Heavy storms have caused the Potomac to swell and even spill over its banks in some places.
Derived from a set of indicators that include water clarity and grass distribution, the report released last week by the University of Maryland and the National Oceanic and Atmospheric Administration found that overall health of the Chesapeake Bay declined by 4 percent. The Potomac River’s overall health declined from a grade of "C" in 2009 to a "D" for 2010.
Scientist Catharine Wicks, coordinator for the report, said increased stream flow contributed to the decline in water quality.
Two major storms, in January and March of 2010 generated an increased amount of runoff, which washed nutrients and sediment into the river. Spring and summer dry spells that followed meant no new supply of water to wash away the runoff, hurting the overall ecosystem.
As the population has increased in the Potomac River Basin over the years, the amount of nutrients and sediment entering the river has increased dramatically. But overall, the quality of the river has more or less "held its own," Wicks said.
That it hasn't worsened dramatically is a fragile sign of progress made over the years.
Conservation efforts such as wastewater treatment plant upgrades and measures to restrict storm water runoff have helped conservation efforts on the Potomac and in the Chesapeake Bay, experts said, but less than half of both bodies of water are within accepted thresholds.
Ed Merrifield, president of conservation group Potomac Riverkeeper, said the science behind the report card is sound, though he's not fond of using grades to express the river's water quality.
The data show that the Potomac is in bad shape, but the grade makes it seem better than it actually is, he said. Every year, the algae is getting worse in the freshwater part of the Potomac upstream of Washington, and Merrifield worries that it and similar threats won't be treated seriously by the public if they see any grade other than an "F".
Though acknowledging that the river's water quality decreased in 2010, Wicks noted that the river has been holding steady over the long term.
It's had several good years, she said, and the 2010 decline is within expectations of the scientists who produced the report. Increases in stream flow, like that caused by the storms in January and March, create water quality variability, though the overall trend for the Potomac remains consistently poor.
"People really care about the Chesapeake, which is great," Wicks said. But she added that the Potomac's water quality is unlikely to make a dramatic improvement any time soon.Chesapeake Bay Report Card