"The water quality of most major rivers flowing into the Chesapeake Bay has significantly improved since cleanup efforts began four decades ago, but the pace of improvement has slowed in many rivers — and even reversed in places.

Recently released U.S. Geological Survey water quality monitoring data from the largest nine rivers feeding the Bay offer a mix of good, cautionary and bad news about the status of the 40-year-old cleanup effort.

The good news is that nitrogen and phosphorus have trended downward since 1985 in the watershed’s three largest rivers — the Susquehanna, Potomac and James — which together account for more than 90% of the water flow into the Bay.

But the story gets murkier when looking at the most recent 10 years. The Susquehanna and Potomac have downward trends for nitrogen, but only the Susquehanna is also clearly improving for phosphorus. The James shows increases for both nitrogen and phosphorus in the most recent decade.

The data is collected from “river input monitoring” sites located just above the tidal reach of nine major Bay tributaries. That’s where water samples can be drawn from free-flowing rivers and estimates made of the load of water-fouling nutrients reaching tidal waters. (Such load estimates are difficult in tidal reaches, where ocean-driven tides slosh water back and forth.)

Water draining from about three quarters of the Bay’s 64,000-square-mile watershed flows past those nine sites, and the land upstream of the sites is estimated to contribute about 60 percent of the nutrients reaching the Chesapeake. . ."