Streams and Rivers..ppt

harrywilliams earthscience 1 streamsandrivers ofalltheagentsoferosion riversarethemostimportant becausetheyarethemostcommonandwidespread virtuallytheentirelandsurfaceconsistsofamosaicofdrainagebasins containingchannelnetworks harrywilliams earthscience 2 adrainagebasinisanareathatdrainsintoachannelnetwork delimitedbyadrainagedivide riverserodelandscapesbycarryingawaysedimentsuppliedbymasswastingorerodedfromslopesbywaterrunningoverthesurface thiscausessheetflow rillsandgullies chris14 4 harrywilliams earthscience 3 rillsformedfromconcentratedsheetflow left furthersurfaceflowconcentrationresultsingullies below cultivationofthefieldpromotedsurfacerunoff harrywilliams earthscience 4 sedimenttransportoncethewaterentersthechannelsystem itbecomesconcentratedanditsabilitytotransportsedimentincreases sedimentissuppliedfromsurroundinghillslopesand orerosionofthestream sownbanksandbed scourandfill harrywilliams earthscience 5 theamountofsedimenttransporteddependsmainlyonthevolumeofflow thisisofcourserelatedtothesizeofthedrainagebasinandwillfluctuateaccordingtoinputsofprecipitation thevolumeofstreamflow ordischarge ismeasuredincubicmeters orfeet persecond agraphshowingthefluctuationindischargethroughtimeisahydrograph harrywilliams earthscience 6 thelargestparticletransporteddependsonthevelocityofflow thegreaterthevelocitythemoreturbulenttheflowbecomesandthegreatertheliftforces generallyspeaking highervelocitiesarefoundatgreaterdischargesandonsteeperslopes sedimentisentrainedatitsentrainmentvelocityandthentransporteduntilthevelocityfallsbelowitssettlingvelocity whendepositionoccurs thesevaryaccordingtosize weight ofsediment flowwidth depth velocityallincreasewithdischarge whichincreasesdownstreamorfollowingrain harrywilliams earthscience 7 modesofsedimenttransportgenerally 3mainsedimenttransportmodesarerecognized 1 solutionload dissolvedrockcarriedintheflow 2 suspendedload finersediment usuallyclayandsilt suspendedbyturbulenceintheflow doesnotcontactbed 3 bedload coarsersediment sand gravel thatslides rollsorskipsalongthestreambed harrywilliams earthscience 8 example mississippisedimentloads1 solution 200milliontons year 2 suspension 500milliontons year 3 bedload 50milliontons year harrywilliams earthscience 9 iftheriverslowsdown forexampleasitsgradientdecreasesdownstream orbyenteringthesea thendepositionoccursforminganalluvialdeposit alluvium riversediment thephotohereisaspecialcaseofalluvialdeposition abraidedriver thisriverdrainsmeltwaterfromglaciersandischokedwithsedimentthatitcan ttransport multiplealluvialbarsresult harrywilliams earthscience 10 rivervalleysgenerallyspeaking itistruethatrivervalleysinupperreachestendtobenarrowandsteep whileinlowerreachesvalleysarewiderandhavemoregentleslopes thesteeperslopesinupperreachespromoteverticalandheadwarderosion ratherthanlateralerosion harrywilliams earthscience 11 inlowerreaches 2importantchangesusuallyoccur theriverbecomesanalluvialchannel beginstoflowthroughitsownalluvium andthechannelhasagreatertendencytoerodelaterally leadingtoawider flatbottomedvalley filledwithalayerofalluvium upperreaches lowerreaches harrywilliams earthscience 12 mostriversaremeandering forthemostpartasinglechannelthattwistsfromsidetosideacrossthevalleyfloor meanderingriversalsohavetheabilitytomigratefromsidetosideandsocauselateralerosionofthevalleywalls harrywilliams earthscience 13 sinceflowisfasteraroundtheoutsideofabend meanderstendtoshiftsidewaysbyerodingtheirouterbank higherflow entrainment formingacutbankandatthesametimedepositingsedimentontheirinnerbankformingpointbars lowerflow deposition harrywilliams earthscience 14 harrywilliams earthscience 15 harrywilliams earthscience 16 theresultingplainisthefloodplain becauseitiscoveredbyfloods floodplainsarebuiltfrompointbardepositsandflooddeposits bedrock harrywilliams earthscience 17 naturallevees duringfloods astheflowoverflowsthechannelandentersthefloodplain itslowsdownandmuchofthesedimentload especiallycoarser heaviersediment isdepositedonthechannelbanks thisformsridgesrunningparalleltothechannel harrywilliams earthscience 18 therearealsoartificiallevees builttoholdbackfloodwaters althoughtheycanfailtodoso aleveebreaksinthe1993midwestfloods monroecounty illinois harrywilliams earthscience 19 oxbowlakes sometimesmeanderstightenandformcutoffs theabandonedmeanderformsanoxbowlake theselakeseventuallyfillwithsediment vegetation becomingmeanderscars arecentcutoffandthebeginningofanoxbowlake harrywilliams earthscience 20 harrywilliams earthscience 21 casestudy dentoncreek dentoncreekdrainsportionsofmontague wiseanddentoncountiesinnorth centraltexas clasticsedimentsourcesinthedrainagebasinincluderills gulliesandchannelbankandbederosion sedimenterodedfromthedrainagebasinisdepositedintograpevinelakewhichwascreatedin1952bydammingdentoncreek harrywilliams earthscience 22 drainagebasinofdentoncreek boldblackline numbers1 7refertofieldtripsitesdescribedbelow bluedashedlineseparatessandstonebedrockunderlyingthenorthernportionofthedrainagebasinfromlimestonebedrocksunderlyingthesouthernportion yellowandorangecolorsindicatestreamfloodplaindeposits harrywilliams earthscience 23 site1 gulliesandrillsonasandstonehillside thecombinationofheavyrain forexampleduringspringthunderstorms andsandygroundwithlowpermeabilitycausessurfacerun offanderosioninthenorthernportionofthedrainagebasin theerosioncreateslargegulliesandsmallerrills harrywilliams earthscience 24 sites2 3 streambankandbederosioninanareaoflimestonebedrock limestonebedrocksinthedrainagebasinproducedarkclayeysoils thesteepbanksofsmallstreamsatsites2 3areevidenceofbankerosion harrywilliams earthscience 25 site4 largegulliesonsandstonebedrock somegullieshavegrownparticularlylargeinthenorthernportionofthedrainagebasin insomecasesmultiplegulliesformbranchesoflargesystemofgullies gullysystem singlebranch harrywilliams earthscience 26 site5 dentoncreekonsandstonebedrock thesmallstreams rillsandgulliesallfeedintodentoncreek themainstreamofthedrainagebasin thechannelhassteepsandybanksandaflatsandybed thesteepbanksareevidenceofongoingbankerosion duringhighstreamflows theflatgrassyareainthedistanceisthecreek sfloodplain constructedfromsandystreamsediment alluvium erodedfromupstream includingthegulliedareaatsite4 thisfloodplainisoneoftheareasshow