WASHINGTON (WJZ) — Despite a cleanup that’s taken decades, the Potomac has been declared the most endangered river in America.

Alex DeMetrick reports the reason is pollution mixing with politics.

The mountain streams that feed the Potomac River eventually carry more than water. In rural areas, fertilizer and animal waste wash off the land. In urban areas, rain carries pollutants off hard surfaces into waterways.

“All that contributes to excess nutrients, which continue to give the Potomac some problems,” said Dr. Donald Boesch, University of Maryland Center for Environmental Science.

But the river running past the nation’s capital used to be in worse shape.

“President Johnson in the 1960s called it a national disgrace because of all the pollution floating on the surface,” said Potomac Conservancy President Hedrick Belin.

The nitrogen and phosphorous pollution that can’t be seen are the focus of a multi-state cleanup. For example, sewage treatment plants are being improved. So why would the nonprofit advocacy group American Rivers call the Potomac the nation’s most endangered river?

“It’s meant to bring attention. This is the nation’s river and members of Congress drink from it and to point out there are actually some plans in Congress to undermine aspects of the Clean Water Act,” Boesch said.

Declaring the Potomac endangered isn’t just about pollution, it’s also about politics.

“There are clearly some major decisions that will be made this year in Congress that could roll back the tremendous progress we’ve seen on the Potomac River over the last 40 years,” Belin said.

In the past, the group American Rivers has also designated the Susquehanna River as endangered.