The volunteers paddled their canoes down the Anacostia River on a recent rainy day, the stench of trash lingering in the air as mud-caked bottles floated on the surface of the murky waters.

Centuries of development around the nation’s capital rendered most of this river too polluted for fishing or swimming. Heavy rains send water rushing from streets and sewers into the Anacostia, dragging debris, bacteria and sediment into the river.

Tucked inside the canoes was the latest hope for turning this river crystal clear: Hundreds of two-inch mussels, piled up in baskets and nestled in gobs of thick mud.

After promising results from a pilot program, the D.C. government in August announced a $400,000 grant to the Anacostia Watershed Society to reintroduce 35,000 mussels into the river. Half of the funding comes from the federal government; the rest from the proceeds of a nickel tax on plastic bags sold in the District.

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The shellfish act as natural filters. They constantly suck in and expel water as they consume plankton and plant particles through their gills. But mussels can also remove bacteria such as E. coli, while trapping microplastics and other sediment clouding up the river.

“People sometimes call them environmental engineers,” said Fred Pinkney, a U.S. Fish and Wildlife Service biologist who has researched mussels. “They are doing what a filtration plant or a wastewater plant would do for water. They are purifying the water.”

Back on the river, the volunteers and staff with the Anacostia Watershed Society steered under a bridge where a plastic garbage can bobbed.

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They paddled past wild rice, part of a wetland restoration effort to provide another natural filter.

They reached a marsh within sight of a high school, wetland that will be the new home for the shellfish.

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“Bye, bye, babies,” Robinne Gray of the watershed society said as she plopped a handful of mussels into the river, their shells disappearing into the murky depths. “They’ll get here right in time for all the pollution.”

“That’s good!” interjected Ariel Trahan, who oversees river restoration programs at the society. “They like eating.”

For a long time, environmentalists thought the Anacostia was too polluted to support mussels.

Then about five years ago, Jorge Bogantes spotted an eastern floater, a species of freshwater mussel, caught on a friend’s fishing line. It was alive.

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Bogantes is a natural resources specialist with the Anacostia Watershed Society and recognized the potential of filter feeders to clean the river.

In the ensuing years, he and other scientists would snorkel and dredge the river beds to identify the native species of mussels. Sometimes they would don shoulder-length gloves and crawl on all fours during low tide to find them.

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With eight species identified, they can get to work revitalizing their populations.

“Each mussel can filter between 10 and 20 gallons a day. Just the 8,700 we are releasing this fall could filter 48 Olympic-size swimming pools a year,” Bogantes said. “It’s also not a silver bullet because if we get a lot of pollution of heavy metals, the mussels are not going to survive.”

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The District has embarked on several strategies to clean up the Potomac and Anacostia rivers, as required by a 2005 federal consent degree.

The most ambitious is a $2.7 billion network of four new tunnels stretching 18 miles, designed to stop overflowing waste from city sewers from discharging into the river. The Anacostia segment of the tunnels is expected to be completed by 2023, but one segment that has been completed already has been able to divert more than 1 billion gallons of sewage in a four-month period.

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The city also provides financial incentives for property owners to install green roofs or to plant trees in an attempt to reduce ­storm-water runoff.

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The mussels fit into a vision of a self-sustaining river as humans do their part to keep pollution out.

“The fact we are able to reintroduce them at all is a huge indicator we are making progress,” said Jeffrey Seltzer, a deputy director at the D.C. Department of Energy and Environment. “It’s a great investment, but it’s one piece of the puzzle.”

Much of the early mussel restoration work has focused on identifying where shellfish can survive and grow in the river and broader watershed. In a good year, they can expand from an inch wide to three inches.

Restoring shellfish as a method for river restoration is not a new idea.

But funding for freshwater mussel research is harder to come by than money for oysters and clams that can both clean water and serve as a delicious meal, scientists say.

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Freshwater mussels are edible but not as desirable as their saltwater cousins, which are often served with butter and beer at restaurants.

“They are essentially as hard as the sole of your shoes,” said Matt Ashton of the Maryland Department of Natural Resources, who helped identify the freshwater mussels native to Anacostia.

Or as Bogantes put it recently, “They taste like mud.”

Without commercial demand for freshwater mussels, funding for their restoration hinges on proof of their ability to save rivers.

Breeding poses another major challenge.

The female mussels lay eggs stored in their gills, which are fertilized when the males release sperm into the water. The eggs hatch into microscopic larvae in the female’s gills. But they require a third party — a fish — to host the larvae.

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The female mussels nudge out the larvae by expelling them or extending a chunk of tissue resembling a worm to lure in fish hosts. Fish come in for a snack and instead leave as the unsuspecting foster parent of mussels growing on their gills and fins. The larvae eventually drop off and form new mussel beds.

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All those steps may not happen in a polluted river, so the Anacostia Watershed Society instead turned to a hatchery at Harrison Lake in Virginia.

Bogantes snorkeled in the Anacostia in search of pregnant mussels. Just eight mussels produced nearly 40,000 larvae.

“The improvement of our technology to propagate and grow them to a large size so we can tag and release them has paved the way for this new way of thinking to release mussels for actual river cleanup,” said Rachel Mair, a Fish and Wildlife Service biologist at the hatchery.

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Not far from the District, advocates have also been experimenting with mussels as a restoration tool in the Delaware Estuary.

Danielle Kreeger, the science director of the Partnership for the Delaware Estuary, said much of the restoration work has been small-scale, and now she’s trying to put a number on the value of mussel restoration to remove nitrogen.

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While nitrogen is essential for plants, too much in waterways can result in algae blooms or oxygen dead zones where wildlife cannot survive.

Based on early estimates, Kreeger projects a full-size mussel hatchery could end up reducing nitrogen at dramatically less cost than other measures such as wetland restoration.

“This is simply a new arrow to put in our quiver,” Kreeger said. “This is a new thing, and it’s still a very rapidly evolving science and there’s a lot we need to learn. But there’s a lot of potential opportunity to marshal these organisms to benefit society.”

The Anacostia Watershed Society is also using its work to help teach environmental awareness and biology. On a recent day, hundreds of high school students from Maryland helped the group with its research.

The students gathered at a riverfront park in Bladensburg to measure and tag the mussels so that the watershed society could track their growth in the years to come.

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A cloudy tank of water brimming with mussels cleared over several hours as the shellfish performed their filtering function. One school administrator warned teenagers to keep their hands out of a bucket of mussels brimming with Anacostia River water, wary of bacteria.

Rhonda Urlin-Knights, an environmental science teacher at Dr. Henry A. Wise Jr. High School in Prince George’s County, looked at the Anacostia where the mussels would soon be dropped.

“Our babies!” she said. “Are we going to come back and see them three years from now?”

“That would be cool,” replied Maddie Koenig, an environmental educator at the society. “Wouldn’t it?”