River Systems Runoff

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River Systems - Runoff
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Hydrologic Cycle

• Precipitation 4.2 trillion gallons per day
• 66 is lost as evapotranspiration
• 31 is runoff
• 3 infiltrates into the subsurface

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Running Water

• Integral part of sculpting the Earths surface
• MOST IMPORTANT AGENT OF EROSION
• Indirectly results in the formation of
  sedimentary rocks

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Stream Formation

• Sheetflow or Sheetwash overland flow of water
• Repeated precipitation events cause a
  preferential channel to form downcutting
• Tributaries form the main channel continues to
  grow up slope headward erosion

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Stream Terminology

• Stream System main stream plus tributaries
• Drainage Basin area drained by main stream and
  tributaries
• Drainage Divide area of higher elevation that
  divides drainage basins
• Function of size/scale

For Example Kickapoo Creek
?Sangamon River
?Illinois River
? Mississippi River
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Stream Subsystems

• Collecting System
• Tributaries is head water region
• Funnel water and sediment to main channel
• Primarily erosion and transport
• Transporting System
• Main tributary
• Main process is the movement of the sediment and
  water
• Erosion, transport, and deposition all occur
• Dispersing System
• Distributaries at mouth region
• Primarily deposition of the sediment
• Coarse sediment along the confluence
• Fine particles carried further in to body of
  water

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Permanent vs. Ephemeral Streams

• Permanent Stream
• Water flows year-round in the stream
• Bed of the stream is below the groundwater table
• Water is from both surface runoff and groundwater
  discharge

• Ephemeral Stream
• Water flows only after precipitation events or
  spring thaws (seasonal)
• Bed of stream is above the groundwater table
• Water is only from surface runoff

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Drainage Patterns

• Function of the underlying geology, the
  topography of the area, the history of the
  stream

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Streamflow Dynamics

• Examine the physical and hydraulic properties of
  the stream
• Discharge
• Velocity
• Gradient
• Channel Properties
• Wetted Perimeter
• Shape
• Size
• Roughness
• Sediment Load

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Discharge (Q)

• Volume of water passing a given point over a
  specified length of time (length3/time),
  generally given in ft3/s or m3/s
• Calculated by
• Where A is the cross-sectional area (length2) v
  is the velocity of the water (length/time)

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Velocity

• The speed of the water at a given point along a
  stream
• Directly related to a streams ability to erode
  and transport material
• High velocity water can carry heavier sediment
• Is a function of
• Gradient
• Channel Properties
• Wetted Perimeter
• Shape
• Size
• Roughness

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Stream Gradient

• Slope of steepness of the stream channel
• Vertical drop (relief) of a stream over a fixed
  distance
• Controls the potential energy of the water
• Steeper the gradient the higher the velocity,
  the lower the gradient the lower the velocity
• Meanders decrease the gradient by increasing the
  horizontal distance of the stream

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Wetted Perimeter

• The area in which water touches the channel walls
• Channel shape and size controls the wetted
  perimeter
• Most efficient streams have small wetted
  perimeters
• Roughness of the channel controls the frictional
  resistance to water movement
• A smooth channel decreases frictional force
• A rough channel increases frictional force

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Channel Shape and Velocity
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Stream Morphology

• Streams increase in length by headward erosion
  erosion occurring at the beginning of the stream
• Streams become wider through lateral erosion
  mass wasting of the stream banks
• Streams become deeper through downward erosion of
  the channel by abrasion of the sand and gravel

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Stream Piracy

• Continual headward erosion of one stream causes
  the intersection of two streams, thus diverting
  the headwater from one stream to another.

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Erosion

• Removal and transport of rock, sediment, soil
• Running water is the major cause of erosion
• Source of Materials in a stream
• Mass Wasting
• Lateral erosion of banks
• Downward erosion of stream channel
• Headward erosion
• Sheetflow
• Chemical Weathering

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Stream Dynamics Sediment Load

• Bed Load
• Suspended Load
• Dissolved Load

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Sediment Load Bed Load

• Particles to large to be lifted into suspension
• Sediment moves along stream bed
• Particles move by sliding, rolling, or saltating
  (short leaps)
• Accounts for 10 to 25 of sediment load
• Agent of downward erosion

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Sediment Load Suspended Load

• Light sediment lifted above stream bed by current
• Mostly silts and clays
• Accounts for most of stream load (? 60)

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Sediment Load Dissolved Load

• Chemical ions produced from chemical weathering
  of minerals
• Ca2, Cl-, Mg2, SO42-, Na, K, HCO3-, etc.
• Acquired from groundwater, sheetflow, or
  
  dissolving rock along a streams
  course.
• Generally ? 10, but can be up to 50 of
  sediment load.

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Effects of Transport on Particles

• Rounding
• caused by abrasion
• Increases with distance particle is transported
• Sorting
• Varies with the competence of the stream
• As competence decreases large particles are
  deposited

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Controls of Sediment Load

• Capacity
• The amount (weight) of sediment a stream can
  carry
• It is a function of discharge (velocity)
• Velocity ? (proportional to) Capacity 3 to 4
• Example, if velocity doubles (2) capacity
  increases between 8 (23) to 16 (24) times

• Competence
• The maximum particle size a stream can transport
• Function of velocity and fluid density
• Velocity ? (proportional to) Competence2
• Example, if velocity doubles (2) competence
  increases 4 (22) times
• As stream density increases with more suspended
  and dissolved sediment, competence increases

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Velocitys role in Erosion

• Threshold Velocity minimum velocity required to
  move grains of a certain size

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QUIZ

• What effect does urbanization have on streams?
• Increase in discharge
• Increases flooding intensity and frequency
• Increases erosive ability of the stream
• All of the Above
• None of the Above

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Urbanization Effects

• Decrease infiltration Increases runoff
• Produces Higher Peak Discharge
• Produces shorter lag time
• Produces more floods
• Higher discharge increased ability to erode

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Deposition of Material

• Occurs when streams lose velocity (competence
  decreases)
• Velocity decreases due to
• Lower gradients
• Flow into still or slow moving waters
  (reservoirs, lakes, etc.)
• Changes in channel shape (widening or narrowing)

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Stream Equilibrium

• Stream want to be at a low energy state
• When the gradient is high (high energy) erosion
  will occur
• When the gradient is low (low energy) deposition
  will occur
• Graded Stream stream gradient is in balance
  (equilibrium) with the volume of water available,
  thus neither erosion nor deposition occur along
  the stream profile

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Longitudinal Profile

• Cross-sectional image showing the variation in a
  streams elevation along its length

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Base Level

• The downward limit of stream erosion lowest
  elevation to which a stream can erode its channel
• Ultimate Base Level sea level
• Local Base Level lakes, resistant layers of
  rock, reservoirs, etc.

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Effects of Changes in Base Level

• Changes in base level change the energy of the
  system, which changes the velocity of the water.
• What would happen if base level was increased,
  say with a dam?

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Because of the reservoir velocity of the water
decreases. Thus competence and capacity decrease
and sediment load is deposited in the
reservoir. Water leaving the reservoir is
starved for sediment erosion occurs below the
dam.
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Processes of Stream Deposition
Processes of Stream Deposition

• Floodplains
• Natural Levees
• Meanders - Point Bars Cut Banks
• Backswamps
• Braided Streams

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Flood Plain

• Flat valley floor created by the side-to-side
  cutting of a stream
• Stream generally confined to channel, except when
  discharge exceeds the volume of the channel

What happens to the competence of the stream when
it floods?
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Natural Levee

• During floods the wetted-perimeter increases, and
  the water moving over the floodplain slows down
  due to frictional forces
• Competence decreases and sediment is deposited
  along the edge of the channel creating natural
  levees
• Coarse sediment is deposited first

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Meanders - Cut Banks Point Bars

• Meander curve in a stream designed to decrease
  the gradient and increase the volume of water a
  stream can carry
• Develop within the flood plain
• Cut Bank outside edge of a meander where
  erosion (cutting) occurs, area of high velocity
• Point Bar inside edge of meander where
  deposition occurs, area of low velocity

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Meander Development

• Meander loops migrate laterally and downstream
• As meanders migrate and streams continues to
  erode and deposit sediment, a meander can be
  cutoff creating an oxbow lake.
• What would happen to the streams gradient?

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Backswamp

• As a consequence of natural levees, the
  floodplain may be lower than the stream
• Areas are poorly drained because water does not
  flow uphill
• Creates marshes and swamps

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Braided Streams

• Stream pattern developed in response to a streams
  lose of competence.
• Generally found in areas draining mountainous
  areas, streams draining actively glaciated areas,
  and semi-arid regions where the volume of water
  is seasonal