For more than a century, the 3.5 million miles of rivers that snake across the U.S. were treated like open sewers and garbage cans, leaving them steeped in oil, paint, fertilizer, feces and other refuse. Fires that ignited riverine trash were widely considered an acceptable cost of industry, a sign of abundant jobs and economic growth. And there was a common (and erroneous) belief that “dilution was the solution to pollution,” meaning that waterways had an endless ability to absorb our waste and flush it away.

But by the 1960s there was a growing realization that the nation’s waterways were in crisis, left unusable for drinking water and toxic to wildlife. To confront the problem, Congress—with strong bipartisan support—enacted the Clean Water Act (CWA) 50 years ago this Tuesday. With the goal of allowing rivers to become swimmable, fishable and drinkable once again, the legislation made it illegal to discharge pollutants into waterways without securing a permit. Though the law has resulted in significantly cleaner waterways, there are still decades of cleanup ahead for many of them. Pollution still washes into streams, rivers and lakes from sources that are not covered by the CWA, such as urban and agricultural runoff. And rising temperatures are stressing aquatic ecosystems.

The CWA is still seen as critical for cleaning up the most egregious and obvious pollution, though. “It’s a legacy for our children,” says Dean Naujoks of the Potomac Riverkeeper Network (PRKN), a nonprofit advocacy group and a member of the Waterkeeper Alliance, a national network of organizations that work to protect the public’s right to clean water. As we mark the CWA’s 50th anniversary, Scientific American takes a look at the state of seven U.S. rivers.