah told the group that frogs and toads need still water like ephemeral pools upland to lay eggs and hatch tadpoles. Bats need woodland trees upland of wetlands for nesting and marsh areas for hunting for food. She is concerned that development proposed for Westgrove Park to the west of Dyke Marsh could destroy or degrade the little nearby suitable habitat that remains. “It’s all interconnected,” she commented.

She offered many interesting facts, among them these:

• The most common native frogs here are the green frog, bullfrog, spring peeper and southern leopard frog. Dyke Marsh is the southern leopard frog’s northernmost habitat.
• Bats can live 20 years on average, are the only mammal that flies and can eat 3,000 insects a night.
• There are 16 bat species in Virginia and 10 in Fairfax County. (Little brown bat, photo by Rick Reynolds.)

Birds are perhaps the most visible and accessible animal species of the marsh. Bird-watching, or birding has made the Dyke Marsh Wildlife Preserve a popular attraction for many visitors. There are resident species in every habitat from emergent wetlands to upland forests as well as migratory species that visit the marsh each spring and fall. FODM has conducted several studies and surveys on birds and bird life and has documented over 270 species in Dyke Marsh. For a complete list including seasonal occurrence, see our Bird List page and see our Breeding Bird Survey page for a report on birds that breed in Dyke Marsh. These studies help to monitor the overall health of the marsh by documenting trends in bird populations such as in The Marsh Wren – Loss of Habitat, Loss of Birds below. FODM also participates in the annual Christmas Bird Count and leads a weekly Sunday morning bird walk all year.

At least three species of owls have been observed in the Dyke Marsh Wildlife Preserve over the years – barred owl (Strix varia), Eastern screech owl (Megascops asio) and the great horned owl (Bubo virginianus).

FODM President Glenda Booth has written an article in the January-February 2019 Virginia Wildlife magazine, titled “Virginia’s Most Mysterious Birds.” You can read the entire article here.

Butterfly checklist - Friends of Dyke Marsh volunteers conduct butterfly surveys from April to October every year. Working with staffers from the National Park Service, George Washington Memorial Parkway unit, we have prepared a butterfly checklist for our members and visitors. The checklist includes butterflies you could observe in Dyke Marsh and many of their host plants. Thank you to the following for helping produce this brochure: Jessica Strother, Jim Waggener, Mark Maloy and Brent Steury.

You can read the checklist here.

FODM participated in the National Park Service’s (NPS) Spotlight on Resources research conference on October 7 and 8, 2020. Jessica Strother and Bob Veltkamp prepared a poster describing our almost five years of volunteer surveys of dragonflies, damselflies and butterflies in Dyke Marsh. The poster, which includes 11 photographs, documents the species surveyed from 2016 through 2019: butterflies, 49 species; damselflies, 12 species; dragonflies, 36 species. Click here to view the poster. Our thanks to the dedicated volunteers and to NPS for the opportunity to share our work.

Since 2016, volunteers from FODM and the Audubon Society of Northern Virginia have conducted surveys of butterflies, dragonflies and damselflies in Dyke Marsh from April to October, led by Jim Waggener and Jessica Strother.

2020 marked the fifth consecutive year. Their volunteer efforts built on previous surveys by Dr. Ed Barrows, Georgetown University, and Christopher Hobson, Virginia Department of Conservation and Recreation, Division of Natural Heritage in 2016.

The surveyors covered a defined area around the Belle Haven Marina and the Haul Road trail, using the same methodology each year, except in 2020, volunteers followed covid-19 pandemic protocols for their protection.

2020 Results

In 15 surveys totaling 33 field hours, the volunteers documented 33 butterfly species and 19 dragonfly species. They added two butterfly species to the previous list: the American snout (Libytheana carinenta) and the great spangled fritillary (Speryeria cybele). They added three species to the dragonfly/damselfly list: the twelve-spotted skimmer (Libellula pulchella), the shadow darner (Aeshna umbrosa) and the swift river cruiser (Macromia illinoiensis illinoiensis) .

A few highlights:

• Butterfly species outnumbered dragonflies (51 to 39), but far more individual dragonflies and damselflies were counted than butterflies.
• The group found new species each year.
• The group observed a more positive trend in monarch butterflies than in the first year of the survey, 2016.
• Species and overall numbers of dragonflies continue to decline.

To read the report covering all five years of surveys, click here. To see the species checklist showing species identified over the five years, click here. To reach the FODM butterfly and host plant checklist, click here.

Eastern tiger swallotail

Eastern tiger swallowtail(Papilio glaucus), black form. Photo by Glenda Booth

Editor’s note: FODM volunteers are working to control invasive plants and add more native plants, especially at the native plant site on the Haul Road trail. Insects and plants co-evolve and insects like butterflies depend on certain native or host plants for food and shelter. We assume that more aggressively controlling non-native plants like English ivy and porcelain berry could provide more support for native insects, like those that these surveys target.

The Friends of Dyke Marsh send athank you to the following volunteers: Jim Waggener, Jessica Strother, Ed Eder, Larry Cartwright, Margaret Fisher, Joel Goldman, Su Kim, Joan Haffey, Ken Larsen, Joanne and Powell Hutton, Larry Meade, Rusty Moran, Gary Myers, Dave Nichols, Kristi Odom, Fred Siskind and Dixie Sommers. Thanks too to Brent Steury, George Washington Memorial Parkway, Natural Resources Program Manager.

FODM volunteers have conducted lepidoptera and odonata (butterflies, dragonflies and damselflies) surveys from April to October ever year since 2016. Here are the total species observed from 2016 to 2019: Butterflies - 49 species; Damselflies - 12 species; Dragonflies - 36 species.

Chris Hobson, with the Virginia Department of Conservation and Recreation, Division of Natural Heritage, conducted a four-day field survey, sponsored by FODM, in 2011 by boat and on foot between May and September.  Dr. Edd Barrows, a Georgetown University entomologist, has identified four additional species. 

Hobson identified the following 16 species in 2011:

Common Green Darner (Anax junius), Orange Bluet (Enallagma signatum), Big Bluet (Enallagma durum), Prince Baskettail(Epitheca princeps), Common Pondhawk (Erythemis simplicicollis), Eastern Forktail (Ischnura verticalis), Fragile Forktail (Ischnura posita), Slaty Skimmer (Libellula incesta), Needham’s Skimmer (Libellula needhami), Great Blue Skimmer (Libellula vibrans), Wandering Glider (Pantala flavescens), Blue Dasher (Pachydiplax longipennis), Eastern Amberwing (Perithemis tenera), Common Whitetail (Plathymis lydia), Russet-tipped Clubtail (Stylurus plagiatus), Black Saddlebags (Tramea lacerate).

On July 22, 2013, Hobson added four more species to the survey:

Widow Skimmer (Libellula luctuosa), Twelve-spot Skimmer (Libellula pulchella), Halloween Pennant (Celithemis eponina), River Cruiser (Macromia sp.).

Any survey is a “snapshot” at a given time and place, not a comprehensive inventory. Total numbers seen on any day can vary tremendously, depending on the weather and time of year. Hobson saw more than 1,000 of one species in one day and hundreds of another during a short paddling trip, for example. His report notes that “there seems to be a core group of species that can be found consistently in and around the marsh” and that a number of other resident and migratory species could occur in the marsh.

Dragonflies and damselflies are in the order Odonata. These insects have two pairs of wings and three pairs of legs, among other characteristics. Dragonflies typically spread their wings to their sides when they land and are typically larger and more robust. Damselflies are usually smaller and they typically hold their wings together over their abdomen. Their eyes are separated. To identify specific species, experts study their wings, wing patterns, colors, tail, thorax, abdomen, genitalia and other features, some under a microscope. Some have bright, lustrous hues and diaphanous wings. Behavior is another important factor in identification of Odonata.

Dragonflies and damselflies are found around rivers, wetlands, seeps, bogs, springs, streams, ponds and lakes because their larvae, known as nymphs, are aquatic. Adults can be from half an inch to five inches long. Some species migrate south and return to Virginia in the spring. In Virginia, at least 194 species of dragonflies and damselflies have been identified. The best time to see them is from April to October.

Hobson submitted a report to FODM and to the National Park Service.

The dominant species of plant is the narrowleaf cattail, a plant more common to salty water. This plant typically develops its characteristic flower spike by June and releases seeds in the fall. Other species include arrowhead, arrow arum, pickerelweed, sweetflag, spatterdock and wild rice.

Unfortunately, many non-native plant species have been introduced into the area, plants that often outcompete beneficial native species. Examples are porcelainberry, Japanese honeysuckle, multi-flora rose and Asian bittersweet.

On a hazy Saturday morning on July 26, 2014, about 30 FODM members and friends turned out for a plant walk led by Dr. Elizabeth Wells (“Call me Beth”), Associate Professor Emerita of Botany at The George Washington University. Dr. Wells’s walks are always engaging and informative, and this one was no exception: the enthusiastic group observed a number of colorful and interesting plants. FODM President Glenda Booth provided the accompanying photos with essay by Pat Salamone.

FODM is partnering with the National Park Service (NPS) on a project to try to save some of the preserve's pumpkin ash trees (Fraxinus profunda) from destruction by the emerald ash borer (EAB) (Agrilus planipennis), an invasive insect that was found in Fairfax County in 2003 and has been documented in Great Falls and Turkey Run Parks. "An EAB infestation is always fatal to ash trees," says the U.S. Department of Agriculture (USDA) website. The insect could kill all of the pumpkin ash trees in Dyke Marsh in 20 years without action.

Dyke Marsh has over 1,000 pumpkin ash trees. These trees thrive in wet areas like swamps, floodplains and bottom lands, can reach 130 feet in height and have a 68-inch diameter. The pumpkin name comes from the trees' usually swollen or pumpkin-shaped butt or base.

FODM's goal is to help NPS maintain a small, healthy breeding stock of these trees. In the spring of 2015, we contracted with Bartlett Tree Experts to conduct an insecticide treatment called Tree-age into the root flares of nine selected trees (Joshua Darkow applies treatment, photo left) in hopes that the treatment will kill any emerald ash borers infesting the trees. NPS chose a small group of trees located close together because ash trees have both male and female flowers and are air pollinated. NPS also tagged and wrapped the trees with beaver-resistant netting. Bartlett's Joshua Darkow injected the trees on May 13. If the treated trees remain healthy for two years, we hope to repeat treatments until the EAB infestation has passed through our area. There is no guarantee that the treatment will be successful.

In September, four North Carolina Botanic Garden staff members, with the assistance of Peter McCallum and Brent Steury from NPS and Robert Smith from FODM, collected pumpkin ash seeds in Dyke Marsh for preservation. They met their goal of 10,000 or more seeds from 25 or more trees. These seeds will be available in the future to help in the marsh's restoration and to replace trees expected to be lost to the EAB.

On May 16, 2016, with support from FODM and NPS, Bartlett Tree Experts treated another four pumpkin ash trees for the emerald ash borer. At that time, there were many dead trees, presumably because of this non-native insect. “Significant numbers of ash trees along the George Washington Memorial Parkway and in Dyke Marsh have failed to leaf out this spring,” said FODM project manager Robert Smith, “presumably due to the emerald ash borer. However, the nine trees we treated last spring are all in good shape.” He added, “Adding these trees to the initial group will greatly enhance the chances that we will be able to maintain a healthy set of pumpkin ash trees in the marsh. Hopefully, they will be able to both propagate themselves and provide seeds for use in the restoration.”

Referring to the trees treated in 2015, NPS Biologist Brent Steury said,” The first nine trees were obviously much healthier than any other ash trees in the area except the four I selected for treatment in this round.”

“Most of the surrounding ash trees are dead with one notable exception and that is a female at the far southeastern edge of the copse that also produced plenty of seeds. Of the study trees, three showed some significant bare areas ranging from approximately 25 percent to 40 percent of the branch area, but this does not indicate a fatal emerald ash borer infestation. Two of these trees were added in 2017 and showed some bare areas at the time of treatment but were determined worth trying to save.”

On September 25, 2017, five FODMers and Brent Steury with the National Park Service surveyed the treated trees in Dyke Marsh. Project leader Robert Smith wrote, “It was very encouraging to see that all of these trees had leafed out this season and that all six of the females had produced plenty of seeds. Six of the trees were originally treated in 2015, four more were treated in 2016 thanks to a dedicated donation, and six more were added by NPS in 2017 and all 16 were treated this spring with NPS funding (mitigation for three that were accidentally removed last year).

On April 11, 2019, FODM and National Park Service (NPS) employees revisited the pumpkin ash preservation site. FODM will finance the treatment of two more trees in May 2019.

In preparation for the treatment, Brent Steury, NPS Natural Resources Manager, Stephen Fagin, NPS Horticulturist, and Robert Smith, FODM Project Manager, surveyed the 16 study trees. They also wrapped trees with wire mesh to prevent beavers from chewing them down and tagged two trees for probable addition to this year’s treatment program. They took diameter measurements, put fresh orange ribbons on the study trees and did some minor clearing of invasive plants.

The success of this project is unknown. The emerald ash borer usually kills the trees it infects. It is too early in the year and in the study to draw any firm conclusions about the treatment’s effect or the health of the trees.

"It is painful to imagine Dyke Marsh without its expanse of ash trees so we are doing what we can to ensure that they will revive after an attack," said Robert Smith, FODM project manager. Photos courtesy of Glenda Booth.