With two-thirds of the U.S. population drinking water from surface sources such as streams, rivers, lakes, and reservoirs, it is very important that these water sources are protected. Part of protecting the water sources and providing cleaner water is protecting the natural land within the watershed. Watershed boundaries often cross State lines. The scientists in this study were interested in conducting a national assessment of drinking water watersheds that crossed State boundaries. Topics: Water, Conservation, Lakes, Land Cover, Rivers, Sedimentation, Water Quality, Watersheds.