Potomac River ecosystem

Category: Inland Aquatic Biomes.

Geographic Location: North America.

Summary: The Potomac River is the second-largest tributary to Chesapeake Bay; its condition reflects the various land uses in its watershed.

The Potomac River is the second-largest freshwater input to Chesapeake Bay. The name Potomac is a European spelling of an Algonquian name for the Powhatan tribe, who inhabited the upper reaches of the Northern Neck Peninsula in the vicinity of modern-day Fredericksburg, Virginia. The spelling of the name has been simplified over the years from Patawomeke, which may mean place of tribute or place of trade. The Potomac begins at Fairfax Stone, West Virginia, and meanders 380 miles (620 kilometers) from the Appalachian plateau through the Ridge and Valley, Blue Ridge, and Piedmont provinces, finally tumbling 76 feet (23 meters) over the fall line at Great Falls to the Coastal Plain, where it empties into the Chesapeake Bay at Point Lookout, Maryland. The river is tidally influenced for the 100 miles (170 kilometers) below Great Falls, and is brackish for the final 50 miles (80 kilometers).

The river has many major tributaries. The Potomac proper forms at the confluence of its North and South branches. From there, it receives water from the Shenandoah and Cacapon rivers to the south; Conococheague and Antietam Creeks and the Monocacy River from the north; and the Anacostia and Occoquan rivers below the fall line.

The 14,700-square-mile (38,073-square-kilometer) watershed encompasses parts of four states—Pennsylvania, Maryland, West Virginia, and Virginia—and the District of Columbia. It is the fourth-largest watershed on the East Coast of the United States. A total 55 percent of the basin today is forested, 16 percent is in agricultural use, and 14 percent, respectively, is suburban or urban development. The remainder comprises wetlands and open water. The Potomac is home to more than 6 million people, and thousands of species of plants and animals.

Biodiversity

The Potomac watershed contains several unique aquatic species and community assemblages. It is home to more than 15 species of freshwater mussels, many of which are the targets of conservation efforts due to their rarity and limited ranges. These bivalves live partially buried in the sediments and feed by filtering particles from the water flowing over them. Young are brooded on the gill surfaces of the parent mussel and then released into the water, where they attach to the gills of fish to complete their development.

The eastern elliptio (Elliptio complanta) is the most common mussel in the watershed, in the rivers and streams of the Piedmont and Coastal Plain. It appears to use the American eel exclusively as a host fish for its offspring. Other mussels exploit several host species during their reproduction. Mussels in the Potomac are negatively affected by land-use activities that alter hydrological regimes, water temperatures, or sediment and nutrient inputs.

Short­nose sturgeon (Acipen­ser breviro­strum), which were placed on the Endangered Species List in 1967, are also notable inhabitants of the Potomac River. Shortnose sturgeon are anadromous fish, spawning in rivers such as the Potomac but spending much of their time in the brackish waters of the Chesapeake Bay and coastal Atlantic Ocean marine habitats. They have a complex life history and are very long-lived, with the oldest recorded female reaching 67 years of age. The fecundity of the species is very low; females are slow to mature and thereafter spawn only once every three years. Their numbers in the Potomac were decimated in the early 1900s, due to by-catch from fishing pressure on a related species, the Atlantic sturgeon.

Several species of shad and river herring also migrate into the Potomac to spawn. American shad (Alosa sapidissima) spend much of their life in the Atlantic and return to natal rivers such as the Potomac to spawn in the spring. Juvenile shad spend their first summer in the river, then migrate to the open ocean by autumn. Shad have a relatively short maturation period of three to six years. Upon reaching sexual maturity, they return to the Potomac to spawn. However, numbers of migrating shad have declined sharply over the past 150 years.

As an economically important seasonal fish, American shad were subjected to extreme fishing pressure and were over-harvested through much of the 1800s and 1900s. The annual harvest for shad went from 17.5 million pounds (7.9 million kilograms) at the turn of the century to less than 2 million pounds (907,000 kilograms) in the mid-1970s. The fishery was closed in 1982. While harvests played a part in the drastic decline of the shad population, poor water quality, the loss of spawning habitat, and the construction of dams that hampered migration also contributed to the fish’s decline.

Since the mid-1990s, shad restoration efforts have included stocking more than 20 million shad fry and the building of a fish ladder at Little Falls, a dam used as a drinking-water supply for Washington, D.C. After more than two decades of restoration efforts, the shad population in the Potomac is still well below its historic levels, but is increasing slowly. In 2003, a very small commercial fishery was re-established in the river, but recreational fishing is still limited to catch-and-release at the present time.

Many exotic plants and animals have been introduced into the Potomac watershed. Some of them have become invasive species that are having significant effects on the ecosystem. The northern snakehead (Channa argus) is an invasive fish that was first discovered in the river in 2004. It is now firmly established, and can be found from Great Falls to the Chesapeake Bay.

Native to northern China and eastern Russia, the snakehead’s introduction was likely related to its popularity as a food fish among Asian communities and its availability in live-food markets. Juvenile snakeheads feed on a variety of benthic organisms (bottom-dwellers); adults are mainly piscivorous, or fish-eaters. While the impacts of the northern snakehead in the Potomac River have not been quantified, it is thought that these invaders may outcompete some native fish species for food and habitat.

The Asiatic clam (Corbicula fluminea), a bivalve mollusk native to Asia, the Middle East, and parts of Africa, was introduced along the west coast of the United States during the 1930s, and arrived in the Potomac River by 1977. Since that time, the clam population has varied widely from year to year, sometimes becoming very large in the tidal portions of the river. However, scientists have yet to discern negative effects due to this invasion. While they may be competing with native mussels for food and habitat, Asiatic clams are also helping filter the water in the Potomac and acting as a food source for aquatic birds and muskrats.

Likewise, a 1980s invasion of the aquatic plant hydrilla (Hydrilla venticillata) has appeared to have had beneficial effects on the tidal freshwater Potomac. A 17-year study indicated that while hydrilla became a dominant macrophyte in the river, it did not completely displace any native species of aquatic vegetation. In fact, the populations of native aquatic grasses expanded. Similarly, waterfowl populations increased with an increase in the growth of hydrilla and other nonnative plant species.

Human Impact

In addition to the introduction of nonnative species, land-based human activities have directly altered water quality in the Potomac watershed. Before 1994, the North Branch of the Potomac River was badly polluted by acidic coal mine drainage, rendering the environment desolate and devoid of all life for many decades. When some of the surface coal mines were abandoned in the 19th and 20th centuries, water flooded the mines. The influx of water and oxygen caused the oxidation of surrounding rock, which produced an array of chemicals (mainly sulfuric acid and iron precipitates) that ultimately decreased the pH level of the water, increasing its acidity. This polluted water flows through the water table and drains into the headwaters of the Potomac.

Remediation of these effects continues to be a huge challenge. The pollution caused by acid mine drainage in the North Branch has not been eliminated, but through a series of lime-dosing treatments (to neutralize the water pH value) that began in the early 1990s, water quality has improved enough for some aquatic life to survive. A trout-stocking program has allowed this portion of the river to become fishable again, but due to current pollution levels above Jennings Randolph Lake, natural populations of trout are not supported in that portion of the river.

By far the major point sources of nutrient pollution into the Potomac are sewage-treatment plants. The largest such facility in the Potomac River watershed is Blue Plains Sewage Treatment Plant, just south of downtown Washington, which processes an average 370 million gallons (140 million liters) of wastewater per day from the city and its Maryland and Virginia suburbs, as well as runoff from city streets.

Conservation Efforts

Blue Plains made dramatic improvements in the way it treats wastewater by adding a biological nutrient-removal step to its processing system. This step has resulted in a dramatic reduction in nitrogen-loading to the tidal Potomac.

Despite these improvements, more work needs to be done to reduce treated effluent entering the Potomac from Blue Plains. Areas within the watershed on combined sewer systems, where storm water and wastewater flow down the same pipe, are faced with the threat of overflow during periods of heavy rain. Some pipes are not big enough to deliver all the water to Blue Plains for treatment during heavy rains. In these cases, overflow from combined pipes is directed to local waterways. Blue Plains has made a pledge that by 2025, it will reduce the number of combined sewage overflows in the Potomac and Anacostia Rivers and Rock Creek by 96 percent.

For years, the Potomac Conservancy and various other environmental groups have been working to restore the river by reducing industrial and agricultural pollution. However, according to the Potomac Conservancy's 2020 Potomac River Report Card, the positive health trends have plateaued, and the river has slipped from a grade of B to a B-. Among the reasons are lower tidal water clarity, which threatens underwater habitats, decreasing juvenile fish rates for smallmouth bass, and an increase is sediment.

It is thought that an additional threat to the health of the Potomac will be from climate change. There will likely be more intense and frequent flooding, and with rising sea levels, existing tidal marshes and coastal habitats may shift. This will impact lower portions of the river and the wildlife there.

In spite of the problems currently facing the Potomac River biome, there is hope that local and federal efforts to restore the river from what President Lyndon Johnson famously labeled “a national disgrace” are slowly working. From the reduction of acid-mine drainage effects in the headwaters, to increases in water clarity and native submerged aquatic vegetation in the tidal freshwater portion of the river, the Potomac is on its way back to improved health and sustainability.

Bibliography

Johnson, D. and K. Hallberg. “Acid Mine Drainage Remediation Options: A Review.” Science of the Total Environment 338 (2005).

Lookingbill, Todd R., et al. “Altered Ecological Flows Blur Boundaries in Urbanizing Watersheds.” Ecology and Society 14, no. 2 (2009).

Odenkirk, J. and S. Owens. “Expansion of a Northern Snakehead Population in the Potomac River System.” Transactions of the American Fisheries Society 136 (2007).

Phelps, H. “The Asiatic Clam (Corbicula Fluminea) Invasion and System-Level Ecological Change in the Potomac River Estuary near Washington, D.C.” Estuaries 17 (1994).

Rice, J. Nature and History in the Potomac Country: From Hunter-Gatherers to the Age of Jefferson. Baltimore, MD: Johns Hopkins University Press, 2009.

Solyst, Jake. "The Health of the Potomac River Plateaus after Decades of Progress." Chesapeake Bay Program, 23 Oct. 2020, www.chesapeakebay.net/news/blog/the‗health‗of‗the‗potomac‗river‗plateaus‗after‗decades‗of‗progress. Accessed 30 Aug. 2022.