Disappearing lakes, shrinking seas.

• Position: Ecology

Mono Lake, North America's oldest, dating back some 760,000 years, is an important feeding stop for a variety of migrating birds, especially as Southern California has lost over 90% of its wetlands. Since the first diversions of its tributaries to quench the thirst of growing Los Angeles in 1941, the lake has contracted dramatically, with water levels dropping by 34 feet and volume down 40%. As a result, its salinity has jumped to three times that of the ocean--far too salty to sustain most fish, reports Janet Larsen, research associate for Earth Policy Institute, Washington, D.C. The lake likely would have died completely had locals not intervened and defeated the city in a legal battle over keeping water for the lake.

For more than 4,000 years, farmers have diverted river water for crops in dry areas and dry seasons, reducing the flow into nearby lakes and seas. Over the last half-century, world water use has tripled, expanding faster than population. Today, irrigation accounts for two-thirds of global water use. With the advent of diesel- and electricity-driven pumps, groundwater extraction in some areas has exceeded recharge from precipitation, also causing water tables and lake levels to fall.

Mexico's largest lake, Chapala, is the primary source of water for Guadalajara's growing population of 5,000,000 This lake's long-term decline began in the 1970s, corresponding with increased agricultural development in the Rio Lerma watershed. Since then, the lake has lost more than 80% of its water. Between 1986-2001, Chapala shrank in size from 405 to 314 square miles. Climbing municipal and industrial water demands now exceed the sustainable supply by 40%. The lake's contraction has come at the expense of several fish species and potentially presages a change in the mild climate that the water supported.

West Africa's Lake Chad has shrunk to an alarming five percent of its former size, while Central Asia's Aral Sea gradually is turning into a desert. In Israel, the receding shores of Lake Tiberias--also known as the Sea of Galilee--sometimes allow mere mortals to walk where the water once was. Thousands of lakes in China have disappeared entirely. The diversion of river water in India and Pakistan that accounted for a doubling of irrigated area over the last four decades has depleted many lakes. All told, over half of the world's 5,000,000 lakes are endangered.

Nestled among deserts, the 5,000,000-year-old Aral Sea is one of the world's most...