Planting her ski pole on the uneven trail, Harriette Phelps begins the half-mile trek down to the Potomac River near Indian Head, Maryland. She's recovering from leg surgery and moves cautiously down the trail, part of Fort Foote Park. Earl Greenidge, her former student and current data technician, carries her field collecting gear and walks nimbly ahead, easily managing the bulky equipment.

By the time Phelps reaches the sandy beach, Greenidge is already hunched over in the knee-deep water of low tide, scooping sand from the bottom using the exterior cage of a fan (the so-called fan guard) as a sieve. With each fan-full, he brings up more than a dozen small clams that remain behind in the fan guard after the sand falls through. Phelps, an emerita biologist at the University of the District of Columbia (UDC), instructs Greenidge to keep sampling until he gets 200 of these small, ridged clams, called Corbicula fluminea.

Greenidge meets the target number in under five minutes — Corbicula thrives at high densities in the Potomac. He wades back to the beach and transfers the clams into shellfish bags made from hard plastic. Phelps and Greenidge pack up their equipment and head back up the trail.

A potent filter feeder, Corbicula may take some of the credit for improving water quality conditions in the Potomac River, according to Phelps. Scientists first identified Corbicula in the Potomac in 1977, and they watched as populations of these clams rapidly increased. By the mid-1980s, Phelps, with help from her students at UDC, calculated that the spring-summer clam population could filter one-third of all the water in this region of the estuary daily. As early as 1981, researchers reported a tripling of water clarity in the region of the clam beds. In 1983, submerged aquatic vegetation (SAV) reappeared for the first time in 50 years. Starting in 1984, the Washington, D.C. Christmas Bird Census reported significant increases in several aquatic bird populations. By 1986, fish populations had increased up to seven times in the newly grown beds of underwater grasses. Corbicula may have helped trigger these system-level changes in the Potomac River, as Phelps reported in a 1994 article in the journal Estuaries.

But the river experienced other changes at the same time, so the role that the clams played proves tough to parse. Improved sewage treatment at the Potomac River-based Blue Plains Waste Treatment Plant, along with a ban on phosphorus in laundry detergent, helped improve water quality by decreasing the flow of nutrients to the river. The fast-growing invasive plant Hydrilla verticillata also appeared in the Potomac in the mid-1980s. Hydrilla may have also helped transform the ecosystem by stabilizing the bottom, producing oxygen, and encouraging the growth of native plants.

Corbicula's positive impact on water quality comes in a complicated package. Like Hydrilla, it is a non-native, invasive species. From its native range in Asia, Corbicula was imported to the west coast of the U.S. in the 1930s for food in Asian markets. When scientists first identified this clam in the Potomac River, they braced for the worst, fearing that it would behave like the dreaded zebra mussel, attaching to every available hard surface, clogging water intake pipes at power plants, and transforming the fundamental who-eats-whom structure of the Chesapeake's freshwater rivers. A research note published by U.S. Geological Survey scientists in Estuaries in 1980 adopted a warning tone, stating that this addition of Corbicula fluminea to the Potomac River ecosystem must be followed attentively. The paper cited another researcher who called the clam "the most costly liability of all exotic mollusks in North America."

Despite these fears, the worst did not come to pass. Unlike the zebra mussel or the native dark false mussel, Corbicula lacks byssal threads and cannot attach to hard surfaces. The clam did cause some problems due to fouling — the Potomac Electric Power Company reported operational problems caused by clamshells and silt clogging their condenser cooling water tubes. As they drift downstream, juvenile clams also get caught inside the intake wells of power plants. But with an open ecological niche available along the sandy bottom sediments of the river, Corbicula could spread rapidly without crowding out populations of native species in the Potomac. As a result, scientists do not think the clam has had a major impact on the river's food web, except as a food source for birds and muskrats.

When Phelps reaches her Honda back in the parking area, she realizes she forgot to bring the ice packs to keep the clams cold while in transit. She makes plans to stop for ice at a nearby fast food restaurant on their way to the Anacostia River.

Phelps is moving these clams from the now relatively unpolluted Potomac to the very polluted Anacostia, to a point in the contaminated northeast branch in Maryland. She uses them as a biological indicator of pollution, a white rat of sorts, harnessing their skill for concentrating chemicals in their body tissue to reveal the source of contaminants from upstream. The clams will stay in the polluted water for two weeks before they are retrieved and analyzed. The transfer carries no risk of invasion to the Anacostia, according to Phelps. The transplanted clams, she explains, are carefully contained in tethered cages and they can survive in the polluted river (just barely) but won't reproduce because the sediments are too toxic.

Rolling down the car window, Phelps pulls the Honda up to the intercom at the takeout window of a nearby Kentucky Fried Chicken and asks for some ice. The voice on the other end tells her to pull around to the service door. A woman soon emerges and gestures for Greenidge to follow her inside with the cooler bag. She reaches for it then hesitates. "What's in the bag?" she asks. "Clams from the Potomac. We are taking them..." Greenidge starts to explain, but she cuts him off and pulls her hands back, giving him a look that says, "Really, I don't need to know." Keeping her distance, the woman scoops ice while Greenidge holds the open bag. The clams, now comfortably cool, continue on their way to the Anacostia.

This page was last modified September 15, 2018