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Water at the Surface
• What happens to raindrops falling to surface?– Some intercepted by vegetation and evaporated• Amount of interception depends on plant structure
– Some infiltrates permeable ground– Some blocked by impermeable surfaces
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Infiltration• Depends– Physcial characteristics of soil
• Coarse soils = greater inflitration rates
– Moisture content of soil• Dry soils absorb more moisture
– Vegetation cover• Dense cover prevents infiltration
– Nature of rainfall• Strong and long storms cause
run-off• Run-off flows to streams or
permeable soil or rock
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Water Flow in Streams
• Flow driven by gravity• Steeper river slopes (gradients)
equal faster flow– Gradient = change in elevation
of 2 points/change in stream elevation between those two points
• Depth of river also important– Steep and deep streams flow
fastest• Rocks & trees & bends in
channel resist flow• Stream moves fastest in deepest
part (thalweg) of channel and slowest on sides and along bottom
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Water Flow in Streams
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Water Flow in Streams
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River Discharge
• Water volume passing channel cross-section in certain amount of time
Q = V X ADischarge (Q), average velocity (V), cross-sectional area
of the flowing water (A) A = average depth of water times the width of the water
surface
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Hydrographs
• Stream discharge varies over time
• Hydrograph is graph of river discharge
• Important for dam construction and flood control
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Instruments for measuring stream flow
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Water Beneath Surface
• 25% of all freshwater is groundwater
• 2 zones in ground may hold water– Vadose (aeration) zone– Saturated (phreatic) zone
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Vadose Zone
• Water movement is by percolation– Water moves under influence of gravity– Percolation fastest in wet soil
• Water movement also by capillary action– Movement is from wetter to drier soil
• Water movement also through evaporation
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Groundwater in Saturated Zone
• Zones in ground exist saturated with water
• Upper part of zone is water table
• Aquifers are fully or partially saturated zones– Porous sandstones and
limestones are good aquifers– Shales and mudstones are
usually aquicludes
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Unconfined Aquifer and Perched Water Table
• Unconfined aquifers receive water from infiltration– Water moves from high to low
areas• Confined aquifers occur
between 2 aquicludes• Perched water tables occur
above main water table– Aquiclude is present in between
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Unconfined Aquifer and Perched Water Table
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Human Use of GroundwaterWell• Wells drilled into
water aquifer to extract water
• Water must be replaced or drawdown occurs– Cones of depression
form around wells– In coastal areas may
get saltwater intrusion
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Slopes and Streams
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Slope Erosion by Water
• Intense rain leads to runoff– Sheet flow and erosion
occurs– Rills and gullies form
moving water/sediment to streams
• Rainsplash erosion moves sediment downslope
• Vegetation impedes erosion
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Drainage Basins
• River system consists of trunk stream and tributaries
• River system occupies drainage basin– Mississippi Basin is an
example• Drainage Divides (high
areas) separate basins
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Drainage Basins & Sediment Yield
• Sediment Yield = sediment mass leaving drainage basin– Sediment yield/ area
basin = erosional effectiveness of river system
– Not all sediment removed. Alluvium deposited
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Drainage Basins & Sediment Yield
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Factors Influencing Sediment Yield
• Precipitation amounts• Relief• Vegetation • Lithology• Human Impac
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Stream Work - Floods
• Defined as peak discharge or when stream overflows channel
• Small floods important in humid regions
• Small floods more frequent than large ones
• Recurrence interval = average time between floods of given magnitude
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Stream Work – River Erosion
• Erosion by Hydraulic Action– Water breaking away and moving rock material
from valley floor and sides• Erosion by Abrasion– Erosion caused by moving sediment
• Corrosion– Minerals and rocks dissolved by water• Least important process
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Stream Work - Transportation• Bedload – particles moved on
stream bottom– Large particles slide and roll– Sand grains move by saltation
• Suspended load –clays and silts suspended in water– Much suspended load leads
to muddy waters• Dissolved load– dissolved rock
material
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Stream Work – River Deposition
Rivers deposit and erode sediment Flooding increases
discharge and depth, increasing stream power and sediment transport.
Receding flood decreases stream power and sediment is deposited
Increased sediment supply from basin leads to sediment deposition
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Stream Work - Base Level
• Base Level = level below which a stream cannot erode– Oceans are ultimate base
levels– Lakes, dams are local
base levels– Large rivers are local base
levels for tributaries
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Landscapes Shaped by Stream Erosion
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Introduction• Rivers erode landscapes
—processes occur at same time– Widening and
deepening of valleys– Slope modification– Mass movements– Exposure of buried
• Stream modified landscapes more extensive than other types
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Factors Affecting Patterns of Stream Erosion--Climate
• Debate continues about role of climate in landscape evolution
• Yet landscapes imprinted by past climate. – For example, glacial period• Rivers with great sediment load drained ice sheets
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Factors Affecting Patterns of Stream Erosion--Lithology
• Rocks have different hardness and resistance to weathering and erosion– Quartzites very
resistant• Mount Monadnock
– Shales/mudstone not resistant
– Limestone resistant in dry climate but not in wet one
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Factors Affecting Patterns of Stream Erosion--Geologic Structure
• Modified by streams to form landscape
– Hogbacks, Cuestas
– Ridges & Valleys—Appalachian Mountains
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Factors Affecting Patterns of Stream Erosion--Geologic Structure
• Modified by streams to form landscape
– Domes—Black Hills
– Faults
• Produce scarps
• Uplifted blocks modified and valley s sediment –filled
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Factors Affecting Patterns of Stream Erosion--Geologic Structure
• Modified by streams to form landscape– Faults
• Produce scarps • Uplifted blocks modified
and valleys filled with sediment
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Drainage Patterns• Influenced by structure and rock type• High drainage density related to high erosion rates
– Badlands• Radial Drainage
– Develop mostly on volcanic cones• Annular Drainage
– Develops on domes• Trellis Drainage
– Develops on parallel folded or dipping sedimentary rocks of differing hardness• Rectangular Drainage
– Forms where joints or faults dominate• Dendritic Drainage
– Forms on extensive batholiths or flat-lying sedimentary rocks
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Landforms of the Fluvial System
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Alluvial Fans• Ephemeral streams
emptying from mountain valleys construct alluvial fans
• Alluvial fans are fan-shaped– Formed of alluvial
material– Debris flows in upper
alluvial fans• More common in deserts– But can form in humid
areas
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Flow and Deposits on Alluvial Fans
• Flow emptying from valley spread– Velocity decreases,
sedimentation occurs– Unconfined water spreads across
fan in shallow channel
• Deposits are poorly stratified– Thick near mountain– Thin outward from mountains
• Alluvial fans coalesce to form bajadas
• Older parts of fan may develop desert varnish/pavement
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Landforms of River Valleys-Meandering Rivers
• Bends in sinuous channel termed meanders– Erosion on outside of
bend– Deposition on inside of
bend—Pt bar– Grow and migrate– Cut-off meanders form
oxbow lakes
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Landforms of River Valleys-Meandering Rivers
• Formation of oxbow lakes
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Landforms of River Valleys - Braided Rivers
• River with multiple channels separated by sand or gravel bars
• Braiding forms by deposition of bar in mid-channel– Water flows around bar
forming two channels• High sediment load, steep
slopes, variable discharge, loose bank materials may cause braiding
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Landforms of River Valleys - Floodplain
• Flat, low-lying ground bordering channel
• Frequently flooded and sediment deposited– Levees form on channel
margin
• Eroded by migrating meandering
• Growing pt. bar rebuilds floodplain
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Landforms of River Valleys - Floodplain
• Widening of floodplain and devleopment of large meanders in meander belts
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Landforms of River Valleys - Floodplain
• Flooding and levee development
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Human Use of Floodplains
• Farming• Development• People do not adjust
well as risk grows• Flood control measures– Artificial levees, dams– Provide mostly artificial
security– Must assume floods will
recur
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Terraces• Benches above modern
floodplain– Are older floodplains
• Develop through stream incision of floodplain– Lower floodplain develops– May have paired or mostly
unpaired terraces• May have aggradation before
terrace formation– Increased sediment supply
causes– Reduced sediment supply
then results in incision– Fill terraces produced
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Terraces
• Aggradation and terrace development
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Deltas
• Triangular shaped deposits where streams empty into oceans or lakes– Coarse sediments deposited at river mouth– Fine sediments carried farther– Distributary channels form transporting water and
sediments
• Delta form depends on:– Discharge and sediment transported– Configuration of seafloor– Strength of currents, waves, tides
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Delta Types