The Potomac River is cleaner now than it has been in decades, thanks largely to upgrades at Washington's sewage plant - and the proof is on the river bottom, where thickets of underwater grass are replacing mud and murk, according to a new scientific study.

The study, released Tuesday, paints an evocative picture of the Potomac's rebound from the 1960s, when its bottom was bare mud, its algae-choked water was AstroTurf green, and President Lyndon B. Johnson called the river a national disgrace.

Today, the river is clearer and heavily carpeted with grass. Scientists found that the Potomac's critical grass beds had doubled in size since 1990.

"These conditions are actually better than they were in the 1950s. The portion of the Potomac that we're talking about was completely devoid of vegetation in the 1950s," said Nancy Rybicki, a scientist with the U.S. Geological Survey and a co-author of the study.

But despite that good news, the Potomac is still full of other pollutants that have led District authorities to warn about swimming in it and eating its fish. Its ecosystems have been scrambled by invasive species, including snakehead fish. And an unknown chemical in the water is making male bass grow eggs.

So, if the Potomac is an environmental success story, that shows how low the bar for success has been set - both for the long-troubled river and for the nation's other polluted rivers and bays.

"When this all started . . . the problems with the water were visible and palpable, for the most part. We had this green goop" on the water, said Ed Merrifield, an environmental activist whose title is Potomac riverkeeper. "What we're left [with] are the invisible problems, which can still be very harmful to us."

The study covered the period from 1990 to 2007 and 50 miles of river, from Chain Bridge downstream to Maryland Point in Charles County. It was an unusually wide-angle look at the Potomac's rebound, which has mirrored gains in Boston Harbor and Cleveland's once-flammable Cuyahoga River.

The Potomac, which begins in Appalachian valleys to the west, once teemed with oysters, sturgeon and shad, but it was poisoned by sewage from the growing capital. By 1969, authorities had pronounced the river "a severe threat to the health of anyone coming into contact with it."

Its comeback has been closely tied to the Blue Plains treatment plant, which handles waste from the District and parts of Montgomery, Prince George's, Fairfax, Loudoun and Arlington counties. Its outflow is a river in itself: about 300 million gallons a day of treated sewage, enough to fill RFK Stadium.

In the past decade, responding to mandates from federal regulators, the plant has added $1 billion in new efforts that allow bacteria to consume the algae-feeding pollutant nitrogen in sewage. The new study determined that between 1990 and 2007, the average level of nitrogen in the river fell by nearly half.

The result, scientists said, was less murk. With less algae in the water, more light gets through to the river bottom. Given the chance, the river's plants came back, the study found: first some nonnative species, then an expanding number of grasses that had always lived in the Potomac. They now cover 8,441 acres of river bottom - and help their own cause by filtering dirt and pollutants out of the current as it passes.