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1、ErosionErosionisanimportantnaturalprocess,resultingintheredistributesoftheproductsofgeologicweathering,andispartofbothsoilformationandsoilloss.Theworld'slandscapeshavesculptedbyerosion.Whentheresultsarespectacularenough,weenshrinetheminnationalparksaswedidwiththeGrandCanyon.Whereerosionhasworndownm
2、ountainsandspreadsoilovertheplains,ordepositedrichalluvialsiltonriverbeds,wegladlyfarmit.Erosionisadisasteronlywhenitoccursinthewrongplaceatthewrongtime.Insomeplaces,erosionoccurssorapidlythatanyonecanseeithappen.Deepgulliesarecreatedwherewaterscoursawaythesoil,leavingfencepostsandtreessittingontal
3、lpedestalsasthelanderodesawayaroundthem・Inmostplaces,however,erosionissubtler.Itisacreepingdisasterthatoccursinsmallincrements.Athinlayeroftopsoiliswashedofffieldsyearafteryearuntileventuallynothingisleftbutpoor-qualitysubsoilthatrequiresmoreandmorefertilizerandwatertoproduceanycropsatall.Theneteff
4、ect,worldwide,ofthisgeneral,widespreadtopsoilerosionisareductionincropproductionequivalenttoremovingabout1%ofworldcroplandeachyea匚Manyfarmersareabletocompensateforthislossbyapplyingmorefertilizerandbybringingnewlandintocultivation.Continuationofcurrenterosionrates,however,couldreduceagriculturalpro
5、duction25%inCentralAmericaandAfricaand20%inSouthAmericabytheyear2000.Thetotalannualsoillossfromcroplandsisthoughttobe25billionmetrictons.Abouttwicethatmuchsoilislostfromrangelands,forests,andurbanconstructionsiteseachyear.Inadditiontoreducedlandfertility,thiserosionresultsinsediment-loadingofrivers
6、andlakes,ofreservoirs,smotheringofwetlandsandcoralreefs,andcloggingofwaterintakesandwaterpowerturbines.Itmakesriversunnavigable,increasesthedestructivenessandfrequencyoffloods,andcausesgulliesthatturngoodlandintouselesswasteland.Windandwaterarethemainagentsthatmovesoilaround・Thin,uniformlayersofsoi
7、larepeeledoffthelandsurfaceinaprocesscalledsheeterosion.Whenlittlerivuletsofrunningwatergathertogetherandcutsmallchannelsinthesoil,theprocessiscalledrillerosion.Whenrillsenlargetoformbiggerchannelsorravines
