Environmental Problems, Their Causes, and Sustainability 1

major rivers are going dry part of the year because offlow reduction by dams (see the case study below).This engineering approach to river managementhas displaced between 40 and 80 million people fromtheir homes and has flooded an area of mostly productiveland roughly equal to the area of California. In addition,this approach often impairs some of the importantecological and economic services rivers provide (Figure13-12, p. 269). In 2003 the World Resources Instituteestimated that dams and reservoirs have strongly ormoderately fragmented and disturbed 60% of theworld’s major river basins.According to water-resourceexpert Peter H. Gleck, at least a fourth of the world’sfreshwater fish species are threatened or endangered,primarily because dams and water withdrawals havedestroyed many free-flowing rivers.Because of evaporation and seepage from theirreservoirs, some dams lose more water than they provide.The reservoirs behind dams also eventually fillup with silt, which makes the dams useless for storingwater or producing electricity.xHOW WOULD YOU VOTE? Do the advantages of largedams outweigh their disadvantages? Cast your vote online athttp://biology.brookscole.com/miller14.Case Study: The Colorado River Basin—AnOvertapped ResourceThe Colorado River has so many dams andwithdrawals that it often does not reachthe ocean.The Colorado River flows 2,300 kilometers(1,400 miles) from the mountains of centralColorado to the Mexican border and eventuallyto the Gulf of California (Figure 15-10).During the past 50 years, this once freeflowingriver has been tamed by a giganticplumbing system consisting of 14 major damsand reservoirs, hundreds of smaller dams,and a network of aqueducts and canals thatsupply water to farmers, ranchers, and cities.This domesticated river provides electricityfrom hydroelectric plants at majordams, water for more than 25 million peoplein seven states, and water used to grow about15% of the nation’s produce and livestock.The river also supports a multibillion-dollarrecreation industry of whitewater rafting,boating, fishing, camping, and hiking.Figure 15-10 The Colorado River basin. The areadrained by this basin is equal to more than one-twelfthof the land area of the lower 48 states.NEVADALas VegasCALIFORNIABoulder CitySan DiegoDamAqueductor canalUpper BasinLower BasinLosAngeles PalmSpringsThe river supplies water to some of the nation’sdriest and hottest cities. Take away this tamed riverand Las Vegas, Nevada, would be a mostly uninhabiteddesert area; San Diego, California, could notsupport its present population; and California’sImperial Valley, which grows a major portion of thenation’s vegetables, would consist mostly of cactusand mesquite plants.There are three major problems associated withuse of this river’s water. One is that the Colorado Riverbasin includes some of the driest lands in the UnitedStates and Mexico (Figure 15-6). In addition, legalpacts in 1922 and 1944 allocated more water for humanuse in the U.S. and Mexico than the river can supply—even in years without a drought. The pacts also allocatedno water for environmental purposes.Finally, because of so many withdrawals, since1960 the river has rarely made it to the Gulf ofCalifornia except during a few years with higher thannormal precipitation. This threatens the survival ofspecies that spawn in the river, destroys estuaries thatserve as breeding grounds for numerous aquaticspecies, and increases saltwater contamination ofaquifers near the coast.Traditionally, about 80% of the water withdrawnfrom the Colorado has been used to irrigate crops andraise cattle. This large-scale use of water for agriculturewas made possible because the government paidAll-AmericanCanalYumaMexicaliGulf ofCaliforniaIDAHOGrandCanyonSalt Lake CityUTAHLakePowellARIZONAPhoenixGlenCanyonDamLOWERBASINColorado RiverTucsonWYOMINGGrand JunctionUPPERBASINCOLORADOAlbuquerque00MEXICODenverNEW MEXICO100 mi.150 km314 CHAPTER 15 Water Resources