The Washington, DC, area is slated to welcome 2 million people over the next 20 years. All of that growth will, if no preventive steps are taken, further damage the already polluted Potomac River.

No one can stop the growth, but the cities and counties that border the river — which range from some of the nation's wealthiest to some of its poorest — can take steps to mitigate the damage.

Those are the findings of "Troubled Waters: State of the Nation's River 2012." The Potomac Conservancy's sixth annual report on the river outlines the pollutants found in the river and suggests ways to mitigate them.

Like every other Chesapeake Bay waterway, the Potomac suffers from excess nitrogen and phosphorus and too much sediment, which reach its waters via agriculture, sewage treatment plants, stormwater runoff and construction projects.

Pathogens also enter the river from old, leaky sewage pipes and failing septics.

Another major contaminant to the Potomac is chemicals, including large concentrations of PCBs that lead to fish-consumption warnings every spring. The river, home to several power plants, has also at times tested high for mercury. The Potomac supplies drinking water to millions of residents in the DC area.

The chemicals have also led to smallmouth bass in the river showing both male and female sex organs, a sign of both endocrine disruption and of emerging chemicals whose damage is not yet fully understood. These changes have shown up in 80 percent of the smallmouth bass sampled.

"Clearly, the damage is present and extensive," said Potomac Conservancy President Hedrick Belin. "And the tapwater comes from the same place. No one really knows what the long-term effect on humans will be."

The Potomac River is the Bay's second largest tributary, after the Susquehanna, and delivers the Chesapeake about one-third of its freshwater. It's a diverse river, swinging through small West Virginia hamlets to sprawling Northern Virginia and Maryland suburbs. The Potomac's tributaries include the rural Shenandoah River and the urban Anacostia River.

Poultry and crop farms contribute some of the pollution, but the biggest source is urban runoff, Belin said.

The District's Department of the Environment has been tackling the issue, promoting green buildings and investing in stormwater solutions along with DC Water.

Recently, the District banned cool tar pavements because they contain toxins that will land in the rivers. Environmental advocates say that a plastic bag fee passed two years ago has made a big difference in trash.

Belin would like to see the watershed's residents do even more, especially because of the onslaught of new growth. He'd like more technical assistance for landowners so they can implement their own solutions, such as rain barrels, rain gardens and better grading to direct the water to landscaping instead of to the street.

The report calls for stronger regulations, including better stream-protection ordinances; more funding for Farm Bill conservation programs and the large-scale adoption of green infrastructure programs.

"It's clear, based on a number of ecological indicators, that we still have a long way to go before we reach a Potomac that is not impaired," Belin said.