Streams: Transport to the Ocean

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Streams Transport to the Ocean
Gary D. McMichael/Photo Researecher
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Rivers and streams

• Stream body of water flowing in a channel
• The floor of the channel is called the bed.
• When rainfall is very heavy or snow melts
  rapidly, bodies of water overflow their banks
  and water covers the adjacent land called the
  floodplain.

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Rivers and streams

• Carry away runoff to lakes and seas
• Erode land (degradation)
• Transport and deposit sedimentary debris

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Stream behavior

• Mostly determined by velocity and shape of
  channel.
• These factors combine to allow either laminar or
  turbulent flow.
• Turbulent flow is much more erosive.
• Stream velocities may vary from 0.25 to 7 m/s.

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Laminar flow

• Smooth sheet-like flow at a low velocity
• Usually confined to edges and top of stream

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Turbulent flow

• Irregular swirling flow
• Occurs at most rates of stream flow
• Keeps particles in suspension

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Laminar to turbulent transition
Laminar flow
Turbulent flow
Fig. 13.1c
ONERA
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Streams move material in three forms

• Dissolved load
• Suspended load
• Bed load (traction and saltation)

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Sediment Transport
Fig. 13.2
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Saltation
Fig. 13.3
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Grain Size and Flow Velocity
Fig. 13.1
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Stream Transport terms

• Competence measure of the largest particles a
  stream can transport proportional to v2
• Capacity maximum quantity of sediment carried
  by stream proportional to Q and v

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Erosion by Rivers

• Abrasion
• Hydraulic force

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Pebbles Caught in Eddies Form Potholes
Fig. 13.6
Carr Clifton/Minden Pictures
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Waterfall Retreating Upriver
Fig. 13.7
Donald Nausbaum
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Parts of a River System
Fig. 13.8
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Shape of River Valleys

• Most river valleys have a characteristic V
  shape
• Produced by mass wasting on the valley sides and
  river erosion in the bottom of the valley

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Two Important Stream Types

• 1. Meandering Streams
• Gentle gradients, fine-grained alluvium
• Minimizes resistance to flow and dissipates
  energy as uniformly as possible (equilibrium)
• Examples point bars,oxbow lake, migrating
  meanders

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Two Important Stream Types (2)

• Braided Streams
• Sediment supply greater than amount stream can
  support.
• At any one moment the active channels may account
  for only a small proportion of the area of the
  channel system, but essentially all is used over
  one season.
• Common in glacial, deserts, and mountain regions.

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Lateral migration by erosion at the outside
deposition on the inside of the river
Fig. 13.10a
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Meandering River Over Time
Fig. 13.10
Cutoff
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Meandering River
Point Bar
Fig. 13.11
Peter Kresan
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Braided River
Fig. 13.12
Tom Bean
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Incised Meanders, Utah
Fig. 13.9
Tom Bean
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Discharge

• Total amount of water that passes a
• given point in a stream per unit time
• Q w d v

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Discharge

• Discharge (m3/s) width (m) ? depth (m) ?
  average velocity (m/s)
• In the U.S., this is expressed as cubic feet per
  second (cfs)
• 1 m3/s 35.9 ft3/s

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River at Low Discharge
Fig. 13.14a
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River at High Discharge
Fig. 13.14b
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Flooding

• Water in the stream is greater than the volume of
  the channel.
• Interval between floods depends on the climate of
  the region and the size of the channel.

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Annual Flood Frequency Curve
Fig. 13.1
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Formation of Natural Levees
Fig. 13.1
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Natural Levee (during flood conditions)
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City Built on a Floodplain
Xie Jiahua/China Features/Sygma
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Longitudinal Stream Profile of the Platt and
South Platt Rivers
Fig. 13.16
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Graded stream

• Stream in which neither erosion nor deposition
  is occurring, due to an equilibrium of slope,
  velocity, and discharge.

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Base level

• Elevation at which a stream enters a large body
  of water such as a lake or ocean

Fig. 13.17
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Effects of Building a DamOriginal Profile Graded
to Regional Base Level
Fig. 13.18a
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Effects of Building a DamDam Forms New Local
Base Level
Fig. 13.18b
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Effects of Building a DamDeposition Upstream
and Erosion Downstream
Fig. 13.18c
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Geologic evidence of changes in stream equilibrium

• Alluvial fans
• Terraces erosional remnants of former
  floodplains

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Alluvial Fans
Fig. 13.19
Michael Collier
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Formation of River Terraces
Fig. 13.20
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Drainage divides separate adjacent drainage basins
Fig. 13.21
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Drainage basin

• Area of land surrounded by topographic divides
  in which all the water is directed to a single
  point .
• Drainage basin also called watershed

Drainage Basin of the Colorado River
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Typical Drainage Networks (Patterns)
Fig. 13.23
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Antecedant Stream
Deformation causes gorge to form
Stream was present before deformation
Fig. 13.24b
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A Superimposed Stream
Downcutting causes gorge to form
Deformation occurred before stream was present
Fig. 13.25
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Delaware Water Gap A Superimposed Stream
Fig. 13.24c
Michael P. Godomski/Photo Researchers
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Delta

• Location of significant sedimentation where a
  river reaches the sea.

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Mississippi Delta
Fig. 13.26
Landsat 2 image annotated by Moore, 1979
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Typical Large Marine Delta
Fig. 13.27
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Shifting Mississippi River Delta Over the Past
6000 Years
Fig. 13.28
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Lower Velocities Form Ripples
ripple
Fig. 13.5a
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Higher Velocities Form Dunes
ripples
dune
dune
Fig. 13.5b
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Recurrence interval

• Average time between the
• occurrences of a given event
• The recurrence interval of a flood of
• a given size at a given place
• depends on
• climate of the region
• width of the floodplain
• size of the channel

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