Scientists grading the Bay's health for 2009 reported there were modest improvements in parts of the Chesapeake, and they gave the Bay an overall grade of C, up from a C minus in 2008.

Overall, 11 of the 16 regions improved, with improvement primarily in the middle regions of the Bay that include the mainstem, Choptank, Potomac and Rappahannock rivers.

Declines were noted in Upper Western Shore rivers, the Patapsco and Back rivers, the James and the Elizabeth.

Once again, the heavily industrialized Patapsco and Back Rivers in Baltimore and the Elizabeth in Norfolk, failed their annual stress test.

Although a small decline was noted there, the Upper Western Shore rivers again received the highest overall grade, a low B. This group of rivers includes the Gunpowder and the Bush. It is the third time those rivers have claimed first place, in large part because their dissolved oxygen meets the healthy threshold. The watershed includes suburban Baltimore and is about half forested, with the other half split between urban uses and agriculture.

The study and report is done by the University of Maryland Center for Environmental Science. It grades each river on water quality, aquatic grasses, benthic communities and dissolved oxygen. It uses data from the Chesapeake Bay Program and is a partnership between the university and National Oceanic and Atmospheric Administration's Chesapeake Bay Office.

The reason for the improvement in the middle sections of the Bay comes down to rainfall. This year's precipitation in many tributaries was heavier than usual. But Pennsylvania and New York saw less rain than usual, which influenced the mainstem, said University of Maryland Center for Environmental Science researcher and project leader Dr. William Dennison.

"Normally, more precipitation means poorer Bay health," Dennison said. "But last year, the Bay benefited from below-average rainfall throughout Pennsylvania, which appears to have reduced the amount of pollutants reaching the open waters of the mainstem Bay."

While agriculture is the main cause of pollution in the Bay watershed overall, it is not a factor in the Patapsco, Back or Elizabeth rivers. The root causes in these waters are legacy pollutants from manufacturing, as well as sewage-treatment, stormwater and impervious surfaces.

Scoring the second lowest were the Lower Western Shore rivers-the Magothy, Severn, South, West and Rhode. These rivers are in the heavily developed Annapolis area and are known for their large homes-many on septic systems-and surrounding commercial developments. A major factor in the decline of the Severn is the number of septic systems, which accounts for about a fourth of the river's pollution (as opposed to about 5 percent Baywide.)

The Patuxent, which begins in the Baltimore suburbs and wends down into Southern Maryland, scored fourth poorest. As with the other low-scoring rivers, the Patuxent is grappling with huge population growth and development.

Rivers in which agriculture is more of a force-the York, Choptank, James and Upper Eastern Shore rivers-improved over previous years. The Potomac notched up to a C. The Tangier area had its highest water quality score since monitoring began in 1986, although it still scored a C.

For more details, visit www.eco-check.org/reportcard/chesapeake/2009/