Chapter 16 The Fluvial ProcessesWord格式文档下载.docx

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a.weardownhillsbyerosion

b.fillupvalleysbydeposition

2.oftencreatedeepcanyonsandsteepslopesaspreliminarystage

II.SomeFundamentalConcepts

A.ValleysandInterfluves

1.allcontinentalsurfacesconsistofthese2topographicelements

2.valley:

thatportionoftheterraininwhichadrainagesystemisclearlyestablished

a.includesvalleybottomandvalleysides

3.interfluve:

higherlandabovethevalleysidesthatseparateadjacentvalleys

a.ridgetops,mountaincrest,orbroad,flattishdividesbetweendrainagesystems

B.DrainageBasins

1.drainagebasin/watershed:

alltheareathatcontributesoverlandflowandgroundwatertoaparticularstream

2.drainagedivide:

lineofseparationbetweenrunoffthatdescendsinthedirectionofonedrainagebasinandrunoffthatflowstowardanadjacentbasin

3.thedrainagebasinofaprincipalriverencompassesthesmallerdrainagebasinsofallitstributaries

III.StreamSystems

A.StreamOrders

1.streamorder:

conceptthatdescribesthehierarchyofadrainagenet

a.smallstreamscometogethertoformsuccessivelylargerones

b.first-orderstream:

astreamwithouttributaries

c.second-orderstream:

formedfromconfluenceof2first-orderstreams

d.thirdorderstream:

formedfromconfluenceof2second-orderstreams

e.joiningofalowerorderstreamwithahigherorderstreamdoesnotincreasetheorderbelowtheconfluence:

i.e.theconfluenceofafirst-orderandsecond-orderstreamdoesnotproduceathird-orderstream

2.inawell-developeddrainagesystem,onecanexpectthefollowing:

a.first-orderstreamsandvalleysaremorenumerousthanallotherscombined

b.first-orderstreamsaretheshortest;

averagestreamlengthincreasesregularlywithincreasingorder

c.first-orderstreamshavethesmallestdrainageareas;

averagewatershedareaincreasesregularlywithincreasingorder

d.averagestreamgradientdecreaseswithincreasingorder

B.ErosionandDeposition

1.intro

a.allexternalforcesremovefragmentsofbedrock,regolith,andsoilfromtheiroriginalpositions（erosion）,transportthemtoanotherlocation,andsetthemdown（deposition）

b.landformsoferosiondifferfromlandformsofdeposition

2.ErosionbyOverlandFlow

a.splasherosion:

erosioncausedbythecollisionofraindropswiththeground

b.sheeterosion:

erosioncausedbywatermovingdownslopeasathinsheet,transportingmaterialloosedbysplasherosion

c.rillerosion:

erosioncausedbythebreakupofthesheetflowintomultitudinoustinyparallelchannels,calledrills

d.gullyerosion:

causedasrillscoalesceintofewerandlargerchannels

3.ErosionbyStreamflow

a.oncesurfaceflowischanneled,itsabilitytoerodeisgreatlyincreasedbytheincreasedvolumeofwater

b.mechanismsforstreamerosion:

1）hydraulicaction:

directpowerofmovingwater;

canexcavateandtransportmaterialatthebottomandsidesofthestream

2）abrasion:

chippingandgrindingeffectofrockfragmentsastheyareswirledorbouncedorrolleddownstreambymovingwater

3）chemicalaction

c.variablesintheerosiveeffectivenessofstreamflow:

1）speedoftheflow

a）steeperthegradient（slopeangleofthestreambed）,fastertheflow

b）greaterthewatervolume,fastertheflow

2）turbulenceoftheflow

a）fasterthestreamflow,greatertheturbulence

b）moreirregularthechannelsurface（channelroughness）,greatertheturbulence

3）resistanceofthebedrock

4.Transportation

a.streamload:

solidmaterialcarriedbyastream

1）streamloadissorted

2）finer,lightermaterialmovesmorerapidlythancoarser,heaviermaterial

b.3categoriesofstreamload:

1）dissolvedload:

material,mainlysalts,thataredissolvedinwaterandcarriedinsolution

2）suspendedload:

veryfineparticlesofclayandsiltthatarecarriedinsuspension

a）veryslowsettlingspeeds,eveninstillwater

b）fineclaymayrequire1yeartosink100feet

3）bedload:

sand,gravel,andlargerrockfragmentsmovinginastreambysaltationandtraction

a）saltation:

smallparticlesaremovedalonginaseriesofjumpsorbounces（appliestowaterandwind）

b）traction:

rollingorslidingalongthestreambed

4）generally,mostmaterialinastreamistransportedbysuspendedload,leastbybedload

c.competence:

measureoftheparticlesizeastreamcantransport

1）expressedbythediameterofthelargestparticlesitcanmove

2）dependsmainlyonflowspeed

3）sizeoflargestparticlecanmovevarieswiththesixthpowerofthewaterspeedevensmallstreamscantransportverylargeboulderswheninfloodstage

d.capacity:

measureoftheamountofsolidmaterialastreamhasthepotentialtotransport

1）expressedasvolumeofmaterialpassingagivenpointinthestreamchannelduringagiventimeinterval

2）canvarytremendouslyovertimedependingon:

a）flowvolume

b）flowspeed

c）loadcharacteristics

3）streamcapacityisgreatlyexpandedduringflooding

5.Deposition

a.erosionisinevitablyfollowedbydeposition

b.ultimatedestination:

ocean,lake,orinteriordrainagebasin

c.depositionoccursaseitherflowspeedorwatervolumedecreases

1）locationsofstreamdeposits:

a）mouthsofcanyons

b）onfloodplains

c）atriverbends

2）mostwaterbornedebrisisdepositedbymovingwater（streamorriver）intoquietwater（oceanorlake）

d.alluvium:

anystream-depositeddebris

1）alluviumissortedbysize–smallerparticlesarecarriedfartherbyrunningwaterthanarelargerparticles

2）alluvialmaterialistypicallysmoothedandrounded

C.TimeandtheRiver

1.uniformitarianismaccountsfortinystreamsinextensivevalleys:

a.extraordinarylengthofgeologictime

b.theepicworkofstreamsisprimarilyaccomplishedbyfloodflows

1）discharge:

volumeofflow

2）moststreamshaveveryerraticregimes,withgreatfluctuationsindischarge

2.typesofstreams:

a.perennialstreams:

apermanentstreamthatcarrieswateryear-round

1）occursinhumidregionsinlargerstreamsandtributaries

b.ephemeralstreams:

streamsthatcarrywateronlyduringandimmediatelyafteralargerain

c.intermittentstreams:

streamsthatflowonlypartoftheyear

1）eveninhumidregionsmanyfirst-andsecond-orderstreamsareonlyintermittent

2）oftenshortstreamswithrelativelysteepgradientsandsmallwatersheds

IV.StreamChannels

A.StructuralRelationships

1.geologic-topographicstructureisanimportantfactoraffectingstreamdevelopment

2.drainagepatternsdeveloplargelyinresponsetotheunderlyingstructureandslopeofthelandsurface

3.Focus:

DrainagePatterns

a.DendriticPattern

1）mostcommonpatternworldwide;

morenumerousthanallotherscombined

2）branching,treelike,randommergingofstreams

3）tributariesjoinlargerstreamsirregularlyatangles<

90o

4）negativerelationshipbetweendrainagepatternandunderlyingstructure

b.TrellisPattern

1）developsonalternatingbandsoftiltedhardandsoftstrata,withlong,parallelstreamslinkedbyshortright-angledsegments

2）ridge-and-valleysectionoftheAppalachianMountains

c.RadialPattern

1）foundwherestreamsdescendfromsomesortofconcentricuplift,suchasanisolatedvolcano

d.CentripetalPattern

1）oppositeofaradialpattern

2）streamsconvergeintoabasin

e.AnnularPattern

1）developseitheronadomeorinabasinwherealternatingconcentricbandsoftiltedhardandsoftrockareexposed

2）softermaterialcurvingcourses;

hardermaterialshort,right-angledsegments

B.ConsequentStreams

1.streamfollowstheinitialslopeoftheland

2.firsttodeveloponnewlyupliftedland

C.SubsequentStreams

1.streamsdevelopalongzonesofstructuralweakness

2.oftentrendatrightanglestootherdrainagechannels

D.AntecedentStreams

1.veryslowupliftoflandinterruptsanestablisheddrainagesystem;

thestreammaintainsitspreviouslyestablishedcoursebydownwarderosion

2.oftenresultsinadeepgorgecarvedthroughhillsormountains

E.SuperimposedStreams

1.originaldrainagepatternbecomeincisedintoanunderlyingsequenceofrocksofquitedifferentstructureafteroriginallandhasbeenerodedaway

2.drainagesystembearsnorelationtothepresentsurfacestructure

F.ChannelFlow

1.watermovesfastestinthecenterofthestream;

slowestlongthebottomandsidesasaresultoffriction

2.anarrow,shallowchannelwitharoughbottomhasagreaterretardingeffectonstreamflowthanawide,deepsmooth-bottomedone

G.Turbulence

1.factorsthatincreaseturbulence:

friction,channelsurfaceirregularities,speed

2.effectofturbulenceisvaried

H.ChannelPatterns

1.highwatertendstoscourstreamchannels;

lowflowstendtofillstreamchannels

2.categoriesofstreamchannelpatterns:

a.straight

1）shortanduncommon

2）indicatesstrongcontrolbyunderlyinggeologicstructure

3）thalweg:

amoreorlesssinuouslineconnectingthedeepestpartsofastreamchannel

b.sinuous

1）windingchannelswithgentleandirregularcurvature

2）foundinalmosteverytypeoftopographicsetting

c.me