Two-thirdsoftheworld’smajorrivershavenowbeendisrupted(中断)withmorethan50000damsinanattempttostolewaterandprovidepower.IntheUS，therearemorethan85，000dams，disruptinglargeandsmallrivesandinmostcasestransformingnaturalflow．Themostfamousofthese，theHooverDamconstructedinthel930's，ismainlyresponsibleforthefactthattheColoradoRivernolongerreachestheocean.

Damsbesidesalltheirattractivebenefitsalsohavenegativeimpacts.Creatingareservoir(水库)meansalargeareamustbeflooded.Communitiesmaylosetheirlandhousesandculturallyimpotentsites.

Environmentallythenewreservoircanbeaparadiseforwildlifeespeciallybirds.Howeveritcancausegreenhousegasemissions(排放)andpoisonthewaterforfish.Alsothedramaticriseandfallofwaterlevelsduringdamreleasesistooextremeforplainsandanimalstocopewithresultingindeadzonesaroundtheshoresofreservoirs.Fishthatlaytheireggsintheshallowsforexamplemayfindafewhourslaterthatthosesitesarehighanddry.

Damsdon’tjustblockwaterflow.Theflowmaybereducedsothatfarmerswillhavegreattroublegettingwaterfortheirfields.Theyalsopreventfishmigrationsandareabarriertosediment(沉淀物)flows.Insteadofrushingdownriversedimentsgetbackedupagainstthedamwallwhichcausethereservoirleveltoincreaseovertime．Howeversedimentswhicharerichinnutrientshavebecomeaproblem.Thefertilityoftheentiresystemcanbeinfluencedwithsoilslostduringseasonalrainsnotbeingreplaced.

Perhapsthebiggestproblemcanbeseenindeltas(三角洲)oftenhosttolargecities，whicharesinkingintotheoceans.Groundwaterisbeingextractedtofeedthecitycausingtheurbanweighttosinkandsedimentswashedawaybytheoceanarenolongerbeingreplaced.TheresultisthatsealevelriseincitiesfromShanghaitoAlexandria.

28.Whateffectsdoescreatingareservoirhaveonpeople’slife?

a.Peoplewillseefewerbirdsneartherivers．

b.Farmersbelowthedammayhavelesswatertoirrigatetheirfields

c.Visitorscan’tseesomeculturalrelicsanymore．

d.Moreconflictsandwarsoverwaterwillbreakout

e.Morekindsoffishwillbecomeextinctinthefuture