Environmental Problems, Their Causes, and Sustainability 1

Oxygenreleased byvegetationDiverseecologicalhabitatTree plantationEvapotranspiration decreasesLeaf litterimprovessoil fertilityForested HillsideTree rootsstabilize soil andaid water flowSteadyriver flowEvapotranspirationTrees reduce soilerosion from heavyrain and windVegetation releaseswater slowly andreduces floodingAgriculturallandAfter DeforestationRoadsdestabilizehillsidesGullies andlandslidesHeavy rain leachesnutrients from soiland erodes topsoilRanchingacceleratessoil erosion bywater and windSilt from erosion blocksrivers and reservoirs andcauses flooding downstreamWinds removefragile topsoilAgricultural landis flooded andsilted upRapid runoffcauses floodingFigure 15-25 Natural capital degradation: hillside before and after deforestation. Once a hillside has beendeforested for timber and fuelwood, livestock grazing, or unsustainable farming, water from precipitationrushes down the denuded slopes, erodes precious topsoil, and floods downstream areas. A 3,000-year-oldChinese proverb says, “To protect your rivers, protect your mountains.”and unsustainable farming on steep and easily erodiblemountain slopes—have greatly diminished theability of its mountain soils to absorb water.Think of a forest as a complex living sponge forcatching, storing, using, and recycling water and releasingit in small amounts. Cut down the forests onthe Himalayan mountains and what happens? Insteadof being absorbed and released slowly, water from themonsoon rains runs off the denuded Himalayan foothills,carrying vital topsoil with it (Figure 15-25, right).This increased runoff of soil, combined with heavier-than-normalmonsoon rains, has increased theseverity of flooding along Himalayan rivers anddownstream in Bangladesh. A disastrous flood in 1998covered two-thirds of Bangladesh’s land area for 9months, leveled 2 million homes, drowned at least2,000 people, and left 30 million people homeless. Italso destroyed more than one-fourth of the country’scrops, which caused thousands of people to die of starvation.In 2002, another flood left 5 million peoplehomeless and inundated large areas of rice fields.Living on Bangladesh’s coastal floodplain alsocarries dangers from storm surges and cyclones. In1970, as many as 1 million people drowned in onestorm, and another surge killed an estimated 139,000people in 1991.In their struggle to survive, the poor in Bangladeshhave cleared many of the country’s coastal mangroveforests for fuelwood, farming, and aquaculture pondsfor raising shrimp. This has led to more severe floodingbecause these coastal wetlands help shelter Bangladesh’slow-lying coastal areas from storm surges andcyclones. Damages and deaths from cyclones in areasof Bangladesh still protected by mangrove forests havebeen much lower than in areas where the forests havebeen cleared.How Can We Reduce Flood Risks?Think about Where You Want to LiveWe can reduce flooding risks by controlling riverwater flows, preserving and restoring wetlands,identifying and managing flood-prone areas, and ifpossible choosing not to live in such areas.We can use several methods to reduce the risk fromflooding. One is to straighten and deepen streams, aprocesscalled channelization (Figure 15-24, middle). Channelizationcan reduce upstream flooding but it removesbank vegetation and increases stream velocity. This increasedflow of water can promote upstream bank erosion,increase downstream flooding and sediment deposition,and reduce habitats for aquatic wildlife.Another approach is to build levees or floodwallsalong the sides of streams (Figure 15-24, middle). Leveescontain and accelerate stream flow, but this increasesthe water’s capacity for doing damage downstream.They also do not protect against unusually highand powerful floodwaters, as occurred in 1993 whentwo-thirds of the levees built along the MississippiRiver in the United States were damaged or destroyed.Building dams can also reduce the threat of floodingby storing water in a reservoir and releasing it328 CHAPTER 15 Water Resources