Yes, the weather is heating up, and everybody is starting to plot their swimming trips (even a humpback whale tried to swim through New York harbor and up the Hudson river today). But for those of us who prefer the cruising currents of fresh water swimming over the eye-stinging waves of salt-water beaches, here’s a little something to think about.

Of all the fresh water in the world only about .008 percent is fit for human consumption, the rest contaminated by various natural and (mostly) man-made pollutants. Most of these unhealthy waterways are in developing nations, but there are many in developed countries too. TreeHugger mentions some in their listicle of America’s 10 Most Endangered Rivers, like the Northwest’s Snake River (pictured at right), whose waters and salmon runs are threatened by four dams (yes, hydroelectric dams have a pretty significant environmental footprint).