The Chesapeake Bay watershed is the largest on the Atlantic seaboard, encompassing most of Maryland and Virginia, along with parts of Delaware, New York, Pennsylvania and West Virginia. More than 150 rivers flow into the system, carrying pollution and nutrient runoff from a 160,000-square-kilometer area into the bay ecosystem. A new study tracking long-term effects of the Clean Air Act has some good news about the often-poor water quality in some areas of the Chesapeake Bay watershed, but the overall picture may be complicated by hydrology.