The Channel Reach

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The Channel Reach

• Lecture Outline
• Discriminating variables
• Straight reaches
• Meandering reaches
• Braided reaches
• Channel metamorphosis
• Case studies
• Snake River metamorphosis
• Supraglacial stream meanders migration

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Discriminating Variables

• The reach in the fluvial system

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Discriminating Variables
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Discriminating Variables

• Instability of reach patterns

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Discriminating Variables

• Flow patterns

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Discriminating Variables
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Discriminating Variables

• Stream power vs. channel sinuosity

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Discriminating Variables

• Channel slope vs. bankfull discharge

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Discriminating Variables

• Unit stream power vs. median bed material size

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Straight Reaches

• Channel reach patterns straight reaches

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Straight Reaches

• Channel reach patterns straight reaches

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Straight vs. Meandering Reaches

• Channel reach patterns straight vs. meandering
  reaches

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Meandering Reaches

• Flow in meanders

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Meandering Reaches

• Flow in meanders

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Meandering Reaches

• Flow in meanders

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Meandering Reaches

• Flow in meanders

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Meandering Reaches

• Flow in meanders

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Meandering Reaches

• Meander geometry

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Meandering Reaches

• Meander geometry

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Meandering Reaches

• Meander geometry

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Meandering Reaches

• Landforms in a meandering river valley

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Meandering Reaches

• Landforms in a meandering river valley

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Meandering Reaches

• Landforms in a meandering river valley

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Meandering Reaches

• Landforms in a meandering river valley

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Meandering Reaches

• Range of meander forms

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Meandering Reaches

• Range of meander forms

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Meandering Reaches

• Fitting a mathematical curve to meanders

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Meandering Reaches

• Channel reach patterns oxbows in meandering
  reaches

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Meandering Reaches

• Oxbows and meander cutoffs

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Meandering Reaches

• Oxbows and meander cutoffs

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Meandering Reaches

• Oxbows and meander cutoffs

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Meandering Reaches

• Entrenched meanders

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Meandering Reaches

• Entrenched meanders

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Meandering Reaches

• Entrenched meanders

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Meandering Reaches

• Natural bridges in meandering reaches

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Meandering Reaches

• Natural bridges in meandering reaches

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Meandering Reaches

• Meander migration

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Meandering Reaches

• Meander migration

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Meandering Reaches

• Entropy and meandering
• Streams evolve toward condition of least rate of
  energy expenditure ? minimum slope
• Streams also evolve toward most probable form
• Random walk models can be used to demonstrate
  that meanders are the most probably pattern
  between two points in a valley
• Energy loss is more equally distributed in
  meanders than in straight channels (where almost
  all is lost in riffles)

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Braided Reaches

• Flow patterns

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Braided Reaches

• Landforms in braided river valleys

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Braided Reaches

• Landforms in braided river valleys

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Braided Reaches

• Landforms in braided river valleys

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Braided Reaches

• Landforms in braided river valleys

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Channel Metamorphosis
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Channel Metamorphosis
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Channel Metamorphosis
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Case Study Snake River Metamorphosis
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Case Study Snake River Metamorphosis
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Case Study Snake River Metamorphosis
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Case Study Snake River Metamorphosis
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Case Study Snake River Metamorphosis
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Case Study Snake River Metamorphosis
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Case Study Snake River Metamorphosis
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Case Study Snake River Metamorphosis
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Case Study 3-D Animated Model of Supraglacial
Stream Migration

• Richard A. Marston
• Oklahoma State University
• William B. Sitterle, Jr.
• Wyoming State Engineers Office

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Juneau Icefield

• 4000 km2 of sub-polar ice in Boundary Coast Range
  of AK-BC
• 30 outlet glaciers from high plateau of ice at
  1400 m
• Supraglacial stream research at confluence of
  Vaughan Lewis and Gilkey glaciers at 1100 m
  elevation
• Research supported by FGER, NSF-REU

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The rates and directions of meander migration
remain poorly understood

• Could study with
• historical aerial photos, maps, field surveys
• lab simulations with stream tables
• Landsat
  Mississippi
• River, AR-MS

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Use Supraglacial Streams as an Analogue

• Best formed in firn below the transient snowline
• Must downcut faster than glacier surface ablation
• Few crevasses

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Use Supraglacial Streams as an Analogue

• Time scale not important in meander development
• Sediment load not necessary to initiate meanders
  but clastics do alter meander dimensions
• Super-elevation of water surface against outer
  bank could provide the incremental frictional
  heat for differential thermal erosion

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Use Supraglacial Streams as an Analogue

• SG streams similar to incised meanders
• Meanders migrate simultaneous with downcutting
• SG streams similar to meanders cut in alluvium
  with high clay-silt
• Hydraulic geometry
• Unit stream power vs. sinuosity
• Discharge vs. meander wavelength
• Channel width vs. meander wavelength

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Daily Formation of Longitudinal Grooves
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Meander Migration Simultaneous with Formation of
Longitudinal Grooves
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1) Describe explain rate direction of
meander migration in supraglacial streams2) Use
VRML to construct a 3-D animated model of SG
stream meander development
Objectives of Study
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Field Methods

• 1) measure geometry for six meanders
• 2) track daily change in position of apex of
  each meander
• 3) measure peak Q and distribution of shear
  stress in each bend

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Width (W) ranged from 18-120cmSinuosity (P)
ranged from 1.07-1.67Channel curvature (r/W)
ranged from 1.8-9.2Peak discharge (Qp) ranged
from 20-240 l/s
Supraglacial Stream Dimensions
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Meander 2 P 1.07 r/W 9.2
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Meander 4 P 1.22 r/W 3.0
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Meander 6 P 1.32 r/W 6.3
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Results

• Extension gt translation in meanders with high P,
  low r/W, low Qp (e.g., meander 6)
• Translation gt extension in meanders with low P,
  high r/W, high Qp (e.g., meander 4)
• Extension and translation both increase as Qp
  increases
• Total rates of migration 8 to 77 cm/d

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Results