Clean water is essential for ecosystem processes and for the maintenance of human populations. However, fresh water accounts for less than three percent of the world's total water volume. Numerous anthropogenic and natural processes impact the quality and quantity of the available resource. The value of high-quality water will likely increase as threats to water resources expand and human demands increase. In the United States, public lands such as forests and grasslands often contain watersheds that have been minimally modified by human disturbances. Wilderness areas in particular often provide disproportionately large volumes of high quality water. Such regions are critically impor-tant for providing water supplies that serve a variety of purposes and uses. The value of water draining these lands is arguably higher now than when the National Wilderness Preservation System was created 50 years ago.