[新闻背景]The conservation group American Rivers says the Potomac River that runs through Washington is the most endangered river this year in the United States. The organization says that pollution in the Potomac is decreasing water quality, threatening marine life and will become worse if Congress rolls back national clean water protections.

The Potomac River is seen by hundreds of thousands each day as it flows under bridges and past memorials.  It is used for recreational activities, and provides drinking water for five million people.

Before the Potomac reaches Washington, it runs through farmland in four states, and continues on to the Chesapeake Bay, the country's largest estuary.  

All along its course, pollution undermines the river's water quality, says American Rivers President Bob Irvin.

"It flows through farmland, where pesticides are running off from the fields, where farm waste is running into the streams," said Irvin.  "Then as it makes its way down through the urban area of Washington, DC, we have runoff from our streets, and when it rains really hard we have raw sewage flowing into the river."

That runoff concerns Don Boesch, head of the University of Maryland Center for Environmental Science.   He says it is creating dead zones where aquatic plants and marine life cannot survive.

"It's got low light penetration and sometimes lowest levels of oxygen in the water," Boesch noted.

He says before the river can become healthier, the nutrient runoff must be controlled.

"These are good things that stimulate life in the river, but they are actually in excess, so they cause lots of negative problems in the river," Boesch added.

A nearby coal powered electrical plant will close soon, but until then, it is putting toxic substances into the water and changing the habitat, says Whit Overstreet, with the environmental group, Potomac Riverkeepers.

"They discharge hot water back into the river that wouldn't be there naturally, which can be problematic for aquatic life," Overstreet noted.

Scientists have also found that chemicals appear to be causing intersexed fish - especially male fish with female traits. But fishing guide Steve Chaconas says he thinks the Potomac has improved from the filthy river it was years ago.

"The fish are very vibrant," said Chaconas.  "People are picking up after themselves, finally!"

American Rivers warns the Potomac and other rivers could backslide if the Clean Water Act of 1972 is weakened, a distinct possibility, says Irvin.

"Congressmen from various parts of the country, representing big agricultural interests and industry interests, are trying to weaken the act, trying to prohibit the federal government from protecting, for example, small headwater streams and wetlands, and from regulating the use of pesticides on farmland," Irvin explained.

Clean water advocacy groups are calling on Congress to kill any legislation that erodes those vital protections.