The four report cards for the Potomac River issued since 2007 by its watershed advocates read like a resume for “most improved” river: The first two grades were Ds, followed by a C in 2013. This year, the river earned its highest grade yet: B-.

The report from the Potomac Conservancy that accompanies the letter grade indicates that the Chesapeake Bay tributary is less polluted, more protected and more widely used as a recreational asset than it has been in decades. But it nevertheless leaves room for improvement.

“Even though we’re celebrating, we still have to remain diligent,” said Hedrick Belin, president of the conservancy.

Other reports, such as those produced by the University of Maryland Center for Environmental Science, grade the health of the nation’s river lower. UMCES’ latest report in 2013 rated the Potomac C- for below-average water clarity, benthic populations and other water-quality factors. These report cards tend to monitor different issues and reflect the interests of the graders.

The conservancy’s latest report looks for progress. It notes that the top three pollutants in the Potomac River — nitrogen, phosphorous and sediment — have been decreasing since 1985, as sources such as agriculture and wastewater treatment plants have reduced their contributions.

But polluted urban and suburban runoff continues to increase, according to the report. It is the only source of pollution still growing in the Potomac and the Chesapeake Bay, as more of the watershed is developed and its population grows.

The spread of suburbia into rural areas, with the loss of forest and farmland and the expansion of buildings and pavement, is a particular threat to the Potomac’s health, the report states.

While more than half of the land in the Potomac watershed is still forested, suburban sprawl is reducing those water-cleaning buffers in places like Loudoun County, VA, and Frederick County, MD.

Belin said his organization is encouraging newly elected leaders of both counties to steer future development toward areas already built up, while protecting remaining farms and woodland.

The report gleaned information from a variety of sources, such as the U.S. Geological Survey’s water quality monitoring stations. Belin said he hopes the report will help advocates determine restoration priorities as they continue to work to improve the river’s health.

If there’s an upside to the region’s growing population, perhaps it’s that more people are recreating in and around the water. Visits to Virginia parks along the river have grown in recent years, and advocates are working to give residents more opportunities. Among them is the Captain John Smith National Historic Trail, which runs nearly the entire length of the tidal portion of the Potomac.

Interest in sport fishing on the river peaked in 2013, when 4,386 individuals purchased fishing licenses, and waned in the next two years. But the report says anglers still have plenty to catch in the nation’s river.

“The Potomac is now healthy enough to support growing populations of game fish including shad and white perch,” the report says.

White perch and American shad both earned As in the report card, reflecting increases in their populations in the river. The Potomac is almost alone in the Chesapeake in seeing a robust comeback of shad.

Martin Gary, executive director of the Potomac River Fisheries Commission, said he initially thought a B- was a high grade for a river that he and so many others are still fighting to clean up. But then he considered how much it has improved, especially when it comes to fisheries.

The numbers of striped bass seen in the river have been on an upward trajectory for several decades, even with declines in some areas over the last 10 years. Though not the focus of the report, populations of oysters and crabs have also ticked upward in recent years in the tidal portions of the Potomac that are Gary’s focus.

“Sometimes people see these reports come out and say, ‘Oh, it’s a B- and it’s improved.’ And there’s an expectation that that will continue,” Gary said. “We can control some things to put us in a better position, but, at the end of the day, the environment will oftentimes decide the outcome.”

That’s particularly true when it comes to fish populations.

Striped bass and smallmouth bass, among the most popular sport fish, each received Bs from the report. Their populations remain robust, but have declined in recent years and they continue to face some pollution issues.

“The abundance of young striped bass in the Potomac has increased significantly over the decades,” the report says, “though we have seen a decline in their numbers over the last 10 years; this trend may result in lower adult populations in the out-years.”

Some anglers remain concerned about largemouth bass, which were not mentioned in the report but have been perceived as declining in population. Gary said that the data collected over the last two years show their numbers are “relatively steady.”

These species and others could be threatened by invasive blue catfish and Northern snakeheads, which have grown in number and spread in the river since the last report card. Commercial catches of blue catfish now exceed by weight that of striped bass, an indicator of their growing acceptance by consumers as an edible white-fleshed fish. It may also be an indicator of their growing dominance in the river food chain — blue catfish have voracious appetites and feed on everything smaller than them.

“State and federal agencies continue to study the ecological impact these predatory fish may have on local fisheries. Once established in a waterway, it is unlikely they can be eradicated,” the report concludes.

The Potomac River’s habitats, impacted by all of the other factors mentioned in the report, seem to be the last area to show improvement. Despite steady pollution decreases, there’s been little expansion of forested buffers along the river’s shorelines, and similarly small gains in underwater and tidal environments. All earned C- grades in the latest report, a slight improvement from Ds in two categories in 2013. The recoveries of underwater grasses and water clarity, in particular, have been slow.

As the report’s tagline states, “The Potomac’s on the mend, but not in the clear.”