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学术交流英语听力原文RoadBuildingGoodmoning,everyone.TodayIlbetalkingabouttherelationshipbetweenwadbuildingandthedevekpnentoftheAmericaneoonontydurnuthe18thcentury.About340)vearsainxtheUnitexStatedccononywaswowingapidly,mainlybecauseofabooingtadeintvoimportantagriculturalproductsganandcotkon.GrainoutputinthecastempartofAmericaincreasedquicklyatthattimeduetotherapidlygowingpopulationandthelargemumberofimmigrantsfionEurope.Asaresult,thedemandforgainalmostdoubled.Forthisrcason,thetradeingrainfirstdevekopedinthispartofthecountry.Atthesametine,therondsystemwasgaduallybuiltupinordertotransportthegminfiomthenumlarcastovariouscitics.TheroadbuildingclearlyhelpeddevekoptheccononyquicklyinthessarcasandinthecitiesaswelDuringthesameperod,fintersintheSouthcoukigelalargeeamoumtofaborersfonAric,andtheystartedtoaowcottonAsthecottonoutputincrcasod,thefanmeesneededtosellitinotherphiwes.Asaesull,manynuudswerebuilltkolinktherumalareastothecilies.Atfist,thistadeofgrainandcottontockplncealongthecosst,ornearriversandlalkes.Ittookplacetherebecauseitwascasyandcheaptotransportgoodsfiomoneplacettanother.Before1700itwasveryexpensivetomovethegoocsbynad.So,ternershadtorelymantyontiverstomosetheircropstomarkets.Atthattime,therewasonlyonecontingousroadthatexistodintheUS.Itranfromnorthtosouthaongcountrytoadswhichwerelinkedtogethertomakeonelongroad.Withinashorttimethefinstenst-to-westroaswerebuill.TheywerecalledtempikesPrivatecomparnesbuilttheseroadsandcollectedfeesfiomallvehiclesthattaveledonthem.Evenlually,becauseofthebonningtrnieofwainandcolon,anelworkofrowyswascompletedthatconpectedsomemajorcitiesandtowns.Akhoughtavelingwasstillcostlyforlartners,theyoonprefenedtonevetheircrspsbociliesandotheraressonroklwysratherthanbyboatbecauseitwasasterandmoreconvenientSoheeewecansoealatherclearpictureofroedbuildineintheUniedStatesanditsinpactoncconomicdevelopmentduringthatperidd.TheMississippiRiverinAmericaGoodafternoon.Todaywe'regoingtotakabouttheMississippiRiver.TheMissssppiRiveristhelarsestriveroftheNorthAmerica.Itsmajortributariesdrainanarenofapproximately3,000,000squnrekilometers.oraboutone-eighthoftheentirecontinent,TheMissssppiRiverliesentirelyintheUnitedStates.FromitssourceatlakeItascainMinncsota.itfloysfomtheNorthalmostducsouthacrossthecontinentainterior,collectingthewatersofitsmajortributaries,downtotheGulfofMexico.TheMightyMssissippiasitisaflectionatelyknown,flowsatotaldistanceabout4.000kilometersfromitssource.Withitstributarics,theMississppidrainsallorpartof31U.s.statesarkitwoCanadinprovincesAseveryoneknows,inthepasttheriverwasthemostimportantwayoftransportinggoodsfromtheNorthtotheSouth,orfromtheSouthtotheNorthintheUnitedStatesTheMississinoiRiverisalsoariverthatconsomctimcsectoutofeontro]andcauscseriousfloodinginsummer.Nowadaysengineersusefbourdifferentmethodstocontroltheloodwatersoftheriver.Theselourwaysare:foodways,dams,leveesandreservoirs.Tudiscusseachofthesehere.FloodwaysarethefirstwayofcontrollingthefoodwatersoftheMississippiRiverFoodwaysareemptychannelsreadytoholdwaterfromtheriver.WhentheMississippiRiveristoohigh,thefloodwaysareopenedandsomeofthewatercanflowintothefloodways.ThereareseveralfloodwaysalongthelowerMississippiRiver,whichhelppreventfloodingeffectivelyThescoondwayofcontrollingthefloodwatersoftheMississippiRiveriswithdams.Damsarewallsthatarebultacrossarivertocontrolthewater.Therearemoreth...