© Craig Chesek

Only 3 percent of the Earth's water is fresh-and less than one-third of that is in a form or a location that makes it readily available for human consumption, such as the lakes at Glendalough in Ireland.

As humans, we have always searched for sources of safe water. Today, with human populations growing and climate changing, that search takes on increased urgency. We have new ways of purifying water--such as disinfection and high-tech filtration--but we have new ways of contaminating it, too. And now we know what our ancestors didn't: Earth has only a limited supply of clean, healthy, fresh water.

How do we make sure there is enough healthy fresh water, not just for ourselves, but for all forms of life? We can start by remembering that what we discard will eventually be in someone--or something--else's water. In the world of water, we're all downstream.