Natural History of the
Potomac Valley

by Philip J. Stone

Those of us who live, work, and play in the Potomac Valley are more or less aware of the natural world around us, depending on our interests and the keenness of our senses. Man has occupied the valley for several hundred years and brought about many changes, yet much undisturbed or slightly disturbed territory remains. This article is an attempt to summarize, in a popular comprehensive way, the natural history of the Potomac Basin.

Physical Geography

The Potomac Basin or watershed is the land drained by the river and it is shaped like a tomahawk or boomerang on the map, with the handle pointing east Its area is 14,670 square miles. 39% is in Virginia, 26% in Maryland, 24% in West Virginia, 11% in Pennsylvania, and a tiny portion in the District of Columbia. About half is mountainous and half forested; of course the two categories overlap. The basin is still predominantly rural, although less so every year. 65% is in farms, although only 29% in crops. This may surprise denizens of the Washington Metropolitan Area who don't often venture into the back country. Elevation varies from sea level at the mouth of the Potomac to 4860 feet at Spruce Knob in West Virginia. The Potomac itself is 385 miles long and 11.3 miles wide at its mouth (7 miles if you measure at right angles to the river instead of from Point Lookout, Maryland to Smith Point, Virginia.).

This watershed lies in four physiographic "provinces": the Coastal Plain or Tidewater from the river's mouth up to the Fall Line at Washington; the Piedmont from Washington to Point of Rocks where the first mountain range is encountered; the Blue Ridge-Great Valley province from Catoctin Ridge to the west side of the Shenandoah Valley; and the Appalachian province. The last is divided into the Ridge and Valley Area extending from the Shenandoah Valley to the Allegheny Front, and the Allegheny Plateau west of the front.

Mountains of the Potomac Watershed are almost all ridges rather than peaks. Some of the more important (from east to west) are South, Sleepy Creek, Tuscarora. Cacapon, and Martin Mountain mountains; the Blue, Tonoloway, and Green Ridges; and Sideling and Town Hills. All of these are intersected by the Potomac as it flows from the Allegheny Plateau to the Piedmont. Allegheny Front, Big Savage and Backbone Mountains rise from the Allegheny Plateau. Massanutten Mountain lies in the middle of the Shenandoah Valley. And Sugarloaf , southwest of Frederick, Md., is a monadnock or single peak.

Streams of the basin will be discussed later in the section on water.


Our basin generally has a moderate climate, with variations between physiographic provinces. Summers are warm and humid, winters moderate.

Mean January temperatures are 38 degrees in the Coastal Plain, 33 in the Piedmont, 34 in the Great Valley, and 30 in the mountains. Mean July temperatures are 78 degrees in the Coastal Plain, 76 in the Piedmont, 75 in the Great Valley, and 72 in the mountains. Garrett County, Md., on the wind-swept Allegheny Plateau, has registered 30 degrees below zero.

Precipitation is generally well distributed throughout the year. Average annual rainfall for the basin is 38 inches. It is 42 inches for the Tidewater and the Piedmont, 37 in the Great Valley, and 36 in the mountains. 54 inches have fallen in a year at the headwaters of the North Branch near the Fairfax Stone, usually considered the source of the Potomac. But in the upper portion of the South Branch, in the "rain shadow" on the eastern side of a mountain ridge, only 30 inches has been reported. Snowfall per year averages 5 inches in Tidewater and 30 inches in the mountains. Summer thunderstorms are common, sometimes with long droughts between them. Tropical hurricanes occur occasionally. And some sort of flood occurs on the Potomac about every two years.

Rocks and Minerals

The underlying rocks of the Coastal Plain are gravel, sand, clay and marl (unconsolidated sedimentary deposits). In the Piedmont they are hard, crystalline rocks -- granite (igneous); gneiss, mica schist, and metagraywacke (metamorphic); and in some places sandstone and shale (sedimentary). In the Blue Ridge they are granite (igneous) and greenstone, quartzite, and phyllite (metamorphic) Limestone underlies the Great Valley with some sandstone and shale (all sedimentary). In the Ridge and Valley Area sandstone predominates on the ridges with shale in the valleys. And on the Allegheny Plateau there is conglomerate (also sedimentary).

What mineral deposits are associated with these rocks? Major materials are coal (from ancient petrified ferns) on the Allegheny Plateau; sandstone in the Ridge and Valley Area; limestone in the Great Valley; and sandstone, gravel and clay in the Coastal Plain.

In these rocks there are numerous caverns, especially in limestone country. For example, there are the Luray, Endless, Grand, Shenandoah, Smokehole, and Seneca Caverns.

Fossils provide a good representation of pre-historic animals. A good example is in the Nomini Cliffs on the lower Potomac in Westmoreland County, Virginia.


Since soil is the result of the weathering of underlying rocks, there are different soils in different provinces. The unconsolidated sediments of the Coastal Plain break down to form sandy or silty soils, generally well drained, usually acidic and with moderate capacity to absorb water. The crystalline rocks of the Piedmont form loam or silty loam which is well drained, moderately acidic, and very fertile. The limestone of the Great Valley weathers into silty loam which is very well drained, slightly acidic and fertile. The sandstone of the ridges west of the Great Valley forms stony soil of low fertility. The shale of the intervening valleys forms well drained soils which are acidic and moderately fertile. And finally, the conglomerate of the Allegheny Plateau breaks down into soil which is poor for agricultural crops but satisfactory for pastures and forests.


There are approximately 700 miles of major streams in the Potomac Valley, plus 3400 miles of streams which are large enough to support fishing. The Potomac's principal tributaries (from east to west) are the St. Mary's, Wicomico, Occoquan, Anacostia, Monocacy, and Shenandoah Rivers, the South Branch and the Savage River.

An estuary is the lower portion of a river where there is tidal action, which is fed by fresh water, and which is connected to the sea. The Potomac's estuary extends from its mouth at Chesapeake Bay to the limit of tide below Chain Bridge. (The tide is about 3 feet at Washington.) The estuary is salty from its mouth to Piney Point, brackish from Piney Point to Marshall Hall, and fresh from Marshall Hall to Washington. Like a marsh, the estuary is more productive than the adjoining land, although productivity has been diminished by pollution and sedimentation.

From Washington to Great Falls the Potomac is rather narrow, the flow fairly swift, and the water cloudy to muddy. Flow at Washington averages 11,000 cubic feet per second, but there are great fluctuations. From Great Falls to Harper's Ferry the river is wider, flows more slowly, and appears cloudy to muddy. At Harper's Ferry the Shenandoah River brings in much silt from erodible farm lands. West of Harper's Ferry the main stream of the river is clearer; west of the Shenandoah Valley it is swifter. Above Green Spring, W. Va. where the South Branch enters the Potomac, the main river (or North Branch) flows rather clear but is polluted by acid mine drainage.

The basin has no natural lakes. There are two large impoundments, the Bloomington Reservoir on the North Branch some distance above Cumberland, and the Savage River Reservoir, upstream from the mouth of the Savage River which enters the Potomac a little below Bloomington. There are a number of small watershed projects with impoundments, and many farm ponds in the watershed which enhance the landscape.

There is a fairly large potential for ground water storage in unconsolidated sediments east of the Fall Line, especially in Prince George's County, Md. Very little ground water storage is found west of the Fall Line except in areas underlaid by limestone.


The Potomac Basin's vegetation is about equally divided between forests and open lands (pasture and cropland). The valley is especially interesting botanically because here there is an overlapping of northern and southern species, including plants from New England and the Carolinas. This is due not only to its intermediate latitude between north and south but because of the variations in altitude. The result is a great variety of plant habitats.

With respect to trees and shrubs, there is virtually no virgin timber remaining but half of the watershed is covered with second growth forests. Large wooded tracts are chiefly publicly owned. Within the basin are the Shenandoah National Park and the George Washington National Forest, plus much of the Monongahela National Forest, as well as a number of state parks, forests, and wildlife areas. Small tracts are mostly farm woodlots. Altogether the Potomac Valley contains more than 100 species of trees, mostly deciduous.

On the Coastal Plain evergreens found are Virginia, pitch, and loblolly pines, cedar and holly. Deciduous species are red, pin, and black oak; sweet and black gum; hickory, soft maple and tulip poplar.

In the Piedmont uplands the evergreens found are Virginia and pitch pine, hemlock and mountain laurel. Deciduous species are white, black, and chestnut oak, red and sugar maple; hickory, tulip poplar and dogwood.

In the mountains the evergreens are red spruce, balsam fir, white pine, hemlock, rhododendron, laurel, and azalea. Deciduous species are chestnut, scarlet and white oak; and hickory.

In the river bottoms one finds sycamore, red birch, red maple, green ash, American hoarbeam, willow, box elder, pawpaw, elm, and spicebush. And in swamps there are pin, willow, and swamp white oak; sycamore, beech, red maple, alder and arrowroot.

Turning to herbaceous (non-woody) plants in the woods of the Coastal Plain and Piedmont, one can encounter such wildflowers as hepatica, bloodroot, spring beauty, violet, trout lily, Dutchman's breeches, red columbine, Jack-in-the-pulpit, red anemone, May apple, Solomon's seal, Virginia bluebell, and wild blue phlox, plus such ferns as maidenhair, Christmas, and cinnamon fern.

In mountainous woods one sees painted trillium, trailing arbutus, Clintonia, bunchberry, wild bleeding heart, Canada mayflower, and fireweed, plus polypody fern.

Herbaceous plants of field and roadside include such composites as daisy, sunflower, aster, and goldenrod; grasses; sedges; legumes (wild pea, lupine, clover, and trefoil); cinqfoil, strawberry, and blackberry from the rose family; and bee-balm and wild bergamot from the mint family.

In the marshes one finds cordgrass, bulrush, cattail, wild rice, duck potato, and sedges.

The Potomac Basin includes a number of interesting, special habitats. One is the sphagnum glade, a wet meadow along a cold stream in the Allegheny Plateau. Examples are Pine Swamp Run in western Maryland, and Stony and Arbams Creek in West Virginia. Here are found sphagnum moss, pitcher plant, cranberry, and bog rosemary.

A second is the heath barren, a high plain on the Allegheny Plateau. Examples are Dolly Sods and Roaring Plains. Here are found azalea, blueberry, huckleberry, mountain laurel, and wild raisin.

Another special habitat is the shale barren, a dry slope in a "rain shadow" on the east side of a ridge. An example is Massanutten Mountain. Here are found prickly pear cactus, bird's foot violet, and creeping phlox.

Still another is the magnolia bog, an area characterized by gravel, diffused springs and acidity. Remaining examples are small bogs near Suitland and Beltsville, Md. Here one may find pitcher plant, sundew, and swamp magnolia.

Finally, there is the phenomenon of plant migration. Willow oak, a Coastal Plain tree, is found in the Piedmont along the Potomac at Seneca, Md. Bladderwort, a Piedmont plant, is found on Theodore Roosevelt Island in the District of Columbia. And southward migration of northern species caused by the glaciers has brought red spruce, balsam fir, Canada mayflower, and Clintonia from the Adirondacks to such places as Cranesville Swamp in the Allegheny Plateau.


The Potomac Valley's wild animals are less obvious than its plants, because many of them try to keep away from human residents. And the mammals are chiefly nocturnal.

Some of the mammals inhabiting the mountain forests are black bear, bobcat, red squirrel, and grey fox. In the Piedmont and Coastal Plain forests whitetailed deer, raccoon, grey squirrel and opossum are common.

In fields and brushy areas red fox, woodchuck, skunk, and cotton-tail rabbit are important denizens. In wetlands and aquatic areas (including the Potomac itself) beaver, otter, mink, and muskrat are important. And generally found in several habitats are the bat, mole and shrew.

Turning to birds, which are usually easier to spot than mammals, the forests harbor wild turkey, ruffed grouse, owls, woodpecker, ovenbird, nuthatch, red-eyed vireo, and warblers.

Open areas (fields, meadows, and pastures) provide homes for bobwhite, mourning dove, woodcock, hawks, meadowlark, bluebird, kingbird, sparrows and crow.

In suburbs and orchards the robin, mockingbird, song sparrow, northern oriole, cardinal, blue jay, chickadee, titmouse, and Carolina wren are common.

If one ventures into a marsh, he may find herons, rails, red-winged blackbird, marsh hawk, coot, Canada goose, and "puddle" ducks. If one goes into a swamp, however, he may encounter the wood duck, water thrush, various warblers (especially prothonotary), Acadian flycatcher, and white-eyed vereo.

In and along rivers and ponds there are water fowl (ducks, geese, and swans), gulls, grebes, the kingfisher, herons, and egrets. And generally distributed in many habitats are the crow, titmouse, chickadee, blue jay, cardinal, and Carolina wren.

Reptiles and Amphibians

Turning to reptiles and amphibians, one can find two species of poisonous snakes in the Potomac Basin -- copperhead and timber rattlesnake -- and many non-poisonous snakes, including the black snake, garter snake, and grass snake. Turtles include the box, snapping, painted, mud, and musk turtle. The blue-tailed skink represents the lizards and there are numerous species of frogs, toads and salamanders.


Turning to fish, in the Tidewater the common finfish are herring, shad, sea trout, rockfish, alewife, croaker, and yellow perch. The shellfish (crustaceans and mollusks) to be found are the crab, oyster and clam. In the middle ranges of the Potomac there are warm-water species of finfish -- black bass (both large-mouth and small-mouth), sunfish, crappie, sucker, catfish, carp, yellow perch and pickerel. In the upper Potomac and its tributaries one finds cold-water finfish -- trout (brook, brown, and rainbow) and pike.

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One special habitat should be mentioned -- caves. Here one may encounter bats, salamanders, snails, and beetles.

Unlike flora, some species of fauna migrate regularly each season. Many birds go north in the spring and south in the fall along the Atlantic Flyway, which crosses the Potomac Basin. Some finfish (shad and herring) are andromous, i.e., live most of their lives in salt water but ascend fresh water streams to spawn. Others (the eels) are catadromous, i.e., live mostly in fresh water but descend to salt water to spawn. The monarch butterfly who flies south to Mexico each fall is another example of migration in the animal world.

Numerous printed references concerning the Potomac Valley's natural history are available. Consult the author for suggestions.