To Understand Potomac River, Study a Clam

Viewing the bottom of the Potomac River through a facemask is a little like peering through a microscope. It’s a different world down there, and you never know what you might find.

I was hoping to find sponges. Last August they were right at the head of Watkins Island, near Pennyfield Lock. It was apparently the . What happened to them? Would they reappear?

I examined the bottom, admiring the shimmering pebbles and the green patina of the mussel shells, poking at anything that looked even the slightest bit spongy.

Then I saw something that made me stop. Wedged among the pebbles was a pair of tiny white tubes, then another, and another. I had come upon a colony of freshwater clams.

This was interesting, but not because clams are unusual in the Potomac. Far from it. The shells of this little bivalve mollusk literally pave parts of the river in Montgomery County. Along some stretches of shoreline, their shells crunch with every footstep.  

But I had never seen these little creatures actually at work. And here they were, drawing water through one tentacle-lined tube, taking in nutrients and oxygen with their gills, and then expelling the residue through the other tube.

It struck me that they might be a little like the Higgs boson of the Potomac. Even though most people don’t know anything about them, their vast numbers could give them a decisive role in the river’s ecology.  

Oysters of the Upper Potomac

Shortly after my sighting, I met with biologist Harriette Phelps, who told me just how critical these little critters are. A professor emeritus at the University of the District of Colombia, Phelps is widely known for her work with this clam species in the tidal Potomac below the Wilson Bridge as well as in the Anacostia River.

The clam’s scientific name is Corbicula fluminea.

“Hard working Corbicula,” she calls them. Put a layer of them on the bottom of a pail of murky water, and an hour and a half later the water will be clear. Multiply this effect by the countless millions of clams in the river, and you have what Phelps calls a “key species.”

Forget the feisty bass, the graceful heron, the majestic eagle—the real drivers of life in the river are humble mollusks, most no bigger than a quarter.

Corbicula almost seems designed to purify water. The particles it extracts from the river are not simply broadcast back again. Instead, they are combined with mucus to build up the sediment in which they live.  

Does this sound familiar? In an earlier column ―or were, until they were nearly wiped out.   

Making the river clear is only the start. Sunlight penetrating the water fuels the growth of aquatic plants. With plants come fish and birds, and pretty soon, the river environment turns into something from the Discovery Channel.  

Phelps saw this happen in the tidal Potomac. There’s “absolutely every reason” to assume that Corbicula are providing the same service in our upstream portion as well, she says.

You can see it for yourself. In some places in the river, plants are so thick that you can barely plow your kayak through them. In the clearings and in the vegetation you see darting shoals of minnows and bass slipping back into the cover. Big catfish chug along the bottom like monster tadpoles.

The Clam's Little Secret

But there is something else: Corbicula is not a native of the Potomac, or anywhere else in North America. Originally from eastern Asia, they arrived on our West Coast around 1930 and showed up in the Potomac in 1977.

Once there, they rapidly increased their numbers, in many places carpeting the river bottom. This would seem to qualify Corbicula for membership in that familiar “genus” of the world of advocacy ecology: an “invasive species.”

But not according to Phelps. “Corbicula were not invaders,” she said emphatically. “They occupied areas that were previously unoccupied.”

Corbicula inhabits the upper layer of the river bottom, she explained. There is a native clam, but it lives buried deeper down―and still does. “Even with Corbicula, the native clams are all over,” said Phelps.

Paradoxically, Phelps worries that this “alien” species might disappear. It already has in her former study areas in the tidal Potomac. The same could happen in our portion of the river.

If Corbicula disappears, the river’s ecological clock could start to go in reverse. With no more clams filtering out all the stuff we put into the river, the water would again lose its clarity, along with its plants, fish, birds, and everything else.  

The clams’ disappearance would also be felt by people here who emigrated from Corbicula’s place of origin. In some place along the river, wielding sieve-like fan covers, they harvest the clams, which they bring home to carry on their culinary traditions.

“They’re tasty,” says Phelps, “but not really clam-like to me.  I like clams that are saltier.”