Disappearing lakes, shrinking seas

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 ancient lakes. As recently as the early 1960s, it covered some 25,483 square miles and held 264 trillion gallons of water. Two rivers, the Amu Darya and Syr Darya, fed the lake with some 17,000,000,000 gallons of water each year. Today, however, irrigation of vast fields of cotton has drained the rivers, reducing the annual inflow to only 396,000,000 gallons. As a result, the Aral has lost four-fifths of its volume and split into two sections.

Growing water demands are causing other lakes around the globe to vanish. Irrigation withdrawals from the waters that feed Lake Chad quadrupled between 1983-94. Water consumption, combined with low rainfall levels since the 1960s, has shrunk the lake by 95%--from 9,653 to 521 square miles--over the past 35 years.

Overpumping groundwater in China's Hebei province has lowered the water table, resulting in the loss of 969 of the province's 1,052 lakes. Madoi County in northwest China's Qinhai province, the first through which the main stream of the Yellow River flows, once had 4,077 lakes. Over the past 20 years, more than half have disappeared.

Lakes not only are being drained dry, they are dying from contamination. Farm wastes, sewage, and nitrogen fallout from fossil fuel burning fertilize lakes, causing excess algal and plant growth that depletes water oxygen levels and kills aquatic animal life. Such eutrophication plagues more than half the lakes in Europe and Asia, 41% of those in South America, and 28% in North America.

Acid precipitation, largely from fossil fuel burning emissions, is killing thousand of lakes as well. An estimated 46,000 square miles of lakes in Norway are acidified to the point where fish stocks have crashed. Sweden has some 4,000 acidified lakes. In Canada, 14,000 lakes are severely acidified.

The Environmental Protection Agency estimates that 70% of sensitive lakes in upstate New York's Adirondack Mountains are at risk of periodic acidification, and that, without further reductions in sulfur dioxide emissions, the rate of acidification will in crease by half or more. In addition, a survey of remote mountain lakes throughout Europe found that even lakes far from human development were acidified by sulfur and nitrogen deposition and that virtually all were contaminated by heavy metals (such as mercury, lead, and cadmimum) and fly ash particles. The sediments and fish in these lakes also contained a wide range of persistent organic pollutants.

Rising global temperatures are predicted to increase average lake temperatures by 3,6-5.4[degrees]F over the next 50 years. As water warms, its natural purification processes can slow down. Climate-related changes in water chemistry and stratification can lead to fish losses, as is already being seen in East Africa's Lake Tanganyika.