A Wild River Runs Through Washington
by Jeffrey P. Cohn

Stephanie Flack stuck her shovel into the ground and gave a good, hard push down with her foot before lifting out a load of dirt. Once the hole she had created was sufficiently deep, Flack picked up a sycamore sapling and carefully placed the young tree in it, then refilled it with dirt. Meanwhile, a dozen staffers and volunteers from The Nature Conservancy, the National Park Service (NPS), and other organizations dug their own holes and filled them with sycamore, paw paw, and spice bush trees.

Mather Gorge
Mather Gorge is part of the Potomac River Gorge, and it features the rocky outcroppings and bedrock terraces (seen here on the left side of the river) typical of the area. (Gary P. Fleming/Va. Dept. of Conservation & Recreation)

"This is a priority conservation site for us," said Flack, The Nature Conservancy's Potomac Gorge project coordinator, of the one-acre plot of land where she and the others worked. Located on the Virginia side of Chain Bridge, the site had been damaged during the construction of a bridge on the nearby George Washington Memorial Parkway. It's a special place, noted Flack, not least because it features several seeps that provide habitat for rare crustaceans called amphipods and isopods. "It's small, but it protects a significant segment of the Potomac River Gorge," she said.

The Potomac River Gorge is a deep, narrow chasm that runs for 15 miles, from Great Falls to Georgetown. Over eons, the river slowly carved it from the rocks, creating steep cliffs that sometimes tower 60 feet above the water. Only a dozen miles from the nation's capital, the gorge harbors a surprising number of plant species and communities in a variety of habitats. These include the river and its tributaries, upland forests, dry bedrock terraces, floodplain woodlands and prairies, and pools, ponds, and marshes that support what is perhaps the most diverse flora on the entire East Coast.

More than 1,400 different plants grow within the Potomac River Gorge, including 300 listed as state species of concern in Maryland, Virginia, or the District of Columbia. Some are common in the gorge, but rare elsewhere. Others are rare species found locally only in the gorge. For still others, the gorge is a last remaining reserve. Beyond individual plants, scientists have identified some three dozen plant communities here, including at least three found nowhere else in the world.

The gorge contains not only a diverse collection of plants, but animals as well. Twenty-six species of amphibians alone can be found here, including 14 salamanders, ten frogs, and two toads. The diversity of plants attracts a variety of bees, butterflies, and other insects as well that, in turn, attract a variety of birds, says Laura Ingram, an NPS resource specialist.

In all, the Potomac River travels 383 miles on its journey from Fairfax Stone in West Virginia to where it joins the Chesapeake Bay at Point Lookout, Maryland. During that course, the river and its tributaries drain nearly 15,000 square miles of West Virginia, Virginia, Pennsylvania, Maryland, and the District of Columbia. It is the fourth-largest watershed on the East Coast and the Chesapeake Bay's second-largest (after the Susquehanna). "This is probably the wildest urban river anywhere in the world," says Matthew Logan, president of the Potomac Conservancy, a local environmental group. "It's unique. It is the nation's river."

One reason for the gorge's wildness is its lack of dams: Unlike most other East Coast rivers, only a few, mostly small, dams disrupt the Potomac's flow. Another is the federal, state, and local parks that protect more than half of the gorge. The 25-mile-long, 7,300-acre George Washington Memorial Parkway, which serves Maryland, Virginia, and D.C., and runs alongside the Potomac River, includes two parks in Virginia: the 800-acre Great Falls Park and the 235-acre Turkey Run Park. Also in Virginia, two county parks, Scott's Run and Riverbend, protect another 750 acres near Great Falls. On the Maryland side, the gorge's natural beauty is preserved by the 184.5-mile-long, 19,682-acre Chesapeake and Ohio (C&O) Canal National Historic Park and Cabin John Stream Park. And the Potomac Conservancy owns all or parts of four islands in the Potomac.

"We are a corridor of biodiversity," says Daniel Sealy, deputy director for natural resources of the NPS' Center for Urban Ecology. The Potomac River Gorge and the parkland that protect it are "strips of green spaces surrounded by suburban development. This is the only place [in the Washington, D.C., area] that some plants and animals can live."

The Potomac River Gorge also owes its biological diversity to the unusual hydrology, geology, and geography of the area. Perhaps most important in promoting diversity in the gorge is the Potomac's propensity to flood. The river can carry 1.4 million cubic feet of water per second during heavy floods. Not only is that 100 times the low flow, but it's enough to send water cascading over banks and tall cliffs. Even lesser floods scour rock terraces, fill old channels and oxbows, rearrange soils and sediments, and wash away years or even decades of old growth. The extensive floodplains on the Maryland side and the hills overlooking the river in Virginia owe their origin to those periodic floods. Other rivers such as the James may flood, but few have the power to create a complex gorge like the Potomac.

Geologically, the Potomac River crosses the "fall zone" that separates the softer rocks of the Atlantic coastal plains from the harder, more erosion-resistant ones of the inland Piedmont Plateau. As a result, the river plunges 76 feet in a series of fast-flowing and dangerous rapids at Great Falls Park. Given the diverse habitats in the Potomac watershed, the river often carries plant seeds from other ecological zones and deposits them in the gorge.

The gorge features what geologists call "bedrock terraces," rock formations that are found in few other rivers. "It looks like someone bulldozed the area," says Scott Southworth, a U.S. Geological Survey research geologist, and adds, "The river did that." Sedimentary and metamorphic rocks cover the river bottom, and on land they form rocky bluffs and outcroppings that create habitats ranging from crevices, potholes, and ponds to prairies, shrub- and tree-covered woodlands, and forests.

In Great Falls Park alone, independent botanist Cristol Fleming found seven distinct plant communities in a 1993 survey, and the Potomac's bedrock terraces support the most unusual of them all. Most other terraces on the East Coast, such as those on the Susquehanna, are flooded as a result of the rivers being dammed, but most of the Potomac's terraces are on dry ground, creating a perfect environment for small plants such as moss phlox, which has little leaves; lyre-leaved rock cress, which has distinctive, four-petaled flowers; and wild columbine. Two of Virginia's 30 species of goldenrod are restricted in the Washington area to the Potomac River Gorge. Trees include white ash, red maple, and sycamore, as well as a holly rare in Maryland. Sugar maples are more common on the Virginia side of the Potomac than on the Maryland side.

The bedrock terraces are found extensively on Bear and Olmsted Islands, two of the most popular and easily accessed tourist areas within the C&O National Historic Park. The 160-acre Bear Island is the largest of the Potomac's many islands. It was once separated from the Maryland mainland by a side channel of the Potomac that has long since become the C&O Canal. Its woodlands include three species of oak—red, white, and chestnut—plus hickory and hop hornbeam, a shrub-like tree. Olmsted Island, whose plant communities are similar to those of Bear Island, is also separated from the mainland by a narrow channel, but it can be accessed via a pedestrian bridge and boardwalk.

The Chain Bridge flats represent yet another interesting habitat within the Potomac River Gorge. Here, in the area underneath Chain Bridge in the District of Columbia, lies a coastal plain being taken over by a sycamore and maple forest. If the river were left to its own devices here, natural flooding would sweep across the flats and wash away tree saplings, giving the land a prairie character, but a man-made berm now prevents most floods by forcing floodwaters back into the river. The Nature Conservancy is hoping to persuade the U.S. Corps of Engineers to remove the berm and fill in a ditch to allow the flats to flood again.

Elsewhere within the gorge, upland forests contain cottonwoods, tulip poplars, black walnut, and chestnut oak, among other trees. Shorter paw paw trees often form thickets because deer will not eat their leaves. "It's important to have an upland forest here because it is mostly gone elsewhere in the D.C. area," says the NPS' Ingram.

Floodplain forests, which cover the lower reaches on the Maryland side of the river, are the most common habitat within C&O Canal National Historic Park. One of these forests can be seen along the wooded Billy Goat Trail, a popular hiking path on Bear Island. Woodland species include sycamore, silver maple, box elder, and red and white oaks. Sycamores, with their peeling bark and whitish winter trunks, are probably the easiest trees to identify, and although most along the Billy Goat Trail are short and thin, sycamores can grow quite large. Wildflowers such as Virginia bluebell, Dutchman's breeches, and toad trillium bloom in the spring. Even rare flowers such as white trout lily and golden Alexanders grow amid the trees.

A large portion of the Virginia side of the Potomac faces north, and its steep slopes and rich soils hold more water than Maryland's south- and west-facing slopes. Consequently, the Virginia side has a much thicker hardwood forest and more open terraces. Its hillsides and deep ravines also discouraged early settlers from clearing land for farming, and most of its land has not been logged since the Civil War. On the other hand, the Virginia side is eroding faster and has fewer bedrock and floodplain terraces than the Maryland side.

Instead of terraces, many of the hillsides on the Virginia side of the gorge feature what Gary Fleming, a vegetative ecologist with the Virginia Division of Natural Heritage, calls "boulder fields." These large areas are strewn with huge boulders and smaller rocks, which break off from bedrock outcroppings as the river erodes the gorge westward faster than it does eastward. Between the rocks grow shrubs such as bladdernut and wildflowers such as jewelweed.

Animals also make their homes in the Potomac River Gorge. Red (Vulpes vulpes) and gray (Urocyon cinereoargenteus) foxes come down to the river and canal to drink, white-tailed deer (Odocoileus virginianus) browse among the upland woodlands and forests, and raccoons (Procyon lotor) and opossums (Didelphis virginiana) go just about anywhere in their search for food. But perhaps the most interesting animal story in the gorge lies within the river itself.

The Potomac River once supported millions of common or American shad (Alosa sapidissima), fish that are closely related to herrings and live in the ocean but spawn in freshwater rivers. Once the most abundant and commercially important fish in the Chesapeake Bay region, shad formed such thick spawning congregations in the Potomac and other Atlantic coast rivers that it was said one could walk across the water on their backs. Overfishing, polluted waters, and the construction of dams on other rivers nearly spelled doom for shad. The commercial catch dropped from a high of 2.4 million pounds in 1950 to less than 25,000 in 1980. As a result, between 1980 and 1992, commercial and recreational shad fishing were banned on the Potomac.

Aided by these protections, less polluted waters, and the stocking of more than 15 million captive-hatched shad from the Harrison Lake National Fish Hatchery in Virginia, shad have rebounded in the Potomac. The number caught per research net has risen from 15 in the 1990s to 50 in 2002 and 94 in 2003. "That's just a beginning," says James Cummins, associate director for living resources of the Interstate Commission on the Potomac River Basin. "I expect those numbers to grow dramatically. I expect the river to turn silver again."

While the gorge's shad numbers are on the rise, its plant biodiversity is increasingly challenged by the invasion of non-native plants. These exotics crowd out native species, including rare or unusual ones, and some have been in the gorge for decades. An overabundance of foraging deer also threatens rare plants and plant communities, and the growing number of visitors to the national and local parks in the gorge stresses the natural resources and the park staffs' ability to manage them. Additionally, the region's growing human population is developing more land near the Potomac River and demanding more water from the river and its tributaries.

Development is the greatest threat to the area's biodiversity, but invasive plants are a close second, says Logan. At least 273 non-native plant species now grow within the gorge, including wide-ranging plants such as Japanese stilt grass (also known as Microstegium), English ivy, Japanese honeysuckle, garlic mustard, and Ailanthus (also called the "tree of heaven"). Some have been carried in as seeds from nearby suburban lawns and gardens by birds, mammals, and the shoes of people, and by the countless creeks and small streams that flow into the Potomac. The problem is greatest along low-lying areas near the river that are subject to more frequent flooding.

To address the problem of exotic plant species, the NPS and private groups such as The Nature Conservancy and the Potomac Conservancy have organized volunteers and staffers who spend weekends removing non-native plants from the gorge and its islands. On Minnie's Island, for example, Potomac Conservancy volunteers and staffers rip out vines, roots and all, and sometimes spray chemicals to prevent their return. The NPS organizes similar efforts along the C&O Canal and George Washington Memorial Parkway. Keeping up with the spread of exotic plants is "a challenge," says Rod Sauter, the C&O Canal's supervisory interpretive ranger.

Toward the same end, the Potomac Conservancy and The Nature Conservancy distribute a "Good Neighbor Handbook" to guide private landowners who want to help preserve the gorge's native biodiversity. It urges people to landscape with native plants and plant species that deer do not eat, reduce the use of fertilizers, and capture and use storm water. It also encourages landowners to consider conservation easements to protect their properties and the gorge from development.

Meanwhile, the number of white-tailed deer residing in the gorge has ballooned in recent years. Biologists estimate there is an average of 85 deer per square mile in the C&O Canal in Maryland and 78 per square mile within the George Washington Memorial Parkway in Virginia, says Scott Bates, an NPS wildlife biologist. More than 40 deer per square mile is usually considered too many, he adds. But given the public outcry over any proposed deer hunt in the area, the NPS has no plans to allow hunting within either the C&O Canal or George Washington Memorial Parkway.

The increasing demand for water from the Potomac and its tributaries is yet another concern to park officials and conservationists. More than five million people live within the Potomac's watershed, some 4.6 million of them in the District and its suburbs. Those figures are up 13 percent since 1990. Washington and its suburbs get 360 million gallons of water a day from the Potomac. As the nation's capital continues to grow, the demand for water is likely to increase too. The result may be insufficient water, especially during droughts and other periods of low flow, to sustain the gorge's natural diversity.

To those problems add the number of people who jog, walk, hike, bike, kayak, fish, or otherwise use the gorge, canal, and river. An estimated 3.5 million people visit the C&O Canal each year, and up to 1.5 million of them go to the gorge. "The park is at risk of being loved to death," The Nature Conservancy's Flack observes. "The gorge is a unique and sensitive place that we all use. It has exceptional natural and cultural resources." Keeping it that way will be a challenge.

—Jeffrey Cohn is a freelance writer who lives in Takoma Park, Maryland, and specializes in conservation subjects.

ZooGoer 34(6) 2005. Copyright 2005 Friends of the National Zoo. All rights reserved.



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