Patches of submerged aquatic vegetation growing in the Potomac. They can seem unsightly, but many experts believe they show that the river is getting healthier. (Dan Rauch/District Department of Energy and Environment)

What is that awful ugly green stuff covering the Potomac River? The whole river near Roosevelt Island, near the airport and south to Alexandria down to National Harbor are covered with green vegetation. I’ve never seen anything like it. What is it? Is anyone trying to get rid of it?

Mary Paris,
Alexandria, Va.

Last month, Joe McMullin, coach of the freshman boys rowing team at Bethesda-Chevy Chase High School, sent his boats out into the Potomac only to have one get stuck in a green mat floating near Roosevelt Island. The mass extended off the island’s bank and into the river.

“It was also growing up near the steps near the Lincoln Memorial,” McMullin told Answer Man as the team assembled this past week at Thompson Boat Center.

A great egret settles itself on submerged aquatic vegetation on the Potomac. (Dan Rauch/ District Department of Energy and Environment)

As Answer Man looked across the Potomac, he could see a thin green margin edging Roosevelt Island.

“Two weeks ago, the heavy rains cleared it out,” Joe said. “It was pretty bad the first two or three weeks of September.”

The tide was rising, which was another reason the green carpet was not as visible as it often is, for it grows up from the river bottom. Biologists call it submerged aquatic vegetation, SAV for short.

The SAV in the DMV includes the invasive species hydrilla along with such mellifluously named native species as small pondweed, spiny naiad, water stargrass, wild celery and coontail. It’s all thriving, which has divided experts.

“In Oronoco Bay, there’s tons of this stuff,” said Dean Naujoks of the Potomac Riverkeeper Network. “It’s so thick you literally cannot paddle through it.”

Some waterfront communities have become so choked that they’ve hired companies to eradicate hydrilla with chemicals, he said.

Dean blames the increased SAV on pollution, namely nutrient runoff: fertilizer, farm waste, pet waste and the overflow from combined storm water and sewage systems in the District and Alexandria. All of it washes into the river and encourages the growth of SAV. In warm, stagnant or slow-moving water, algae will often bloom atop the vegetation.

“We’re always concerned about that,” said Hedrick Belin, president of the Potomac Conservancy. “That’s a very visible sign that we’re overfeeding the plants in the river. And that’s causing this excessive growth of algae.”

But others feel that the increase in the Potomac’s SAV — including hydrilla, a once much-maligned species whose reputation has improved in recent years — is because the river is actually cleaner. Improvements in wastewater treatment have reduced the overall nutrient load. And a Potomac strewn with vegetation is actually more natural, said R. Christian Jones, a freshwater ecologist and director of George Mason University’s Potomac Environmental Research and Education Center.

“Our best guess is that all of the shallow areas of the Potomac were covered with submerged aquatic vegetation when the Europeans first came to the area,” Jones said.

The SAV is so noticeable now because it’s doing the same thing trees do this time of year. “The root stock stays in place, but of most of them just shed the leaves and the shoots and they start drifting around,” Jones said. “Algae grows on top of them.”

Beds of submerged aquatic vegetation filter the water, cleaning it. They provide habitat for aquatic life such as fish and eels — and the animals that eat them.

“It’s a fantastic little ecosystem,” said Daniel Rauch, a wildlife biologist with the District’s Department of Energy and Environment. “There are thousands and thousands of dragonflies using it. You can go out and see great blue herons and lesser yellowlegs walking right on top of it, it’s so thick. I can’t wait for the ducks.”

Jones admits that it can be aesthetically unpleasant, but he said that it is a natural process.

“I think it’s a small price to pay for the huge benefits of both hydrilla and native plants,” he said. “I don’t have anything against hydrilla. Hydrilla improves the water quality to the point that the natives can come back in, which is a good thing. . . . I think we’re in a really good place. I think we’re moving toward an even better place. You just have to put up with a little bit of a nuisance in the fall when they break up.”

So, the vegetation is back. But you still shouldn’t overfertilize your lawn.

Twitter: @johnkelly

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