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Chapter 05
Secondary education (bio sci)
Eastern Visayas State University
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Whyisgroundwaterimportant?
Alltheriversrunintothesea;yettheseaisnotfull;untotheplacefromwhencetherivers come,thithertheyreturnagain.”(Ecclesiastes1:7) AstheperceptivewriterofEcclesiastesindicated,wateriscontinuallyonthemove,from theoceantothelandandbackagaininanendlesscycle- I logiccyclethatreturnswaterto thesea,andsome movesmoreslowlybelowthesurface,streamsandground- , waterrepresentbasiclinksintheconstantcyclingoftheplanet’swater,weexaminethe l factorsthatinfluencethedistributionandmovementofwater,aswellaslookathowwatersculptures thelandscape,theGrandCanyon,NiagaraFalls,OldFaithful,andMammothCaveall owetheirexistencetotheactionofwateronitswayto thesea.
Toassistyouinlearningtheimportantconceptsinthischapter,focusonthefollowingquestions: in Whatisthehydrologiccycle?Whatisthesourceofenergythatpowersthecycle? Whatarethethreemainzonesofariversystem? Whatarethefactorsthatdeterminetheflowvelocityofastream? anTheworkofastreamincludeswhatthreeprocesses? 51$Whatarethetwogeneraltypesofstreamvalleysandsomefeaturesassociatedwitheach? Howaredeltas,naturallevees,andalluvialfanssimilar?Howaretheydifferent? 113:Whydofloodsoccur?Whataresomebasicflood-controlstrategies?
Whatisgroundwater,andwhatfactorsinfluenceitsstorageandmovement? Howdosprings,geysers,wells,andartesiansystemsform? Whataresomeenvironmentalproblemsassociatedwithgroundwater? Whatfeaturesareproducedbythegeologicworkofgroundwater?Whatiskarsttopography?
Earth as a System:
The Hydrologic Cycle
SculpturingEarth’sSurface U5F"G3Q>Fm"'9:g2-lfiHI1'71 9 RunningWater WaterisjustabouteverywhereonEarth—intheoceans,glaciers, rivers,lakes,air,soil,andinlivingtissue i).Allofthese reservoirsconstituteEarth’shydrosphere,thewatercontent ofthehydrospherecomprisesabout1 (326millioncubicmiles).Thevastbulkofit,about9'7, isstoredintheglobaloceans another2,leavingonly0 amonglakes,streams,groundwater,andtheatmosphere(see Figure1,p].AlthoughthepercentagesofEarth’stotalwater foundineachofthelattersourcesisbutasmallfractionofthe totalinventory,theabsolutequantitiesaregreat. Watercanreadilychangefromonestateofmatter(solid, liquid,orgas)toanotheratthetemperaturesandpressures occurringatEarth’ssurface,waterisconstantlymov-
ingamongtheoceans,theatmosphere,thesolidEarth,andthe biosphere’swatersupplyis calledthehydrologiccycle interrelationshipsamongdifferentpartsoftheEarthsystem. Thehydrologiccycleisagiganticworldwidesystempowered byenergyfromtheSuninwhichtheatmosphereprovidesthe vitallinkbetweentheoceansandcontinents A.).Water evaporatesintotheatmospherefromtheoceanandtoamuch lesserextentfromthecontinents- ladenair,oftengreatdistances,untilconditionscausethemois- turetocondenseintocloudsandtoprecipitateandfall precipitationthatfallsintotheoceanhascompleteditscycleand isreadytobeginanother, however,mustmakeitswaybacktotheocean. Whathappenstoprecipitationonceithasfallenonland? Aportionofthewatersoaksintotheground(calledinfiltra- tion),slowlymovingdownward,thenlaterally,finallyseeping intolakes,streams,ordirectlyintotheocean rainfallexceedsEarth’sabilitytoabsorbit,thesurpluswater flowsoverthesurfaceintolakesandstreams,aprocesscalled runoff
120 CHAPTER 5 RunningWaterandGroundwater
returnstotheatmospherebecauseofevaporationfromthe soil,lakes,andstreams,someofthewaterthatinfiltrates thegroundsurfaceisabsorbedbyplants,whichthenreleaseit intothe atmosphere calledtranspiration (trans=across,spire=tobreathe).Becausewecannotclearly distinguishbetweentheamountofwaterthatisevaporatedand theamountthatistranspiredbyplants,thetermevapotran- spirationisoftenusedforthecombinedeffect. Whenprecipitationfallsinverycoldareas—athighelevations orhighlatitudes—thewatermaynotimmediatelysoakin,run off,orevaporate,itmaybecomepartofasnowfieldora glacier,glaciersstorelargequantitiesofwateronland. Ifpresent-dayglaciersweretomeltandreleasealltheirwater,sea levelwouldrisebyseveraldozenmeters manyheavilypopulatedcoastalareas, overthepast 2 millionyears,hugeicesheetshaveformedand meltedonseveraloccasions,eachtimechangingthebalanceof thehydrologiccycle. Figure5’soverallwaterbalance,orthevol- umeofwaterthatpassesthrougheachpartofthecycleannually. Theamountofwatervaporintheairatanyonetimeisjustatiny fractionofEarth’stotalwatersupply thatarecycledthroughtheatmosphereoveraone-yearperiod areimmense—-some380,000cubickilometers—enoughtocover Earth’sentiresurfacetoadepthofabout 1 meter(39inches).Esti- matesshowthatoverNorthAmericaalmostsixtimesmorewater iscarriedbymovingcurrentsofairthanistransportedbyallthe continents’rivers. Itisimportanttoknowthatthehydrologiccycleisbalanced. Becausethetotalamountofwatervaporintheatmosphere remainsaboutthesame,theaverageannualprecipitationover Earthmustbeequaltothequantityofwaterevaporated- ever,forallofthecontinentstakentogether,precipitationexceeds evaporation,overtheoceans,evaporationexceeds precipitation- ping,thesystemmustbeinbalance,the36,000cubic kilometersofwaterthatannuallyrunsofffromthelandtothe oceancausesenormouserosion,thisimmensevolumeof movingwateristhesinglemostimportantagentscalptingEartlfs landsurface. Tosummarize,thehydrologiccycleisthecontinuousmove- mentofwaterfromtheoceanstotheatmosphere,fromthe atmospheretotheland,andfromthelandbacktothesea land-back-to-the-seastepistheprimaryactionthatwearsdown Earth’slandsurface,wefirstobservetheworkof waterrunningoverthesurface,includingfloods,erosion,and theformationofvalleys laborsofgroundwaterasitformsspringsandcavernsandpro- videsdrinkingwateronitslongmigrationtothesea.
CONCEPTCHECK 5. QDescribeorsketchthemovementofwaterthroughthe hydrologiccycle, whatpathsmightittake? Q“Thatiseuapotranspiration?
Running Water
SculpturingEarth'sSurface
‘Rm"'202-: 01 fl‘! 9 RunningWater Recallthatmostoftheprecipitationthatfallsonlandeitherenters thesoil(infiltration)orremainsatthesurface,movingdownslope asrunoff intothegrounddependsonseveralfactors:(1)intensityanddura- tionofrainfall,(2)amountofwateralreadyinthesoil,(3)nature ofthesurfacematerial,(4)slopeoftheland,and(5)theextentand typeofvegetation/Vhenthesurfacematerialishighlyimperme- able,orwhenitbecomessaturated,runoffisthedominantprocess. Runoffisalsohighinurbanareasbecauselargeareasarecovered byimpermeablebuildings,roads,andparkinglots. Runoffinitiallyflowsinbroad,thinsheets floweventuallydevelopsthreadsofcurrentthatformtinychan- nelscalledrills,whichjointoform streams,butasoneintersectsanother, largerandlargeronesform waterfromabroadregion.
DrainageBasins Thelandareathatcontributeswatertoariversystemiscalled Eldrainagebasin iiiI;< E151).Thedrainagebasinofonestream isseparatedfromthedrainagebasinofanotherbyanimaginary
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RunningWater 121
Students SometimesAsk...
Istheamountofwatervaporthatplantsemitintothe atmospherethroughtranspirationasignificantamount? 60 centimeters(2feet)deepover theentirefield oftreesmaypumptwicethis amountintotheatmosphere.
Usethisexampletojudgefor yourself cropsmaytranspirethe equivalentofawaterlayer
linecalledadivide- ratingtwosmallgulliesonahillsidetoacontinentaldivide, whichsplitswholecontinentsintoenormousdrainagebasins. TheMississippiRiverhasthelargestdrainagebasininNorth America(:-w;—f';).ExtendingbetweentheRockyMountains inthewestandtheAppalachianMountainsintheeast,the MississippiRiveranditstributariescollectwaterfrommorethan 3(1)ofthe continent.
RiverSystems Riversystemsinvolvenotonlyanetworkofstreamchannels,but theentiredrainagebasin- atingwithinthem,riversystemscanbedividedintothreezones: sedimentproduction—whereerosiondominates,sedimenttrans- port,andsedimentdeposition Itisimportantto recognizethatsedimentisbeingeroded,transported,and
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depositedalongtheentirelengthofastream,regardlessofwhich processisdominantwithineachzone. Thezoneofsedimentproduction,wheremostofthewater andsedimentisderived,islocatedintheheadwaterregionofthe riversystem bedrockthatissubsequentlybrokendownbyweathering,then transporteddownslopebymasswastingandoverlandflow erosioncanalsocontributesignificantamountsofsediment addition,scouringofthechannelbeddeepensthechanneland addstothestream’ssedimentload.
§_=;.,-2%-,regardlessofsize thatcontributewaterto theMissouriRiver,whichinturnisoneofmanythatmakeupthedrainagebasinoftheMississippiRiver basinoftheMississippiRiver,NorthAmerica'slargest,coversabout 3 millionsquarekilometers.
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FIGURE5.‘?Rapidsarecommoninmountainstreamswherethe gradientissteepandthechannelisroughandirregular moststreamflowisturbulent,itisusuallynotasroughasthat
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124 CHAPTER 5 RunningWaterandGroundwater
TABLE5. World’sLargestRiversRankedbyDischarge DrainageArea AverageDischarge Square Square Cubicmeters Cubicfeet Rank River Country 1 Amazon Brazil 2 Congo 3 Yangtze China 4 Brahmaputra Bangladesh 935 5 Ganges India 1,
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Russia UnitedStates
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9 Lena Russia 10 Parana Argentina
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J].I Alongitudinalprofileisacross-sectionalongthe lengthofastream-upwardcurveof theprofile, withasteepergradientupstreamandagentlergradient downstream'sKingsRiver originatesintheSierraNevadaandflowswestwardintotheSan JoaquinValley.
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featureofatypicallongitudinalprofileisitsconcaveshape—aresult ofthedecreaseinslopethatoccursfromtheheadwaterstothe mouth,localirregularitiesexistintheprofilesofmost streams—theflattersectionsmaybeassociatedwithlakesorreser- voirs,andthesteepersectionsaresitesofrapidsorwaterfalls. Thechangeinslopeobservedonmoststreamprofilesisusu- allyaccompaniedbyanincreaseindischargeandchannelsize, andareductioninsedimentparticlesize 1.--Ir).Forexam- ple,datafromsuccessivegagingstationsalongmostriversshows that,inhumidregions,dischargeincreasestowardthemouth. Thisshouldcomeasnosurprisebecause,aswemovedown- stream,moreandmoretributariescontributewatertothemain channel,forexample,about1,000trib- utariesjointhemainriveralongits6,500-kilometercourseacross SouthAmerica. Inordertoaccommodatethe growingvolumeofwater,channel size typically increases down- streamaswell velocitiesarehigherinlargechan- nelscomparedtosmallchannels. Furthermore,observationsshow ageneraldeclineinsedimentsize downstream,makingthechannel smootherandmoreefficient. Although the channelslope decreasestowardastream’smouth, theflowvelocitygenerallyincreases. Thisfactcontradictsourintuitive
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/57-= -
126 CHAPTER 5 RunningWaterandGroundwater
TransportationofSediment
Allstreams,regardlessofsize,transportsomerock material transportbecausefiner,lightermaterialiscarried morereadilythanlargerparticles- porttheirloadofsedimentinthreeways:(1)insolu- tion(dissolvedload),(2)insuspension(suspended load),and(3)slidingorrollingalongthebottom (bedload).
Dissolved Load Mostofthedissolvedloadis broughttoastreambygroundwaterandisdis- persedthroughouttheflow dissolvedloadissmallandthereforeisexpressed aspartsofdissolvedmaterialpermillionpartsof water(partspermillion,orppm).Althoughsome riversmayhaveadissolvedloadof1,000ppmor more,theaveragefigurefortheworld’sriversis estimatedat 115 to 120 ppm. Thevelocityofstreamflowhasessentiallyno effectonastream’sabilitytocarryitsdissolvedload. Oncematerialisinsolution,itgoeswhereverthe streamgoes,regardlessofvelocity occurs onlywhenthe chemistry ofthe water changes.
Suspended Load Mostlarge rivers carry the largestpartoftheirloadinsuspension,the muddyappearancecreatedbysuspendedsediment is the most obvious portionofa stream’s load (Figure5).Usuallyonlyfineparticlesconsisting ofsiltandclaycanbecarriedthisway,butduringa flood, sand and even gravel-size particles are transportedaswell,duringaflood,thetotal quantityofmaterialcarriedinsuspensionincreases dramatically,ascanbeverifiedbyanyonewhose Wm homehasbeena siteforthedepositionofthis material. Thetypeandamountofmaterialcarriedinsuspensionare controlledbytwofactors:theflowvelocityandthesettling velocityofeachsedimentgrain thespeedatwhichaparticlefallsthroughastillfluid theparticle,themorerapidlyitsettlestowardthestreambed. Inadditiontosize,theshapeandspecificgravityofparticles alsoinfluencesettlingvelocity moreslowlythandosphericalgrains,anddenseparticlesfall towardthebottommorerapidlythandolessdenseparticles. Theslowerthesettlingvelocityandthehighertheflowveloc- ity,thelongerasedimentparticlewillstayinsuspension,and thefartheritwillbecarrieddownstream.
Bed Load Aportion of a stream’sload ofsolid material consistsofsedimentthatistoolargetobecarriedinsuspension. Thesecoarserparticlesmovealongthebottom (bed)ofthe streamandconstitutethebedload
ZF‘it‘%'iitTi§.i TheColoradoRiverinGrandCanyonNationalPark. Themuddyappearanceisaresultofsuspendedsediment.(Photo byMichaelCollier)
work accomplishedby a downcuttingstream, the grinding actionofthebedloadisofgreatimportance. Theparticlesthatmakeupthebedloadmovebyrolling,sliding, andsaltation(saltare=toleap) appearstojumpor skipalongthestreambed- clesarepropelledupwardbycollisionsorliftedbythecurrentand thencarrieddownstreamashortdistanceuntilgravitypullsthem backtothebedofthestream movebysaltationeitherrollorslidealongthebottom,dependingon theirshapes. Thebedloadusuallydoesnotexceed 10 percentofastream’s totalload’sbedloadshouldbeviewedcau- tiously,however,becausethetransportofsedimentasbedload orassuspendedloadchangesfrequently dependonthecharacteristicsofstreamflowatanytimeandthese
StreamChannels 127
mayfluctuateovershortintervals, partsofthebedloadarethrownintosuspension,when velocitydecreases,aportionofthebedloadisdepositedandsome sedimentthathadbeensuspendedchangestomovingasbedload.
Competence andCapacity Astream’sabilitytocarrysolid particlesisdescribedusingtwocriteria:capacityandcompetence. Capacityisthemaximumloadofsolidparticlesastreamcan transportperunitoftime,thegreater thestream’scapacityforhaulingsediment,large riverswithhighflowvelocitieshavelargecapacities. Competenceisameasureofastream’sabilitytotransport particlesbasedonsizeratherthanquantity key—swiftstreams have greater competencies than slow streams,regardlessofchannelsize’scompetence increasesproportionatelytothesquareofitsvelocity,if thevelocityofastreamdoubles,theimpactforceofthewater increasesfourtimes;ifthevelocitytriples,theforceincreases ninetimes,andsoforth,largebouldersthatareoften visibleduringlowwaterandseemimmovablecan,infact,be transportedduringexceptionalfloodsbecauseofthestream’s increasedcompetence. Bynowitshouldbeclearwhythegreatesterosionand transportationofsedimentoccurduringfloods indischargeresultsingreatercapacityandtheincreasedveloc- ityproducesgreatercompetency watermoreturbulent,andlargerparticlesaresetinmotion justafewdays,orperhapsafewhours,astreamatfloodstage canerodeandtransportmoresedimentthanitdoesduring manymonthsofnormalflow.
DepositionofSediment Wheneverastreamslowsdown,thesituationreverses velocity decreases,its competenceisreducedandsediment beginstodropout,largestparticlesfirst size hasa settlingvelocity. Asstreamflow drops belowthe settingvelocityof a certainparticle size, sedimentin that categorybeginstosettleout,streamtransportprovidesa mechanism by which solid particles of various sizes are separated,calledsorting,explainswhyparticlesof similarsizearedepositedtogether. Thematerialdepositedbyastreamiscalledalluvium,the generaltermforanystream-depositedsediment depositionalfeaturesarecomposedofalluvium withinstreamchannels,someoccuronthevalleyflooradjacent tothechannel,andsomeexistatthemouthofthestream considerthenatureofthesefeatureslaterinthechapter.
CONCEPTcnscx 5. 0 Listtwowaysinwhichstreamserodetheirchannels. QInwhatthreewaysdoesastreamtransportitsload’? QVllhatisthedifferencebetweencapacityandcompetency? QWhatissettlingvelocity?Whatfactorsinfluencesettling velocity?
Stream Channels
Abasiccharacteristicofstreamflowthatdistinguishesitfrom sheetflowisthatitisusuallyconfinedtoachannel channelcanbethoughtofasanopenconduitthatconsistsof thestreambedandbanksthatacttoconfinetheflow,except duringfloods. Althoughsomewhatoversimplified,wecandividestream channelsintotwotypes streamsareactivelycuttingintosolidrock,whenthe bedandbanksarecomposedmainlyofunconsolidatedsediment, thechanneliscalledanalluvialchannel.
BedrockChannels In theirheadwaters,where thegradientisusuallysteepest, manyriverscutintobedrock(oftenmountain streams)typicallytransportcoarseparticlesthatactivelyabrade thebedrockchannel erosionalforcesatwork. Bedrockchannelsoftenalternatebetweenrelativelygently slopingsegmentswherealluviumtendsto accumulate,and steepersegmentswherebedrockisexposed maycontainrapidsoroccasionallyawaterfall- ternexhibitedbystreamscuttingintobedrockiscontrolledby theunderlyinggeologicstructure uniformbedrock,streamstendtoexhibitawindingorirregular patternratherthanflowinginastraightchannel goneonawhitewaterraftingtriphasobservedthesteep,winding natureofastreamflowinginabedrockchannel.
AlluvialChannels Manystreamchannelsarecomposedoflooselyconsolidated sediment (alluvium) andtherefore canundergosignificant changesinshapebecausethesedimentsarecontinuallybeing eroded, transported, and redeposited. The major factors affectingtheshapesofthesechannelsistheaveragesizeofthe sediment beingtransported, thechannelgradient, andthe discharge. Alluvialchannelpatternsreflectastream’sabilitytotrans- portitsloadatauniformrate,whileexpendingtheleastamount ofenergy,thesizeandtypeofsedimentbeingcarried helpdeterminethenatureofthestreamchannel typesofalluvialchannelsaremeanderingchannelsandbraided channels.
Meandering Streams Streamsthattransportmuchoftheir loadinsuspensiongenerallymoveinsweepingbendscalled meanders. These streams flow in relatively deep, smooth channelsandtransportmainlymud(siltandclay),sand,and occasionallyfinegravel channelofthistype. Meanderingchannelsevolveovertimeasindividualbends migrateacrossthefloodplain theoutsideofthemeander,wherevelocityandturbulenceare greatest,theoutsidebankisundermined,especiallydur- ingperiodsofhighwater
StreamChannels 129
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dumpedintothemeltwaterstreams flowingawayfromtheglacier- ever, when flow is sluggish, the streamisunabletomoveallofthe sedimentandthereforedepositsthe coarsestmaterialasbarsthatforce theflowtosplitandfollowseveral paths channelscompletelyreworkmostof thesurface sediments eachyear, thereby transforming the entire streambed, however,thebarshavebuiltupto formislandsthatareanchoredby vegetation. Insummary,meanderingchan- nelsdevelopwheretheloadconsists largelyoffine-grainedparticlesthat aretransportedassuspendedloadin deep,relativelysmoothchannels contrast,wide,shallowbraidedchannelsdevelopwherecoarse- grainedmaterialistransportedasbedload.
####### comcnrrcnncn 5. 5
QArebedrockchannelsmorelikelytobefoundnearthehead ornearthemouthofastream? QDescribeorsketchthedevelopmentofameander,including howanoxbowlakeforms. FIGURE5 QDescribeb _dd asituationthatmightcauseastreamtobecome occupyabandonedmeanders,oxbow ml e' lakesgraduallybecomeswampymeanderscars oxbowlakecreatedbythemeanderingGreenRivernearBronx, Wyoming.(PhotobyMichaelCollier)
Fin-i'_lRE5 TheKnikRiver isaclassicbraidedstreamwith multiplechannelsseparatedby migratinggravelbars chokedwithsedimentcontributed bymeltwaterfromfourglaciersin theChugachMountainsnorthof Anchorage,Alaska.(Photoby MichaelCollier)
130 CHAPTER 5 RunningWaterandGroundwater
Base Level and Stream
Erosion
Streams cannotendlessly erodetheir channels deeper and deeper, andthatlimitiscalledbaselevel straightforward,itisneverthelessakeyconceptinthestudyof streamactivity whichastreamcanerodeitschannel atwhichthemouthofastreamenterstheocean,alake,or anotherstream profiles havelow gradients neartheir mouths, becausethe streamsareapproachingtheelevationbelowwhichtheycannot erodetheirbeds. Twogeneraltypesofbaselevelarerecognized consideredtheultimatebaselevel,becauseitisthelowestlevelto whichstreamerosioncouldlowertheland,orlocal, baselevelsincludelakes,resistantlayersofrock,andmainstreams thatactasbaselevelsfortheirtributaries,whena streamentersalake,itsvelocityquicklyapproacheszeroandits abilitytoerodeceases,thelakepreventsthestreamfrom erodingbelowitslevelatanypointupstreamfromthelake- ever,becausetheoutletofthelakecancutdownwardanddrain thelake,thelakeisonlyatemporaryhindrancetothestream’s
1'i;>.;i111?. _ 5 .5Aresistantlayerofrockcanactasalocal(temporary) baselevel,itlimits theamountofdowncuttingupstream.
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abilitytodowncutitschannel,thelayerof resistantrockatthelipofthewaterfallin .rr-Yactsasatem- porarybaselevel,itwill limittheamountofdowncuttingupstream. Anychangeinbaselevelwillcauseacorrespondingread- justmentofstreamactivities, thereservoirthatformsbehinditraisesthebaselevelofthe stream Upstreamfromthedamthegradientis reduced,loweringthestream’svelocityand,hence,itssediment- transportingability,nowhavingtoolittleenergyto transportitsentireload,willdepositsediment channel stream’sgradientincreasessufficientlytotransportitsload.
CONCEPTcnrzcx 5. QWhatisbaselevelandhowdoesitinfluencestreamactivity? QDistinguishbetweenultimateandtemporary(local)baselevel.
Shaping StreamValleys
fifi SculpturingEarth’sSurface mm9->"buZ-ln1IT! iRunningWater Streams,withtheaidofweatheringandmasswasting,shapethe landscapethroughwhichtheyflow,streamscontinu- ouslymodifythevalleysthattheyoccupy. Astreamvalleyconsistsofachannelandthesurrounding terrainthatcontributeswatertothestream thevalleyfloor,whichisthelower,flatterareathatispartially ortotallyoccupiedbythestreamchannel,andthesloping valleywallsthatriseabovethevalleyflooronbothsides streamvalleysaremuchbroaderatthetopthanisthewidthof theirchannelatthebottom t iftheonlyagentresponsibleforerodingvalleyswerethe streamsflowingthroughthem areshapedbyacombinationofweathering,overland flow,andmasswasting,where weatheringisslowandwhererockisparticularlyresis- tant, narrowvalleyshavingnearlyvertical wallsarecommon continuumfromnarrow,steep-sidedvalleys Rflljlafii '. -Eie.:-1i.-lwj-.~li%:. tothosethataresoflatandwidethattheval-
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132 CHAPTER 5 iRunningWaterandGroundwater
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ChangingBaseLevel
andIncisedMeanders
Weusuallyexpectastreamwithahighlymeanderingcoursetobe onafloodplaininawidevalley,certainriversexhibit meanderingchannelsthatflowinsteep,narrowvalleys meandersarecalledincised(incisum=tocutinto)meanders (Figure5:19).Howdosuchfeaturesform? Originally,themeandersprobablydevelopedontheflood- plainofastreamthatwasrelativelynearbaselevel,achange inbaselevelcausedthestreamtobegindowncutting eventscouldhaveoccurred uponwhichtheriverwasflowingwasuplifted. AnexampleofthefirstcircumstancehappenedduringtheIce Agewhenlargequantitiesofwaterwerewithdrawnfromtheocean andlockedupinglaciersonland (ultimatebaselevel)dropped,causingriversflowingintotheocean tobegintodowncut,thisactivityceasedatthecloseof theIceAgewhenicesheetsmeltedandsealevelrose. Regionalupliftoftheland,thesecondcauseforincised meanders,isexemplifiedbytheColoradoPlateauinthesouth- westernUnitedStates, astheplateauwas gradually uplifted,numerousmeanderingriversadjustedtobeinghigher abovebaselevelbydowncutting.
concern‘cnscn 5. QExplainwhyV-shapedvalleysoftencontainrapidsand/or waterfalls. QDescribeorsketchhowanerosionalfloodplaindevelops. QRelateincisedmeanderstochangesinbaselevel.
T referredtoasbars,how- , ever,areonlytemporary,becausethe M I materialwillbepickedupagainand eventuallycarriedtotheocean- its I tiontosandandgravelbars,streamsalso
- createotherdepositionalfeaturesthathave asomewhatlongerlifespan deltas,naturallevees,andalluvialfans.
Deltas Deltasformwheresediment-chargedstreams entertherelativelystillwatersofalake,an I inlandsea,ortheocean(Figure5).Asthe stream’sforwardmotionslows,sedimentsare depositedbythedyingcurrent outward,thestream’sgradientcontinuallylessens. Thiscircumstanceeventuallycausesthechanneltobecome chokedwithsedimentdepositedfromtheslowingwater- sequence,theriverseeksashorter,higher-gradientroutetobase level,asillustratedinFigure5 main channel dividing into several smaller ones, called distributaries channelsthatactinanoppositewaytothatoftributaries.
FIGURE5 theColoradoRiverinCanyonlands NationalPark,Utah, themeanderingriveradjustedtobeinghigherabovebaselevelby downcutting.(PhotobyMichaelCollier)
DepositionalLandforms 133
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Ratherthancarryingwaterintothemainchannel,distribu- tariescarrywaterawayfromthemainchannel shiftsofthechannel,adeltamaygrowintoaroughlytriangular shapeliketheGreekletterdelta(A),forwhichitisnamed, however,thatmanydeltasdonotexhibittheidealizedshape- ferencesintheconfigurationsofshorelinesandvariationsinthe natureandstrengthofwaveactivityresultinmanyshapes
largerivershavedeltasextendingoverthousandsofsquarekilo- meters(see Box5).Itresultedfromtheaccumulationofhugequantitiesof sedimentderivedfromthevastregiondrainedbytheriverandits tributaries(seeFigure5,p).Today,NewOrleansrestswhere therewasoceanlessthan5,000yearsago. showsthat portionoftheMississippideltathathasbeenbuiltoverthepast 6,000years,thedeltaisactuallyaseriesofseven coalescingsubdeltas channelinfavorofashorter,moredirectpathtotheGulfofMex- ico anothertoproduceaverycomplexstructure, calledabird-footdeltabecauseoftheconfigurationofitsdis- tributaries,hasbeenbuiltbytheMississippiinthelast 500 years.
NaturalLevees Someriversoccupyvalleyswithbroadfloodplainsandbuildnatrual leveesthatparalleltheirchannelsonbothbanks -: *- ‘ti 1).Nat- uralleveesarebuiltbysuccessivefloodsovermanyyears streamoverflowsitsbanks,itsvelocityimmediatelydiminishes,leav- ingcoarsesedimentdepositedinstripsborderingthechannel thewaterspreadsoutoverthevalley,alesseramountoffinesedi- mentisdepositedoverthevalleyfloor materialproducestheverygentleslopeofthenaturallevee. ThenaturalleveesofthelowerMississippirise 6 meters (20feet)abovethefloodplain- acteristicallypoorlydrainedfortheobviousreasonthatwater cannotflowuptheleveeandintotheriver
Duringthepast6,000years,theMississippiRiverhasbuiltaseriesofsevencoalescingsubdeltas numbersindicatetheorderinwhichthesubdeltasweredeposited(number7)representsthe activityofthepast 500 years.(ImagecourtesyofJPL/CalTech/NASA)Withoutongoinghumanefforts,thepresentcoursewill shiftandfollowthepathof theAtchafalayaRiver somedaybreakthrough(arrow)andtheshorterpathitwouldtaketotheGulfofMexico.(AfterC. R.KolbandJ)
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DrainagePatterns 135
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-";-iti%;é.f-if-ii Naturalleveesaregentlyslopingdepositsthatarecreatedbyrepeatedfloods ofdevelopment,backswampsandyazootributaries maydevelop.
backswampsresult becauseleveesblockthewayoftenhastoflowparallelto the riveruntilitcanbreachthelevee tributariesaftertheYazooRiver,whichparallelstheMississippi forover 300 kilometers(about 190 miles).
AlluvialFans Alluvialfanstypicallydevelopwhereahigh-gradientstreamleaves anarrowvalleyinmountainousterrainandcomesoutsuddenly ontoabroad,flatplainorvalleyfloor(seeFigure6,p).Allu- vialfansforminresponsetomeabruptdropingradientcombined withthechangefromanarrowchannelofamountainstreamto lessconfinedchannelsatthebaseofthemountains dropinvelocitycausesthestreamtodumpitsloadofsediment quicklyinadistinctivecone-orfan-shapedacctunulation- tratedbyFigure6,thesurfaceofthe fanslopesoutwardina broadarcfromanapexatthemouthofthesteepvalley, coarsematerialisdroppedneartheapexofthefan,whilefine materialiscarriedtowardthebaseofthedeposit.
CONCEPTcnacx 5. QWhathappenswhenastreamenterstherelativelystill watersofalake,aninlandsea,ortheocean? QBrieflydescribetheformationofanaturallevee featurerelatedtobackswampsandyazootributaries? QHowdoesanalluvialfandifferfromadelta?
Drainage Patterns
SculpturingEarth'sSurface
mm9:6“"'112-ha?nu:ITI PRunningWater
Drainagesystemsarenetworksofstreamsthattogetherformdis- tinctivepatterns fromonetypeofterraintoanother,primarilyinresponsetothe kindsofrockonwhichthestreamsdevelopedorthestructural patternoffaultsandfolds. Themostcommonlyencountereddrainagepatternisthe dendriticpattern Thispatternofirregularly branchingtributarystreamsresemblesthebranchingpatternof adeciduoustree,theworddenclriticmeans“treelike.”The dendriticpatternformswheretheunderlyingmaterialisrelatively uniform itsresistancetoerosion,itdoesnotcontrolthepatternofstream- flow,thepatternisdeterminedchieflybythedirectionof slopeoftheland. Whenstreamsdivergefromacentralarealikespokesfrom thehub ofawheel,thepatternissaidtoberadial(Figure5). Thispatterntypicallydevelopsonisolatedvolcanicconesand domaluplifts. Figure5,inwhich manyright-anglebendscanbeseen whenthebedrockiscrisscrossedbyaseriesofjointsand/or faults unbrokenrock,theirgeometricpatternguidesthedirections ofvalleys.
136 CHAPTER 5 RunningWaterandGroundwater
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- iV ’ -_, 1 .-- -tP1:' '15. - D 1 "i '. .=:’7/ developsonisolated F i‘- ‘= a devempsinareasof jr volcanicconesordomes alternatingweakand resistantbedrock Drainagepatterns.A.Dendritic.B.Radial.C.Rectangular.
Figure5,arectan- gularpatterninwhichtributarystreamsarenearlyparalleltoone anotherandhavetheappearanceofagardentrellis formsinareasunderlainbyalternatingbandsofresistantand less-resistantrock.
CONCEPTcnscx 5. 9 QMakeasimplesketchofthefourbasicdrainagepatterns.
Floods and Flood Control
Whenthedischargeofastreambecomessogreatthatitexceeds thecapacityofitschannel,itoverflowsitsbanks asaflood. Floodsareamongthemostdeadlyandmostdestructiveofall geologichazards. Theyare, nevertheless, simplypartofthe naturalbehaviorofstreams.
CausesofFloods
Riversfloodbecauseoftheweather thespringand/ormajorstormsthatbringheavyrainsovera largeregioncausemostfloods 1997 floodalong theRedRiverofthenorthisanexampleofaneventtriggeredby rapidsnowmelt intheupperMississippiRiverValleyduringthesummerof 1993
Unliketheextensiveregionalfloodsjustmentioned,flash floodsaremorelimitedinextent warningandcanbedeadlybecausetheyproducearapidrisein waterlevelsandcanhaveadevastatingflowvelocity- torsinfluenceflashflooding andduration,topography,andsurfaceconditions areasaresusceptiblebecausesteepslopescanquicklyfunnel runoffintonarrowcanyons Urbanareasaresus- ceptibletoflashfloodsbecauseahighpercentageofthesurface areaiscomposedofimpervioussurfacessuchasroofs,streets, andparkinglotswhererunoffisveryrapid,astudyindi- catedthattheareaofimpervioussurfacesintheUnitedStates (excludingAlaskaandHawaii)amountstomorethan112, squarekilometers(nearly44,000squaremiles),whichisslightly lessthantheareaofthestateofOhio. Humaninterferencewiththestreamsystemcanworsenor evencausefloods artificiallevee. Theyaredesignedtocontainfloodsofacertainmagnitude largerfloodoccurs,thedamorleveeisovertopped leveefails oriswashedout,thewaterbehinditisreleasedto becomeaflashflood 1889 ontheLittle ConemaughRivercausedthedevastatingIohnstown,Pennsyl- vania,floodthattooksome3,000lives occurredthereagainin 1977 andcaused 77 fatalities.
TC,etal.“U,”inE08, Trrmsacrions,zlmericrmGeophysicalUnion85,No. 24 (lune15,20
Chapter 05
Course: Secondary education (bio sci)
University: Eastern Visayas State University
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