Water-snake hunter, opposite page at left, brings his catch to market on Tonle Sap Lake in 

 Cambodia. The capture of water snakes — mainly to feed crocodiles raised for their hides — has 

 reached unsustainable levels. Workers tend a rice paddy in Laos, top center of these two pages; 

 agricultural runoff degrades the Mekong's water quality. About a dozen dams now partition the 

 river and its tributaries and several more are under construction, including the Nam Theun 2 

 dam in Laos, above right. Dams provide irrigation water and electricity for the region's growing 

 population, but can harm wildlife. 



large-dam construction in the United States and Europe. 

 In the Mekong River Basin and elsewhere, however, big 

 dams continue to rise. 



Species along the Mekong, as in other freshwater systems, 

 depend on natural flood cycles for nutrients and for trans- 

 portation to and from spawning grounds. More than 90 

 percent of the fish species in the Mekong watershed spawn 

 not in rivers, but in seasonal lakes or periodically flooded 

 forests and fields. Flow patterns altered by dams and other 

 projects could prevent those species from reproducing. In 

 addition to building dams, countries along the Mekong are 

 destroying or modifying rapids and other natural features 

 to improve navigation — changes that will disturb critical 

 fish habitats and alter downstream water flow. 



Another destructive practice is crop irrigation, the biggest 

 consumer of freshwater both along the Mekong and 

 worldwide. Most of the water withdrawn from the 

 Mekong goes to irrigating crops, mainly rice. Demand for 

 irrigation water has risen dramatically in the past decade, 

 as new acreage has come under cultivation and new irri- 

 gation schemes have enabled farmers to produce a second 

 or third rice crop each year. Removing so much water 

 from freshwater systems can be devastating for wildlife, 

 exacerbating flow problems caused by upstream dams. 



Worldwide, irrigation guzzles about 70 percent of the 

 freshwater people use. To grow food for expanding human 

 populations, people divert rivers, drain inland seas, and 

 extract fossil groundwater collected over thousands of years, 

 often at unsustainable rates. Worse, current agricultural 



practices often waste as much water as they use: about half 

 the water that flows through conventional irrigation systems 

 never actually reaches a crop plant. A lesser — though still 

 formidable — amount of water is siphoned off to slake the 

 thirst of cities and industry, and when you add it all together, 

 it's clear that people are using more than their fair share. 

 The Mekong still manages to reach the sea. But at least 

 ten other major rivers, including the Colorado, Ganges, 

 Jordan, Nile, Rio Grande, and Yellow, now regularly run 

 dry before they reach their outlets. 



Agriculture, in addition to being the greatest consumer 

 of freshwater, is also a major polluter — another bane for 

 wildlife. In the Mekong River Basin, agriculture relies heav- 

 ily on pesticides and fertilizers; it also drives deforestation, 

 which causes erosion. Chemical, nutrient, and sediment 

 runoff from farms winds up in the Mekong River Delta, 

 where it degrades water quality, shifts natural nutrient cycles, 

 and alters wildlife habitat. The six nations in the Mekong 

 watershed have initiated a regional program to encourage 

 agricultural development. If not done mindfully, the ac- 

 celerated development could worsen water quality. 



Other countries are already contending with the effects 

 of major pollution. Fertilizer, pesticide, and livestock-waste 

 runoff from farms in the American Midwest, for example, 

 have created a dead zone at the mouth of the Mississippi 

 River in the Gulf of Mexico. There, coastal algae populations 

 thrive on the influx of nutrients and the misfortune of then- 

 natural predators, which are often curtailed by the pesticides. 

 From spring until late summer, immense algal blooms rob 

 the Gulf's water of oxygen. Such hypoxic conditions chase 



November 2007 natural HlSTORA 



4 3 



