Rowers on the Potomac River. (Astrid Riecken/For The Washington Post)

THE POTOMAC RIVER has been a national embarrassment for decades. Even in recent years, after stronger cleanup efforts, the Nation’s Triathlon has had to cancel its swimming event several times because of high bacteria levels. But the picture is changing, and a bit faster than those watching the cleanup had expected. The Potomac Conservancy, a regional environmental group, this week upgraded the river’s health to a B . With evidence that a federally backed regional cleanup is working, this is not the time to decelerate or defund the effort.

The river is on track to meet its goal in reducing nitrogen and to surpass its goal in reducing phosphorous. Bald eagles are nesting near the river in the District for the first time since 1946, and dolphin sightings are becoming more common. The populations of American shad and white perch are well over target; installation of a fish ladder in 2000 and other measures are paying off. More help will come when the District completes a tunnel system that will divert raw sewage and overflow from the river to a treatment plant during storms.

Even so, there is work left to do. Runoff still brings too much pollution into the water, with wastewater and septic systems increasingly the sources. Billions of dollars in investment in wastewater treatment plants have been key to cutting pollution, but more regulation and better filtering may still be needed. There may still be unsettlingly high amounts of dangerous bacteria in the Potomac because of farm and urban runoff.

Fish continue to face threats from nutrient pollution, but also from warming water and the seepage of chemicals that alter the animals’ hormones. Striped bass, the official state fish of Maryland, has made progress but is not doing as well as some other species; the smallmouth bass has a ways to go. Invasive species, particularly catfish and snakeheads, pose a continuing threat.

Measures of water clarity and algae blooms are still quite negative. Clarity is needed for crucial underwater grasses to thrive, and algae blooms eat up needed oxygen and introduce toxins into the river. The region is also far behind in building forested buffers that filter water streaming toward the Potomac.

But the biggest threat may be the Trump administration. The region’s Chesapeake Bay cleanup program has driven many of the improvements in the Potomac’s water quality. Yet President Trump has proposed to hollow out funding for the effort. Congress rejected the defunding in its latest omnibus spending bill, but that saves the program only through September. Republicans cannot argue that the program is failing. The Potomac’s progress is yet more proof that the effort is producing tangible results.