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R ivers Instruct iona l Case : A ser ies o f s tudent-centered sc ience lessons

TeacherBackgroundErosionandDepositionErosionisaprocessthatinvolvesthephysicalremovalandtransportofmaterialbymobileagentsattheEarth’ssurface.ThethreecommonagentsoferosiononEartharewater,wind,andice.Wateristhemostefficientandeffectiveagentforerosion(throughprocessessuchaserosionbystreamsandriversandcoastalerosion).Erosionresultsinthedepositionofsedimentsinadifferentlocation.Depositionoccurswhentheforcesresponsibleforerosionarenolongersufficienttoovercometheweightoftheparticlesorfriction.Forexample,considersedimentthatiscarriedbyaswiftriver.Ifthewatervelocitydrops(duetowideningofthechannelorchangeintheslopeoftheterrain),thelarger(heavier)particlesmaydropoutandbedepositedintheriverchannel.However,thewatervelocitymaybesufficienttocontinuetocarrysmallerparticles.Inthisway,sedimentsmaybesortedbysizeduetochangesintheenergyofthedepositionalenvironment.Masswasting(commonlyreferredtoaslandslides)issimilartoerosioninthatitinvolvesthemovementofrockandsoilattheEarth’ssurface.However,masswastingisadistinctlydifferentprocessthanerosioninthatitinvolvesthedownslopemovementofsurfacematerialsundertheinfluenceofgravity.Erosion,incontrast,requiresanagent(suchaswater,windorice)thatisphysicallyinvolvedinthemovementofthesediment.Inmasswasting,gravityisthecontrollingfactoranddoesnotinvolveanerosionalagent.Althoughscientistsconsidermasswastinganderosionasseparateprocesses,theycommonlyworktogetherinprocessessuchascoastalerosionandthewideningofrivervalleys.Forexample,theformationandwideningoftheGrandCanyon(Fig.1)isaccomplishedbyerosionandmasswastingworkingtogether.Masswasting(landslides)transfersmaterialtothecanyonfloor(wideningthecanyon)wherethematerialisthentransporteddownstream(erosion)bytheriver.Mostrivervalleysaremuchwiderthantheyaredeepandwiderthanthestreamsthathelpedtoformthem.Thisisevidenceofmasswastingprocessesinsupplyingmaterialtothestreaminforminglargevalleysandcanyons.

Figure1.GrandCanyon.Source:NPS.

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WeatheringisthephysicalbreakdownandchemicalalterationofrocksandmineralsatorneartheEarth'ssurfaceduetointeractionswithlivingorganisms,waterandtheatmosphere.Weatheringisdistinctlydifferentfromerosioninthatweatheringoccursinplaceanddoesnotinvolvethemovementortransportofsediment.Theweatheringofrocksresultsintwotypesofproducts:

1.solidparticles(sandgrains,clayminerals,etc.)2.dissolvedconstituentsinwater

Theproductsofweathering(sediment)providethematerialsthatareinvolvedinerosionanddeposition.Theweatheringofrockscommonlyresultsintheproductiondifferentsizesofsedimentsbeingproducedrangingfrommicroscopic(clays)tosand-andgravel-sizedparticles.Thereare3typesofsedimentcarriedbyariver:

1. dissolvedload-transportofionsandchemicalsinsolution–partiallyresponsibleforthesaltinessofthesea.

2. suspendedload-typicallythelargestloadandiscomposedclays,siltandfinesand.Thismaterialsettlesoutwhentheriverslowsusuallyinanoceanorlake.

3. bedload-thematerialthatistransportedalongtheriverbedbyrolling,slidingandsaltation(wheretheparticlesbounceorskipalongtheriverbed).

Thesedimentthatiscarriedbyariverisdeterminedbythelandscapethatitflowsthrough.Mostofthesedimentloadofriversissuspendedload.Asanexample,thesedimentloadoftheMississippiRiveriscomposedof26%dissolvedload,67%suspendedloadand7%bedload.Duringerosionanddepositionofsediment,thesizesoftheparticlescanprovideusefulinformationabouttheenvironmentofdeposition.Lowenergyenvironmentstendtoinvolvesmallersedimentaryparticleswhereashighenergyenvironmentscommonlydepositlargersediments(suchasgravelandboulders).

• Graveldepositsarefoundinhighenergyenvironmentssuchasmountainstreamsandsomebeaches.

• Sandsarefoundinintermediateenergyenvironmentssuchasbeachesandriverdeposits.• Claysarefoundinquietenvironmentssuchaslakesandthedeepocean.

Becauseofchangesinenvironmentalenergy(suchaswatervelocity),differentsizesofsedimentsmaydepositandoccurinassociationwithoneanother.Forexample,thesatelliteimageoftheMississippiRiverdelta(Fig.2)showsthetransportanddepositionofsedimentswheretheriveremptiesintotheGulfofMexico.Sincenearshoreenvironmentshavehigherenergy(duetocurrentsandwaves),onlycoarsersediments(suchassand)aredepositedalongtheshoreandinshallowwater.However,finesedimentsaresuspendedinthewatercolumnandaredepositedinquieteroffshoreenvironmentssuchasinthedeeperocean.

Figure2.SatelliteimageoftheMississippiRiverdelta.Source:NASA.

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Thus,differentsedimentaryenvironmentsoccuroverabroadareaatthesametimeandsedimentsmaybesortedbysize.Thetermfaciesisusedtodescribesetsofsedimentaryrocksthatgradeintooneanotherduetothissortofsortingofsedimentsbysize.

Inadepositionalenvironment(suchasariver),theenvironmentalenergymayexperiencelargechangesovertime.Gradedbeddingofsedimentsischaracterizedbyachangeingrainsizewithcoarsersedimentsatthebasethatgradeupwardintoprogressivelyfinerones.Gradedbeds(Fig.3)representdepositionalenvironmentsthatdecreaseinenergyastimepasses.Gradedbeddingiscommoninstreamdeposits(commonlyassociatedwithstormsurges)andotherdepositionalenvironmentssuchasturbidites(submarinelandslidedeposits).

RiversAstreamisageneraltermforanychannelizedbodyofrunningwater.Streamsmayrangeissizebutthereisnoestablishednomenclaturefornamingastreambaseduponitssize.Althoughthetermriverisgenerallyusedforlargerstreams,othertermsforsmallstreamsincludecreek,brook,orrill.

Mostwaterinstreamsistheresultofsurfacerunoffinthewater(hydrologic)cycle.Othersourcesofstreamwaterincludegroundwaterandthemeltingofglacialice.Themajorityofstreamsemptyintotheocean,alakeoranotherbodyofwater.Somestreamsmayterminatewheretheyemptyintoaclosedinteriorbasinonlandwithoutanoutlet(suchasattheGreatSaltLakeorDeadSea).Ariverbeginsatasource(commonlyinamountainousregion)andendsatitsmouth(Fig.4).Theriverisconfinedtoflowinachannelthatevolves(insizeandshape)alongthelengthoftheriver.Thepointwheretworiversmergeiscalledaconfluence.Atributaryisthetermforariverthatmergesintoalargerriver.

Figure3.Gradedsedimentarybeds.

Profileofariverfromitssourcetomouth.

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Thevelocityofwaterflowingthroughthechannelisafunctionofthesize(cross-section)ofthechannelandthesteepnessoftheterrain.Inmountainousregions,theterrainissteeperandhasahigherstreamgradient(ratioofthedropinelevationperdistanceperdistancetheriverflows).Forexample,astreamwithagradientof1m/kmmeansthattheelevationofthesurfaceoftheriverdecreasesonemeterforeverykilometerthattheriverflows.Inmountainousregions,atypicalstreamgradientmaybegreaterthanseveralmetersperkilometer.Sincewaterflowsundertheinfluenceofgravity,watervelocityinaregionwithasteepgradientisgreaterandcancarrylargersedimentparticles.Inregionsthatthelandscapeisrelativelyflat,thestreamwillhaveasmallgradientandlowerwatervelocity;itscapacitytotransportlargersedimentaryparticlesisreduced.ForexamplethegradientneartheendoftheMississippiriverisonly~7cm/km.AsindicatedinFig.5,waterinthemiddleofastreamchannelhasthehighestvelocity.Thelowestflowvelocityoccursalongthestreambankandalongthestreambedduetofriction.Whenthereisabendinthestream,thewaterwiththehighestvelocitymovesforwardinastraightlinewhereitencountersthebankon

theoutsideofthebend;thisresultsinazonealongthebankofthestreamthatispronetoerosion.Ontheoppositebank,thewatervelocityislowest.Thelowestvelocityislocatedontheinsidebankwheredepositionofsedimentoccurs.Pointbarsaredepositsthatformontheinsidecurveofmeanders,wherethewatervelocityislowest.Overtime,pointbarsaccumulateontheinsideanderosionoccursontheoutsideofriverbends,causingthechanneltomigrateandchangeshape.Theconcurrenterosionanddepositionthatoccursonoppositebankscausesthestreamchannelbecomesmorecurvedinshapecalledameander.Asthechanneloftheriverbecomesmoresinuousandcurved,theendsofthemeandermaymeetandcutoffaportionofthechannelforminganoxbowlake.Fig.6showsthedevelopmentofameanderandhowitmayevolvetoformanoxbowlake(bottomdiagram).Riversareshort-livedfeatures(ingeologictime)andcanchangeontimescalesof10’sto100’sofyears.Fig.7showsasatelliteimageofamatureriverwithmanyabandonedmeandersandoxbowlakesasithascontinuedtoevolveovertime.

Figure5.Schematicdiagramshowingwatervelocityinastreamchannelwhenitencountersabend.Thelongerthearrow,thehigherthewatervelocity.

Figure6.Schematicdiagramshowingtheevolutionofmeandersandtheformationofanoxbowlake.Source:WikipediaUser:maksim

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Atypicalstreamhasaprofilewhereitssourceislocatedinamoremountainousregionwithsteepgradients.Asastreamflowsawayfromitssource,itgenerallymovesintoaregionwithlowgradients(flatplains)andthereisthedevelopmentofextensivemeanders.Streamsneartheirsourceinaregionwithahighgradientarecommonlyreferredtoas“young”or“immature.”Streamsinregionswithlowgradientsandextensivemeandersarecalled"old"or"mature."Matureandimmaturestreamsevolvedifferently.Immaturestreamswithasteepgradientputmostoftheirerosionintodownwardcuttingmakingtherivervalleydeeperwithtimeandthechannelsthatdonotshiftmuchlaterally.Inmaturestreamsystems,thereislessdown-cuttingoftheriverchannelbecauseofthelowgradient.Thus,maturestreamsputmoreoftheirerosiveactionintosidetosideerosionleadingtothedevelopmentofmeanders.

DeltasandAlluvialFansAtitsmouth,ariverwilldropitssedimentduetoadecreaseinthevelocityofthewater.Alluvialfansanddeltasformfromthedepositionofthesedimentcarriedbytheriver.Theyaresimilarstructuresanddiffermainlybybeingdepositedeitheraboveorbelowwater.Deltasformbythedepositionofsedimentbelowwaterwhenthemouthofariverentersanoceanoralake.Deltaformationisacontinuousprocessaslongastheriverifflowingtoitsoutlet.Alluvialfansforminaridregionsinaterrestrialsettingfromthedepositionofthesedimentcarriedbytheriverastheflowentersacanyonorflatterplain.Theformationofalluvialfansiscommonlyandepisodicprocesswheremajordepositionofsedimentsmayoccurduringstorms.Generally,riversmergetogetherwheretributariesjoinwithariver.Itiscommonindeltasandalluvialfansforastreamtobranchoffintomultiplesmallerchannelsknownasdistributarychannelsthatflowawayfromthemainchannel.DistributarychannelscanbeseeninFigs.8and9.

Figure8.SatelliteimageoftheMississippiRiverdelta.Source:NASA

Figure9.AerialphotographoftheHanaupahalluvialfaninDeathValley.Source:USGS

Figure7.SatelliteimageshowingtheRioNegroRiver(SouthAmerica).Theimageshowsthecurrentriverchannelandmanyabandonedmeandersandoxbowlakesastheriverhasevolved.Source:NASA.