Environmental Problems, Their Causes, and Sustainability 1

15-8 TOO MUCH WATERWhat Causes Flooding? Rain and PeopleHeavy rainfall, rapid melting of snow, removingvegetation, and destroying wetlands cause flooding.Heavy rain and rapid snowmelt are the major causesof natural flooding by streams. A flood happens whenwater in a stream overflows its normal channel andspills into the adjacent area, called a floodplain (Figure15-24).People settle on floodplains because of their manyadvantages: fertile soil, ample water for irrigation,availability of nearby rivers for transportation andrecreation, and flat land suitable for crops, buildings,highways, and railroads.Floods have several benefits. They provide theworld’s most productive farmland because the land isregularly covered with nutrient-rich silt left afterfloodwaters recede. They also recharge groundwaterand help refill wetlands.But each year, floods kill up to 25,000 people andcause tens of billions of dollars in property damage.Floods, like droughts, are usually considered naturaldisasters.Since the 1960s several types of human activitieshave contributed to the sharp rise in flood deaths anddamages. One is removal of water-absorbing vegetation,especially on hillsides (Figure 15-25, p. 328). Another isdraining wetlands that absorb floodwaters and reducethe severity of flooding. Living on floodplains also increasesthe threat of damage from flooding. Floodingalso increases when we pave or build and replace water-absorbing vegetation, soil, and wetlands with highways,parking lots, and buildings that cannot absorbrainwater.In developed countries, people deliberately settleon floodplains and then expect dams, levees, and otherdevices to protect them from floodwaters. However,when heavier-than-normal rains occur, these devicescan be overwhelmed. In many developing countries,the poor have little choice but to try to survive inflood-prone areas (see the Case Study below).Case Study: Living on Floodplainsin Bangladesh—Danger for the PoorBangladesh has increased flooding because ofupstream deforestation of Himalayan mountainslopes and the clearing of mangrove forests on itscoastal floodplains.Bangladesh is one of the world’s most densely populatedcountries, with 141 million people (projected toreach 280 million by 2050) packed into an area roughlythe size of Wisconsin. It is also one of the world’s poorestcountries.The people of Bangladesh depend on moderateannual flooding during the summer monsoon seasonto grow rice and help maintain soil fertility in the deltabasin. The annual floods deposit eroded Himalayansoil on the country’s crop fields.In the past, great floods occurred every 50 years orso. But since the 1970s they have come about every 4years. Bangladesh’s increased flood problems begin inthe Himalayan watershed, where several factors—rapid population growth, deforestation, overgrazing,ReservoirDamLeveeFloodwallFloodplainFigure 15-24 Land in a natural floodplain (left) often is flooded after prolonged rains. When the floodwatersrecede, deposits of silt are left behind, creating a nutrient-rich soil. To reduce the threat of flooding and thus toallow people to live in floodplains, rivers have been narrowed and straightened (channelized), equipped withprotective levees and walls, and dammed to create reservoirs that store and release water as needed. Thesealterations can give a false sense of security to floodplain dwellers. In the long run, such measures can greatlyincrease flood damage because they can be overwhelmed by prolonged rains (right), as happened along theMississippi River in the midwestern United States during the summer of 1993.http://biology.brookscole.com/miller14327