It wasn’t so long ago that city rivers were some of the more polluted bodies of water on the planet. In 1969 for example, the Cuyahoga River famously caught fire after years of unabated pollution, spurring an environmental movement to clean up the nation’s waterways.

Now after decades of effort and a generational movement of gentrification across the U.S. driving urban renewal, cities are turning these once-polluted rivers into swimming pools.

For example, the Charles River Conservancy in Boston is leading the effort to get swimmers into its waters. Swimming had been prohibited in the Charles but since 2013, the Conservancy has been hosting public swimming days and an annual one-mile swim race with over 1,000 participants. The Conservancy’s goal is to build a permanent swimming facility in the river. If implemented, the swim-park would be located within North Point Park, along the border of Cambridge and Boston.

Similarly, a group called +Pool has proposed a floating pool in New York’s East River that would filter polluted water, providing a place for residents to swim and help clean the river at the same time. As +Pool points out, “Like a giant strainer dropped in the river, + POOL filters river water within its walls, removing bacteria, contaminants and odors, leaving only safe, swimmable water that meets local and state standards.”

“ndbjj-kids-pool-party-2013.f,” Jennifer Opra © 2013

And in Portland, the once unhealthy Willamette River besieged by weekly sewage overflows has been holding The Big Float, a river float parade that gets people on the water, ever since a $1.4 billion sewage pipe was installed in 2011.

What’s interesting about all these projects is the grass-roots nature of each, kick-starting projects through crowd sourcing and community activism.

Arguably the most advanced urban swimming pool is the Bassin de la Villette which opened in a canal as part of the Paris Plages festival this past summer. Inspired by this effort, Paris is pledging to make the Seine officially swimmable in time for the 2024 Olympics. And in London, the Thames Baths project would filter River Thames water to provide open-air pools for its residents.