ANNAPOLIS, MD — Less pollution runoff into Chesapeake Bay in 2015 earned the country's largest estuary a grade of C for its overall health, an improvement over recent years, scientists said.

The overall health of Chesapeake Bay improved last year, according to scientists at the University of Maryland Center for Environmental Science. The C grade in 2015 is one of the three highest scores since 1986. Only 1992 and 2002 scored as high or higher, both years of major sustained droughts. "The high score for 2015 indicates that we're making progress reducing what's coming off the land," said Bill Dennison, vice president for Science Applications at the University of Maryland Center for Environmental Science.

Cleaner air, cleaner water and cover crop programs that protect against wind and water erosion are the keys to the steps forward. The Clean Air Act has cut nitrogen in the air by pushing the installation of smokestack scrubbers at power plants and catalytic converters on cars. Money spent by the states of Maryland and Virginia to upgrade sewage treatment plants is also curbing pollutants, he told CNN.

»What do you think of the pace and efforts to clean up Chesapeake Bay? Tell us in comments below. The overall score for the Chesapeake Bay Health Index for 2015 was 53 percent, compared with 50 percent in 2014 and 45 percent in 2013. Conditions improve in many regions throughout the Bay, including the Choptank River, Upper Eastern Shore, Lower Western Shore, and the Rappahannock River. There were no regions that had lower scores in 2015 compared to 2014.

The Lower Western Shore, which includes Anne Arundel waters like the Magothy, Severn, South, Rhode and West rivers, scored a D, reports The Capital-Gazette. The area has "poor ecosystem health," the report says, although there were significant improvements in chlorophyll a and ocean floor communities.

Factors in the Bay's overall health include several years of moderate weather, sewage treatment upgrades, use of winter cover crops by farmers, and reductions in atmospheric nitrogen deposition, the study says.

"We know why the Bay became degraded and what we need to do to restore it. This report card shows what's possible when we take action," said Donald Boesch, president of the University of Maryland Center for Environmental Science. "The positive results give us confidence that even greater improvements will be realized if pollutant loads are further reduced as committed." While the scorecard shows progress, the bay is not on track to meet long-term goals to meet cleanup goals set by the Environmental Protection Agency to reduce the volume of contaminants and sediment flowing into the bay by 2025, said Chesapeake Bay Foundation Vice President Kim Coble.