It was the advocacy group’s highest rating in its 10 years of monitoring river conditions, up from a B-minus last year and a D in 2011. One biologist working with the group declared a new “golden age” of eagles, osprey and other waterfowl thriving within the tidal reach of the Chesapeake Bay, which includes the Potomac up to Washington.

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“The comeback from where the river was just 10 years ago has been tremendous,” Potomac Conservancy President Hedrick Belin said in an interview. He cited decades of recovery initiatives — including waste-treatment upgrades and agricultural-pollution controls — that may be nearing an ecological tipping point.

“The speed at which Mother Nature, once given the space, has rebounded is remarkable,” Belin said. “It’s happening a little faster than we expected, but we can’t take it for granted.”

The Potomac is now on the verge of being one of the nation’s great river recovery stories, Belin said, putting it in the ranks of Boston’s Charles River and Portland’s Willamette as formerly no-go rivers that now invite residents into the water.

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“In the next 10 years, the Potomac is poised to be the next urban river where you can go swimming,” Belin said. Conditions are already okay for a dip on many days, but not when rains flush more pollution off the banks.

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To arrive at an overall grade of river health, the conservancy each year reviews more than 20 indicators in five categories: pollution, fish, habitat, land and people. While not every indicator edged up over last year’s report, the overall improvement was enough to declare the river better off.

Fish surveys in the past year record booming populations of American shad — some of the fastest growing on the East Coast — along with striped bass and white perch. Higher up the food chain, reports of bottlenose dolphin have increased in recent years. The river was once home to a robust population of the marine mammal, with sightings common as far upriver as Georgetown.

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Birds are back in many parts of the river basin. The report notes that the number of breeding pairs of bald eagles jumped by 50 percent in the past year. The 214 pairs included the first known to nest in the District since the end of World War II.

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No marker may be more important than the reduction of runoff pollution from streets and sewers throughout the basin. Agricultural runoff and industrial discharge are falling, and even though development continues to add more runoff in the region, pollution levels have dropped thanks to better water treatment.

The completion of huge sewage tunnels now under construction beneath the District will provide another boost to water quality, planners say. The network will contain the untreated waste and storm water that now cascades into the Potomac and Anacostia rivers during downpours.

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Not all the news in the report is good. Pollution runoff rates continue to grow. Invasive species, such as flathead and blue catfish, are growing in number and preying on native fish. Forest and farmland, while more often protected in the watershed, are increasingly at risk in the critical narrow stretch along the river itself. Most worrying, the report said, are threats to the federal funding that have driven many of the improvements.

“We’ve made great progress,” Belin said, “but it’s certainly frightening how the progress could be undone.”