As population grows, so does demand for freshwater for food pro-duction, household (municipal) consumption, and industrial uses. The availability of freshwater limits the number of people that an area can support and affects standards of living. In turn, population growth and density typically affect the availability and quality of water resources in an area, as people attempt to assure their water supply by digging wells, constructing reservoirs and dams, and diverting the flow of rivers (101, 105). If needs consistently outpace available supplies, at some point overuse of water leads to the depletion of surface and ground-water resources, triggering chronic water shortages (114) (see Figure 1).

Scarce and unclean water supplies are critical public health problems in much of the world (199). Polluted water, water shortages, and unsanitary living conditions kill over 12 million people a year (35, 190) (see Chapter 5).

Competition for freshwater supplies breeds social and political tensions. River basins and other water bodies do not respect national borders. For example, one country's use of upstream water often subtracts from the supply available for use by downstream countries. As the 21st century dawns, there is a growing risk that wars will be fought over access to freshwater supplies (17, 41, 140, 170) (see side-bar, Wars over Water?).

If a crisis is to be averted, the world's overuse and misuse of freshwater must end as soon as possible. We cannot afford to keep wasting and fouling our precious supplies of freshwater. Increasingly, human activities are altering the flow of water and drawing down freshwater supplies faster than they can be replenished. Throughout the world enormous amounts of water are wasted due to inappropriate agricultural subsidies, inefficient irrigation systems, leaky municipal pipes, improper pricing of municipal water, poor watershed management, and other imprudent practices (63, 75, 142). It is time for widespread conservation measures, effective water management policies, and growing attention to assuring freshwater supplies and decent sanitation as part of development and public health projects.