Channel Patterns: Classification of Form or Process

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Channel Patterns Classification of Form or
Process?
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William Morris Davis

• Cycle of erosion (Davisian cycle)
• Classified stream channels as
• Young
• Mature
• Old Age

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IMNH
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IMNH
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IMNH
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Leopold and Wolman, 1957

• Channels classified as
• Straight
• Meandering
• Braided
• Boundaries are arbitrary and indistinct straight
• For example the distinction between straight and
  meandering is based on sinuosity (a ratio of
  stream length to valley length), transition
  usually placed at 1.5

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Channel Pattern is a function of

• Discharge
• the volume of water that passes a given location
  within a given period of time. Usually expressed
  in cubic feet per second (cfs)
• Load
• The sediments and dissolved ions carried by the
  stream are collectively the stream's load.
  Divided into three parts
• Dissolved load
• Suspended load Bed load
• Bed load

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Dissolved (solution) Load

• Primarily from groundwater seepage into the
  stream.
• Also come from the weathering and dissolution of
  materials that line the channel.

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Suspended Load

• Sediment suspended and transported through the
  stream.
• Turbulent flow suspends clay and silt in the
  stream.
• Suspended load comes from material eroded from
  the surface bordering the channel and deposited
  in the stream, as well as, erosion of the channel
  itself.

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Bed Load

• Is moved across the bed of the channel.
• Bed load is transported in two ways,
• Traction, which is a scooting and rolling of
  particles along the bed.
• Saltation, which occurs when particles are
  suspended in the stream for a short distance
  after which they fall to the bed, dislodging
  particles from the bed. The dislodged particles
  move downstream a short distance where they fall
  to the bed, again dislodging particles upon
  impact.

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Stream Capacity

• The maximum load of sediment a stream can carry
  for a given discharge.
• Stream capacity increases with increasing flow
  velocity.
• Increased water velocity imparts a greater
  frictional drag on bed to erode it.
• Turbulent flow occurs under higher velocity thus
  increasing the water's ability to dislodge
  material from the bed or sides of the stream.

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Stream Competence

• The largest size material the stream can move
  under a given discharge.

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The Quasi-Equilibrium Model
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Lanes Channel Stability Model
FISRWG, 1998 Lane, 1955
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Straight Channels

• Uncommon in natural systems, where found tend to
  be short reaches with steep slope, or bedrock
  controlled
• In straight reaches bed material frequently has
  features associated with meandering channels

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Rosgen
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USEPA
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Fishback headwaters
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Meandering Channels

• Most common river form
• Meandering rivers shift their positions across
  the valley bottom by eroding on the outer banks
  of meander bends and simultaneously depositing
  point bars on the inside of meander bends
• Pool and riffle sequences

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Rosgen
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Braided

• Division of a single trunk channel into a network
  of channels
• Development of braid bars
• Differences between divided reaches and single
  channels include
• Braided zones usually steeper and shallower
• Total width usually greater
• Changes in channel position can occur quickly

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Origin of braided channels

• Origin of braids
• Erodible banks
• Sediment transport and abundant load
• Rapid and frequent variations in Q

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Rosgen
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Anastomosed Streams

• Primary characteristic is that individual
  channels (anabranches) are separated by islands
  of floodplain material
• Characterized by a interconnected network of
  low-gradient, relatively deep and narrow channels
  with variable sinuosities, and stable banks
• Tend to not migrate laterally
• Predominately transport suspended load

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Schumms Fluvial System
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Montgomery and Buffington , 1998
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Rosgen
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Most Probable Form