In 1996 the world's human population was using an estimated 54% of all the accessible freshwater contained in rivers, lakes, and underground aquifers. This percentage is conservatively projected to climb to at least 70% by 2025, reflecting population growth alone, and by much more if per capita consumption continues to rise at its current pace (93, 145). As humankind withdraws a growing share of all water, less remains to maintain the vital ecosystems on which we also depend (11, 140, 145).

A substantial portion of the total freshwater available in the hydrological cycle is needed to sustain natural aquatic ecosystems—marshes, rivers, coastal wetlands—and the millions of species that they shelter (32, 132). Healthy natural ecosystems are indispensable regulators of water quality and quantity. For example, flood plain wetlands soak up and store water when rivers flood their banks, reducing downstream damage.

The value of these environmental services to humankind is immense. One estimate, made by Robert Costanza, director of the Institute of Ecological Economics at the University of Maryland, puts the global value of wetlands at close to US$5 trillion dollars a year, based on their value as flood regulators, waste treatment plants, and wildlife habitats, as well as for fisheries production and recreation, among other uses (92). New York City is spending US$1 billion to conserve and protect water catchment areas in upstate New York—the source of the city's drinking water. The alternative would be to spend $5 billion on a state-of-the-art water filtration plant that would cost an additional $300 million a year to operate (11, 28, 155).

In virtually all regions of the world, careless use of water resources is harming the natural environment. Globally, over 20% of all freshwater fish species are either endangered or vulnerable or recently have been made extinct (98). As the following examples demonstrate, overusing and misusing freshwater resources carries serious consequences for natural species as well as for human populations:

• Diverting water from the Nile River, along with build-up of sediments trapped behind dams and barrages, has caused the fertile Nile delta to shrink. Of 47 commercial species of fish, about 30 have become extinct or virtually extinct. Delta fisheries that once supported over a million people have been wiped out (1, 90).
• Lake Chad, in Africa's Sahel region, has shrunk from 25,000 square kilometers to just 2,000 square kilometers in the last three decades from periodic droughts and massive diversions of water for irrigation. The lake's once rich fisheries have entirely collapsed (1, 97).
• Despite cleanup efforts, the Rhine River, which runs through the industrial heartland of Western Europe, is so polluted that it has lost 8 of its 44 species of fish. Another 25 species have become rare or are endangered (1).
• In Colombia fish production in the Magdalena River plunged from 72,000 metric tons in 1977 to 23,000 metric tons by 1992—a two-thirds drop in 15 years—as a result of agricultural, urban, and industrial development and deforestation in the river's watershed (1).
• Southeast Asia's Mekong River has had a two-thirds drop in fisheries production due to dams, deforestation, and conversion of 1,000 square kilometers of mangrove swamps into rice paddies and fish ponds (1).
• The US state of California has lost over 90% of its wetlands. As a result, nearly two-thirds of the state's native fish are extinct, endangered, threatened, or in decline (1, 154).