Earth Environments GEOL 104
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Date Created: 09/26/15
Geol 104 Deserts Deserts 0 Any region that receives lt25 cm 10 in of rain annually and can support little or no vegetation due to low soil moisture 0 Cover 725 of Earth s land surface outside of polar regions 0 Deserification is increasing amount of desert land annually Conditions Leading to Deserts Latitude and prevailing wind direction 0 Topography and prevailing wind direction 0 Orientation of coast and prevailing wind direction 0 Distance from coast and prevailing wind direction 0 Cold ocean currents and prevailing wind direction Wind 0 Wind is the flow of air from areas of higher pressure to lower pressure 0 The ultimate driving force for wind is solar energy 0 Wind is controlled by 7 Pressure gradient 7 Coriolis effect 7 Friction Geol 104 Deserts Pressure Gradient Effect 0 The greater the difference in air pressures the greater the wind speed 0 lsobars lines on weather map connecting areas of equal air pressure 0 Pressure gradient amount of pressure change over a given distance The closer the isobars the greater the pressure gradient and the stronger resulting winds Coriolis Effect 0 All freemoving uids are de ected on the globe due to the rotation of the Earth 0 Wind de ection due to the Coriolis effect 7 is always directed at right angles to the direction 0 air ow 7 affecw Wind direction but not Wind speed 7 is affected by the Wind speed ie stronger the Wind greater the de ection and 7 is strongest at the poles weakening to zero at the equator Friction Effect 0 Friction slows surface winds which weakens the Coriolis effect causing the winds to cross isobars Roughness of terrain dictates angle of air ow across isobars little effect over water but large over rough terrain Geol 104 Deserts Global Air Circulation 0 Combination of Coriolis effect and the differential heating of equator vs poles and land vs water 0 General pattern consists of fig 133 A Equatorial Lows Near the equator hot moist air rises resulting in abundant precipitation and tropical rain forests B Subtropical Highs As upperlevel ow from equatorial lows sinks back 0 associated heating produces hot arid regions subtropica deserm centered at about 30quotN and S latitudes Global Air Circulation cont C Subpolar Lows Form at convergence of Subtropical High and Polar i Warm moist midlatitude air rises and meets cold dry polar air in zone of atmospheric instability at about 60quotN and S latitudes Polar Highs U Extremely cold dry polar air subsides spreading toward the equator due to greater density of cold air Prevailing Winds A Easterly Trade Winds 7 Easterly owing winds between the Subtropical highs and Equatorial lows B Prevailing Westerlies 7 Westerly owing winds between the Subpolar lows and Subtropical highs C Polar Easterlies 7 Easterly owing winds between the polar highs and Subpolar lows Geol 104 Deserts Global Precipitation Patterns 0 Maj or precipitation patterns are in uenced by 1 Global Circulation Patterns latitude High precipitation at zones of convergence low at zones of divergence 2 Coastline vs Prevailing Wind Direction Wind moving o water yields high precipitation 3 Topography and Extent of Landmass Rainshadow effect low precipitation on leeward side of mountains g 13 4 Precipitation greatly decreases with distance from shore source of moisture Global Distribution of Deserts 0 Regions of low precipitation or high evaporation 0 See fig 133 0 Note relationship between location of deserts and global air circulation and precipitation patterns Wind Action in Deserts Winds are very strong in desert areas because 1 Low humidity 2 Great temperature range 3 Little or no vegetation Geol 104 Deserts Wind Erosion De ation removal of loose materials by wind often resulm in lowering of land surface 0 Desert Pavement pebbles and cobbles left behind as wind erodes smaller particles Wind Transportation 0 Dust Storms 0 Most sand transport is near the surface by saltation Ventifact cobble of boulder with flattened Polished faces due to windblown sand Wind Deposition Dunes 7 Mound or ridge of Well sorted rounded Wind bloWn sand 7 Types of Dunes Barchan Transverse Parabolic Longitudinal Loess 7 Homogeneous unlayered deposit ofWind blown silt and clay usually of glacial origin Geol 104 Deserts Desert Landforms 0 Desert Streams seasonal flow 7 Wash stream bed that is dry most ofthe time 0 Desert Lakes seasonal due to seasonal stream flow 7 Playa lake bed that is dry most ofthe year 0 Debris flows desert soils can become saturated and flow 7 Flash oods slow in ltration of rain leads to rapid increase in discharge and short lag time Desert Landscapes 0 Plateau 7 Large elevated area of fairly at land capped by resistant rock 0 Mesa 7 Broad attopped hill With steep cliff faces capped by resistant rock smaller than plateau 0 Butte 7 Narrow attopped hill With very steep cliffs capped by resistant rock smaller than mesa See fig 137 Deserti cation 0 Process whereby productive potential of arid or semiarid land falls by 10 or more 1 025 moderate 2550 severe and gt50 very severe 0 Estimates are 7 31 million mi2 area 12 times size ofTexas have become deserti ed in past 50 years 7 Current trend could threaten livelihood of 12 billion peopleEach year 23000 mi2 undergo ow to moderate and 81000 mi2 area the size of Kansas undergo severe deserti cation Geol 104 Glaciers Glaciers 0 Large longlasting mass of ice formed on land that moves 0 Driving force for glacier movement is gravity 0 Importance ofglaciers 7 Record of climate atmospheric change 7 Economic deposits of sand and gravel 7 Freshwater reservoirs and aquifers 7 Landscape formation Distribution of Glaciers At present there are two major glacial regions 1 Antarctica 7 785 of glacial ice is on Antarctica 7 Antarctica ice sheet is up to 5 km thick and is as large as the Us and Mexico combined Greenland 7 710 ofglacial ice is on Greenland 7 Greenland ice sheet is 715 km thick N Types of Glaciers A Confined to valleys 7 Valley glaciers Associated with Alpine mountain glaciation Flow from higher to lower elevation like water in a stream channel B Not confined to valleys 7 Ice cap covers area lt50000 km2 7 Ice sheet covers area gt50000 km2 Associated with continental glaciation Flow downward and outward from central highpoint largely uncon ned by underlying topography 7 See g 123 Geol 104 Glaciers Formation Glacial Ice 0 As Show is buried and compressed by the weight of overlying Show it changes from snow gt granular snow firn glacial ice 0 During the transformation the density increases and the amount of trapped air decreases 0 See fig 124 Formation of Glaciers Glaciers form in regions where the amount of snow that falls during winter exceeds amount that melts during summer Need combination of cold w moisture 7 Polar regions high latitudes 7 High elevations high altitude 7 Area of high Winter snowfall Glaciers form when Accumulation exceeds Ablation 7 Accumulation is addition ofsnoW by precipitation 7 Ablation wasting is loss ofsnoWice by Melting Calving Sublimation Geol 104 Glaciers Glacial Budget 0 Zone of Accumulation 7 Upper portion ofglacier where some snow remains from previous year 0 Zone of Ablation Wasting 7 Lower art of lacier where there is net loss of glacial snowice during the year 0 Snowline 7 Boundary between permanent snow and seasonal snow zones of accumulation and Terminus 7 Extremity or lower edge ofglacier See fig 1216 Glacial Budget and Movement 0 Advancing glaciers 7 Positive budget terminus advancing Receding glaciers 7 Negative budget terminus retreating Glacial Movement 0 Glaciers flow due to gravitational forces acting on em 0 Flow at the base and sides ofglaciers is greatly inhibit by friction 0 Rates of flow vary with 7 Slope steepness 7 Precipitation 7 Temperature 0 Typical rates are cm s to m s per day but glaciers can surge at speeds of hundreds of meters per day Geol 104 Glaciers Mechanisms of Movement 0 Basal slip 7 Entire glacier slides over bedrock Water at base can grea y accelerate rate 0 Plastic internal flow 7 acier moves by plastic deformation of ice grains in response to the stress of overlying mass 0 Brittle fracture 7 Upper part ofglaciers top 50 In represents a rigid zone W are glacial ice fractures due to tensional forces forming open ssures called crevasses 0 See figs 1281210 Glacial Erosion 0 At base of glacier 7 Plucking 7 Abrasion Glacial striations Rock our 7 See g 1214 0 At top and sides of glacier 7 Frost Wedging and erosion of slopes 7 See g 1221 Erosional Landforms 0 Alpine Glaciers 7 Ushaped valleys 7 ging Valleys 7 Cirque 7 Tarns 7 Horn 7 Arete 7 See g 1216 0 Continental Glaciers 7 Striations and grooves in rock 7 Rounded hills and mountains Geol 104 Glaciers Glacial Deposits Drift all rock and sediments transported and deposited by a glacier 7 Till dri deposited directly from glacial ice Typically poorly sorted 7 Striated drift Outwash dri tmnsported by glacier but deposited by streams Tend to be Well sorted layered deposiw T111 Landforms 7 Erratics Large boulders transported by glacier and deposited some distance from original outcrop 7 Moraines gs 1226 1229 End or terminal moraines deposited at terminus Lmeral or medial morm39nex deposited along sides Ground or recessional moraines deposited at base during meltingretreat 7 Drumlin g 1230 Elongate hill formed when ows over and reshapes mound of previously deposited drift Strati ed Drift Landforms 7 Outwash g 1232 sediment deposited by streams beyond terminus of g acrer Outwash plain Broad level surface composed of outwash 7 Kame small mound or ridge of sediment layers deposited by stream at glacier mar in 7 Esker g 1231 long ridge formed by stream that owed within on eneath g acier 7 Kettle depression in outwash cre chunk 0 ice left buried in Kettle lake kettle lled with water ated by melting large drift Geol 104 Glaciers Other Glacial Deposits 0 Lake Deposits 7 Varves paired light and dark layers deposited annually in a glacial lake g 1233 0 Loess deposim 7 Wind blown glacial rock our In US deposits range from 15 to 30 In thick and form some of the most fertile soils in Midwest g 1319 Effect of Glaciers on North America 0 Much of Canada scoured Formed Great Lakes amp Finger Lakes 0 Deposited till and attened much of Midwest especially Wood County Extensive alpine glaciers shaped mountains especially Rocky Mountains What Causes Glacial Ages 1 Astronomical factors 2 Atmospheric factors 3 Tectonic factors Geol 104 Glaciers Astronomical Factors 0 Milankovitch Theory 0 Idea that glacial ages are relate to decreases in amount of solar radiation received by the Earth due to cyclical orbital variations 7 Orbital Essentricity 100000 year cycle 7 Tilt of axis 41000 year cycle 7 Precession ofaxis 23000 year cycle Atmospheric Factors 0 Glacial ages are due to decreases in the amount of solar radiation that reaches the Earth s surface Decreases in solar radiation reaching the Earth s surface may be due to increases in 7 CO2 in atmosphere 7 Volcanic ash 7 Dust and ash from meteorite impact Tectonic Factors 0 Glaciation is related to latitude altitude and moisture budget Hence glacial ages may correspond to changes in 7 Position latitude of continents through time 7 Mountain building and erosion 7 Presence and magnitude of deep ocean currenw Geol 104 Floods Flooding I Flooding refers to over owing of a stream s channel due to an excessive discharge of water down the stream I Floods are a normal natural event that benefit the productivity of floodplains by periodic addition of nutrientrich sediments Floodplains 7 Floodplains are the oors ofstrearns during oods 7 Historically the combination of I Flat land I Fertile agriculmrally productive soils I Abundant water for drinking irrigation domestic use etc I Ready access to water transportation have led to heavy building on and develop of oodplains and river valleys 7 But oodplains are an important component of the dynamic and o en unpredictable stream system Terminology I Stage height ofa body ofWater above a locally de ned reference surface I Bankfull discharge discharge thatjust lls a stream channel any greater discharge would result in ooding I Natural levee boundary between channel and oodplain built by deposition of sediments due to rapid decrease in stream velocity when ow is spread out over large area of oodplain g 1028 Geol 104 Floods Causes of Floods Abnormally heavy precipitation 0 Rapid melting of snow cover 0 Failure of dams manmade and natural Coastal ooding munamis hurricanes over ow of dikes Types of Floods Riverine oods 7 Upstream floods due to locally intense rainfall in portion of drainage basin Waters rise and fall rapidly with little effect downstream 7 Flash oods exceptionally short duration between precipitation and ooding due to heavy rain 39 39 ge area 7 Downstream oods related to largerscale Weather events Water rises more slowly but large areas of drainage basin may be inundated Types of Floods 0 Coastal Floods 7 Stormsurge oods result when onshore Winds and lower barometric pressure in storms cause sea level to rise above local coastal lowlands 7 Tsunami wave surge oods occur in response to submarine earthquakes that produce large Wavelength seawaves Geol 104 Floods Hazards Associated with Flooding 7 Primary Death by drowning Structural damage to building bridges and dams Crop and livestock loss Erosion of ooded areas 7 Secondary Disruption ofmunicipal services Disease starvation and homelessness Loss of wildlife habitat hanges in ch Sediment deposition or erosion degrading agricultural lands annels hindering navigation Runoff Runoff is water that flows across the land The amount of runoff is related to the amount of precipitation and the infiltration rate Runoff Precipitation Infiltration Lag time is the duration between rainfall and stream discharge controlled by relative amounm of infiltration and runoff Flood Hydrograph Plot of stream discharge or height as a function of time After precipitation begins there is a lag time b an water owing over the ground surface reaches a stream channel Once surface runoff reaches a stream channel discharge usually rises quickly After a ood crest passes downstream the stream level drops off much slower than it o r se See fig 1035 Geol 104 Floods In ltration Infiltration rate is in uenced by 7 ysical characteristics of soil and rock Amount of soil moisture Particle sizes and shapes Fractures pores 7 Amount and type ofVegetation 7 steepness ofslope 7 Climatic factors Arid re ions often impermeable cap of hardpan or caliche39orms on surface Cold regions frozen ground acts as impermeable zone Flood Severity Dictated by amount and rate of runoff 0 Natural conditions that influence severity 7 Rainfall amount and rate 7 In ltration rate 7 Vegetation 7 Clirn ate S eason Flood Severity 0 Human Activities that influence severity 7 Urbanization Increases peak discharge and decreases lag time 7 Agricultural practices 7 Timbering deforestation 7 Flood control structures Often makes matters worse downstream Geol 104 Floods Flood Control Structures 7 Floodways Areas on oodplains where no new structures or homes are permitte 7 Floodwalls Reinforced concrete structures parallel to river banks 7 Flood impoundment dams Built to impound water allowing for more controlled release Tennessee Valley Authority TVA maintains 650 miles of the Tennessee River through a series of dams that slow oodwaters and produce hydroelectric power Flood Control Structures 7 Channelizing Modi cation of stream channel by straightening clearing deepening widening and lining with concrete or boulders 7 Artificial levees Manmade ridges of sedimentearthen materials or reinforced concrete caps over a core of ll Problems of Levees 0 Stop periodic deposition of nutrientrich sediments on ood plain 0 Create false sense of security 0 Upstream levees increase discharge downstream 0 Once overtopped water ponds behind levee greatly extending duration of ooding Geol 104 Floods Levee Failure 0 Three major mechanisms 7 Overtopping When river overtops levee erosion during the over ow may breach part of the levee 7 Slump Watersaturated earth materials along the sides ofthe levee may slump and oW decreasing the levee thickness and its strength 7 Sand boils uid pressure from stream forces Water through pores in material at levee bottom and up on the other side Prediction of Stream Flooding 0 Three main approaches are employed 7 Frequency of ooding 7 Flood hazards mapping 7 Monitoring progress of a ood Frequency of Flooding 7 Based upon statistical analysis of hydrological records at speci c stream gauging stations an annual maximum discharge is determined and recorded each year 0 Recurrence Interval 7 average interval between occurrences of two hydrological events of equal magnitude Box 102 table 1 7 Recurrence Interval R is calculated as R n 1 In Where n ofyears recorded m rank of oad by size Geol 104 Floods Frequency of Flooding 0 Annual Excedence Principle 7 reciprocal of recurrence interval gives probability that a discharge of a certain magnitude Will be exceeded in a given year 0 Flood Frequency Curves 7 determined from plot of maximum annual discharge values against recurrence interval calculated for a ood of that magnitude at a particular locality see Box 102 g 3 Flood Hazard Mapping 0 Use mapping to determine the impact and aerial extent of a particular ood 0 Indicates area that would be ooded by stream discharges of a given magnitude typically for ood discharges associated with recurrence intervals of l 0 25 50 and 100 years Monitoring Progress of Flood 0 Realtime monitoring of storms and upstream gauging stations provide means to forecast early warning of downstream ooding often giving several hours to days advance warning 0 In areas prone to ash oods forecasm of heavy precipitation in the area serve as warnings for ash ood potential Geol 104 Floods Clues to Past Floods 7 Physical Clues Stranded debris indicating high water level Bare sandy tracts indicating scouring above stream level Ripple marks in sand above present channel Lines of driftwood Trees bent downstream Terraces or cut surfaces Breaks in slope in sediment Erosional niches in bedrock Scour 0 es Clues to Past Floods 7 Geological clues Coursegrained stream deposim well above low water channel level out on ood plain 7 Ecological clues Different plant species develop higher above riverbed Case Studies 0 Flash Flooding 7 Big Thompson River CO 0 Downstream Flooding 7 Midwest ood of 1993 Geol 104 Floods Big Thompson River 0 Causes 7 Torrential rainfall from station ary thunderstorms dropped 19 cm 75 in of rain from 730 PM to 840 PM on July 311976 Rainfall over four hour period equaled average yearly total 7 Steep rocky sparsely vegetated slopes resulted in heavy rapid runoff Big Thompson River 0 Effects 7 Flash ood With initial Wall ofWater 20 high moved through the canyon at 15 m h 7 Flow increased from 137 cfs at 6 PM to 31200 cfs at 9 PM Flow velocity Was 4 times greater than estimate for 100year ood 7 Damages exceeded 35 million 139 people died and 5 missing 418 houses Were stroyed along With 52 businesses and more than 400 vehicles Midwest Floods of 1993 Abnormally large amount of snow cover Abnormally wet spring J Severe storms across the upper Midwest in early July ie 4 in Cedar Rapids ofJuly 34 6 in Bismark and 7 in amestown ND 7 Januaryrluly total preclpltatlon was nearly all tlm record or much of the reglon an well over 30 year averages Table l Tobln amp Moritz The Great Midwestern Floods af1993 Harcourt Brace 199 ow at gauging statlons on upper Mlsslsslppl mar basln were all at gt100 year recurrence levels Flg 6 Tobln ampMon 7 The 1993 r w st ood ls a clasle sample of downstream ood Flg 7ToblnampMo tz Geol 104 Floods Midwest Floods of 1993 mPrimary Effects e 20 mllllon acres offarrnland were adversely affected 7 gt75 towns were mun ed 748 drrectly attrrbuted deaths rProperty damage ofslzeslo bllllon Humcane Andrew 1992 had damages of30 bllllon 1994 Northndge earthquake had 10 bllllon ln damages rAglculLure suffered losses of1 bllllon ln Iowa and 600 mllllon ln SD NOTE the ruse 15 eumpared tu prevmus year but eaeh 1W Er uvemment uup subsrdres by ssuu mllllun su taxpayers actually saved mu mllllun due tu th grerpnee 756295 famlly dwellrngs were adversely affected rRed Cro s spent 30 mllllon ln relrefoperatrons nee uf eum Midwest Floods of 1993 msecondary Effects 7 Water Qu lrty e 388 wastewater treatment plaan were rnundated by ood waters relea rng raw sewage In Des Memes 800 mlles of water plpes were penetrated by mrcrobes Des Molnes wth a populatron ofz5oooo was wrthout dnnklng water for 19 days 7 Agneultural Rune e o al atrazrne load to Gulf of Mex co from t 1993 was 80 hl ov trme s n 9 ln many areas lncludlng Iowa Crty 7 Hazardous wastes were released from 54 rnundated Superfund srtes e Duratron offloodrng le some homeless forweeks 7 All barge traffre on e M sslppl was halted for t Louls at revenue losses of1 to 2 months above 2 mllllon per day Many bndges and roads were also damaged ln Mlssourl estrmates of road repalrwere 91 5 mllllon and m Iowa 500 mllllon Midwest Floods of 1993 0 Response to Flood Disaster 7 Three issues were identi ed by this ood 1 Levee effect 7 much ofthe fioodrngresulted from falled levee systems 2 Impact on sfream ow 3 Wetland drainage Geol 104 Floods Levee Effect Levee systems are devised to protect a commun39ty fro ertain recurrence interval typically 100 year ood security and intensi cation of construction occurs behind the levees on the oodplain Impact on Stream Flow Construction of levees alters the hydrology of the river t ows are constricted at particular points along the stream thus raising the ood levels in some places This e ect was significant on the local scale but overall the in uence of levees were minor in the 1993 Midwest ood peak stage at St Louis would have been raised 2 3 feet if all of the agricultural levees had held 39 no levees the tage would have been 2 5 feet lower but still 17 feet above ood stage Wetland Drainage Drainage of wetlands for agricultural and industrial use decreases the ood storage component of the oodplain Impacm of wetlands on a ood the size of the 1993 Midwest oods is not significant Geol 104 Streams Streams 0 Why study streams Running water is Q most important geologic agent in erosion transportation and deposition of sedimenm Water 7 The unique physical and chemical properties of Water make it the single most important chemical compound in de ning our existence on this planet 7 These properties include Water exists in all thr solid at Earth s surfa Water is a polar molecule that makes it a nearly universal solvent ee states vapor liquid and ce Unlike most Crnpounds water expands its volume up on freezing Hydrologic Cycle 0 Represenm the circulation of water in its three physical states vapor liquid and solid through the Earth s atmosphere hydrosphere and lithosphere See figure lOl Geol 104 Streams Driving Force for Hydrologic Cycle 0 The hydrologic cycle is driven by radiant energy from the Sun and Earth s gravity Radiant energy of the sun is converted into stored energy of water vapor through W 0 Stored energy of water vapor is converted to kinetic energy through condensation and precipitation Distribution of Water 0 Distribution of water in the hydrosphere N97 Oceans N2 Glaciers N06 Groundwater N002 Streams and lakes Terminology of Streams Stream body of surface water owing in a confined channel independent of size 0 Stream channel long narrow depression eroded by the stream into rock and sediment Stream banks are the sides of the channel 0 Stream bed is the bottom of the channel Geol 104 Streams Terminology of Streams 0 Flood plain is the stream valley inundated during oods Tributary stream that flows into a larger stream 0 Sheetwash thin layer of unchanneled water owing downslope Stream Pro le 0 Longitudinal profile fig 102A 7 Headwaters upper part of stream near its source 7 Mouth place Where stream enters ocean lake or larger stream 0 Crosssectional profile fig 102BampC 7 Near headwaters steep mountain Vshaped Valley cut into solid rock 7 Near mouth broad at oored Valley lled with sediment Channel Patterns 0 Straight channels 7 Rare most o en streams with steep gradients in resistant rock 0 Meandering channels 7 Frequently found in streams With gentle gradienw in easily eroded sediments Braided channels 7 Flow along many shallow interconnected channels Where more sediment is supplied to the stream than it can carry Geol 104 Streams Drainage 0 Drainage basin total area drained by a stream and its tributaries Drainage Pattern can reveal information about underlying rock type andor structure 0 See fig 104 Common Drainage Patterns Dendriu39c 7 Tree branchesrllke dralnage pattem 7 Common m unlformly eroded reglons Rectangular e Trlbutarles have frequent rlght angle bends Common ln regularly fractured rocks Tre is e Trlbutarles perpmdreular to parallel maln streams mmon ln folded strata ofvarlable resistance Radial e streams dlverge outward llke spokes ofa wheel 7 Common around composlte volcanoes and domes See g 10 5 Stream Erosion Transport and Deposition 0 Kinetic energy of water owing downslope performs work in the form of eroding regolith and transporting sediments Governed by stream velocity and particle size 7 Velocity the rate at which Water ows 0 See fig 107 Geol 104 Streams Stream Velocity 0 Three factors control stream velocity 7 Stream gradient Steepness of the stream vertical drop over a horizontal distance 7 Discharge Volume of water owing past given point in unit time Discharge 7 velocity x crosssectjonal area or stream 7 Shape and roughness of channel Friction between owing water and stream channel slows velocity Velocity in a Stream Channel 0 In straight sections 7 Velocity is greatest near middle of channel least friction with channel 0 In curved sections 7 Velocity is greatest on outside of curves Cutbank steep slope caused by erosion along outer bank of curve due to greater stream velocity Point bar series of arcuate ridges of sand or gravel deposited on inside of curves due to lower velocity 0 See fig 106 Stream Erosion Mechanisms Hydraulic action 7 Ability of owing water to li and move rock and sediment 0 Solution 7 Chemical dissolution mineralsrocks Abrasion 7 Wearing away of channel and grinding of sedimenw by friction and impact ofload Geol 104 Streams Stream Load 0 Load all material transported by a stream 0 Capacity quantity of sediment that stream carries in a given time 0 Competence largestsized particles a stream can carry at a given time Stream Transport 0 Transport of sediment load in a stream occurs by fig 1014 7 Bed load large or heavy particles that travel along the bed Traction moving by rolling sliding or dragging Salmtjon sandsized particles that move by series of short leaps or bounce along stream bed 7 Suspended load ner claysilt sized particles carried by turbulence of owing Water 7 Dissolved load dissolved ions canied in solution Stream Deposition Deposition of sediments depends on the type of load 0 Deposition of clastic materials 7 Decrease in stream velocity as it approaches mouth decreases ability to transport larger particles Results in Wellsorted deposits 0 Deposition of dissolved load 7 Dissolved ions are deposited by evaporation or precipitation by chemical reactions Geol 104 Streams Depositional Features 0 Three types of stream deposits 0 Channel deposits 7 Form in stream channel itself 0 Flood plain deposits 7 Accumulation ofsediments on a ood plain adjacent to the stream channel 0 Alluvial fans and delta 7 Form Where stream s gradient rapidly decreases like When it empties into a lake or at plain Channel Deposits 0 Bar elongated bar of sedimenm generally a transient feature 0 Point bar deposit on inside of curve in stream 0 Midchannel bar sandgravel deposit in middle of stream channel 7 Braided stream ow along many shallow interconnected channels instead of single en a1 c en sediment supply is greater than stream can carry g 1018 Meandering Streams Meanders pronounced sinuous curves in stream channel fig 1022 7 Form by erosion on outside and deposition on inside of stream curves 0 Oxbow lakes form when stream meanders become cutoff during ooding fig 1024 Geol 104 Streams Flood Plain Deposits 0 Flood plain broad strip of land built up by sedimentation on either side of a stream 0 anne 0 Natural levees low ridges of flood deposited sediment that form on either side of stream c nne 0 See fig 1028 Alluvial fan and Delta Deposits 0 Delta triangularshaped pile of sediment that orms where stream enters a calm body of water lake ocean fig 1030 7 Distributaries form when stream feeding a delta or fan spliw into many channels 0 Alluvial fan triangularshaped pile of sediments that forms where a stream enters a flat plain or valley floor fig 1032 Stream Valley Development 0 Downcut ting 7 Downward erosion of stream bed 0 Base level 7 Deepest level to which a stream can erode its bed 7 Ultimate base level for most streams is sea level 7 Temporary or local base levels occur Where streams empty into lakes or particularly resistant rock layers 7 See g 1044
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