Fluvial Landforms

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Fluvial Landforms
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Channel Morphology
What variables control channel morphology?
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Channel Morphology

• What variables control channel morphology?
• Discharge (Q)
• Sediment load
• Sediment size
• Valley slope

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Channel Morphology

• What variables control channel morphology?
• Discharge (Q)
• Sediment load
• Sediment size
• Valley slope

What factors can the channel adjust?
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Channel Morphology

• What variables control channel morphology?
• Discharge (Q)
• Sediment load
• Sediment size
• Valley slope

• What factors can the channel adjust?
• Width and depth
• Channel slope
• Sinuosity (one way to adjust slope in short
  term)
• Bed roughness

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Sinuosity
How do you change slope with sinuosity?
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Sinuosity
How do you change slope with sinuosity?
meandering
(straight channels often have a low valley
gradient and are in fine-grained sediments)
braided
straight
sinuosity
Unit stream power (w/m2)
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Width to depth ratios w/d
Low w/d ratio
high w/d ratio
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Width to depth ratios w/d
Low w/d ratio
high w/d ratio
How does bank material control channel form?
Cohesive banks have lower w/d ratio
Log w/d
Low silt/clay
High silt/clay
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Fluvial Landforms
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Alluvial Channel Patterns
straight
meandering
anastamosing
braided
Thalweg deepest part of flow
Alternate bars
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Step-pool morphology
Pool-riffle morphology
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Pool-riffle morphology
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Alluvial Channel Patterns
straight
meandering
anastomosing
braided
Uncommon for streams to be straight for any
significant distance Often characterized by
pool-riffle sequences Riffles rapid, steep flow,
coarser particles Pools low velocities, low
gradient In steep, gravel bedded streams, often
get step-pool structure.
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Alluvial Channel Patterns
straight
meandering
anastamosing
braided
Thalweg deepest part of flow
Point bar
Cutbank
Alternate bars
Former point bars
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Meandering Rivers
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Alluvial Channel Patterns
straight
meandering
anastomosing
braided

• Most common type
• Same physical components as you see in straight
  channels (thalweg, pool, riffles, bars)
• Low w/d ratio
• Dominated by suspended load
• Low to moderate channel slopes
• Vertically stable (no net aggradation/
  degradation
• Avulsion rapid shift to new channel

Uncommon for streams to be straight for any
significant distance Often characterized by
pool-riffle sequences Riffles rapid, steep flow,
coarser particles Pools low velocities, low
gradient In steep, gravel bedded streams, often
get step-pool structure.
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Braided Channel
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Alluvial Channel Patterns
straight
meandering
anastamosing
braided
Thalweg deepest part of flow
Point bar
Cutbank
Alternate bars
Former point bars
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Alluvial Channel Patterns
straight
meandering
anastomosing
braided

• Most common type
• Same physical components as you see in straight
  channels (thalweg, pool, riffles, bars)
• Low w/d ratio
• Dominated by suspended load
• Low to moderate channel slopes
• Vertically stable (no net aggradation/
  degradation
• Avulsion rapid shift to new channel

Uncommon for streams to be straight for any
significant distance Often characterized by
pool-riffle sequences Riffles rapid, steep flow,
coarser particles Pools low velocities, low
gradient In steep, gravel bedded streams, often
get step-pool structure.

• High w/d ratio
• dominated by bedload transport
• Non-cohesive bank materials
• Moderate to high stream power in relation to
  sediment size
• Moderate-high channel slopes
• High peak flood discharge
• High bedload supply
• aggradation and incision of bedload (locally
  vertically unstable)

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Alluvial Channel Patterns
straight
meandering
anastamosing
braided
Thalweg deepest part of flow
Point bar
Cutbank
Alternate bars
Former point bars
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Anastomosing river
The Columbia River in British Columbia, Canada
(photo by H.J.A. Berendsen).
http//www.geo.uu.nl/fg/palaeogeography/results/fl
uvialstyle
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Alluvial Channel Patterns
straight
meandering
anastomosing
braided

• Most common type
• Same physical components as you see in straight
  channels (thalweg, pool, riffles, bars)
• Low w/d ratio
• Dominated by suspended load
• Low to moderate channel slopes
• Vertically stable (no net aggradation/
  degradation
• Avulsion rapid shift to new channel

• One type of an anabranching channel pattern
  (table 6.6)
• vegetated islands
• Low gradient and low w/d ratios
• cohesive bank material
• high sediment supply, but low slope due to
  downstream base level control
• Stable islands

Uncommon for streams to be straight for any
significant distance Often characterized by
pool-riffle sequences Riffles rapid, steep flow,
coarser particles Pools low velocities, low
gradient In steep, gravel bedded streams, often
get step-pool structure.

• High w/d ratio
• dominated by bedload transport
• Non-cohesive bank materials
• Moderate to high stream power in relation to
  sediment size
• Moderate-high channel slopes
• High peak flood discharge
• High bedload supply
• aggradation and incision of bedload (locally
  vertically unstable)

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Channel pattern and sandbody geometry. Different
fluvial styles are associated with different
sandbody geometries (after Törnqvist)
http//www.geo.uu.nl/fg/palaeogeography/results/fl
uvialstyle
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http//www.geo.uu.nl/fg/palaeogeography/results/fl
uvialstyle
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http//www.geo.uu.nl/fg/palaeogeography/results/fl
uvialstyle
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How can a stream adjust vertically?
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How can a stream adjust vertically? Degradation
(incision) or aggradation (deposition)
What causes vertical changes in channel
elevation?
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How can a stream adjust vertically? Degradation
(incision) or aggradation (deposition)

• What causes vertical changes in channel
  elevation?
• Changes to discharge (QVA can change velocity
  by changing slope, can increase discharge by
  increasing water supply)
• Changes in sediment supply

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What can cause changes in discharge or sediment
supply?
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• What can cause changes in discharge or sediment
  supply?
• Climate changes
• Glaciation usually increases sediment supply
• Climate change can cause changes in
  precipitation/evaporation/snowmelt, etc changes
  in Q

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Denali National Park, Alaska
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• What can cause changes in discharge or sediment
  supply?
• Climate changes
• Glaciation usually increases sediment supply
• Climate change can cause changes in
  precipitation/evaporation/snowmelt, etc changes
  in Q
• Base level changes
• Sea level changes (related to climate)
• Sea level or other base level changes associated
  with tectonics

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Base level changes climate
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Base level changes tectonics
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• What can cause changes in discharge or sediment
  supply?
• Climate changes
• Glaciation usually increases sediment supply
• Climate change can cause changes in
  precipitation/evaporation/snowmelt, etc changes
  in Q
• Base level changes
• Sea level changes (related to climate)
• Sea level or other base level changes associated
  with tectonics
• Complex response combination of aggradation and
  degradation

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Complex response

• Example
• Decrease in base levelgt downcutting of main stem
  river
• Tributaries still unaffected, and remain in
  equilibrium
• With time, channel incision moves upstream,
  lowers base level of tributaries gt incision
• BUT, incision of tributaries provides sediment
  for the main stem gt aggradation of mainstem
• Different part of the system are out of phase
  with each other, so different processes are
  interacting at the same time.
• Can happen with decrease in sea level during
  glaciations mouth of Mississippi feels drop in
  base level before tributaries.
• Also can result from anthropogenic influences

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• What can cause changes in discharge or sediment
  supply?
• Climate changes
• Glaciation usually increases sediment supply
• Climate change can cause changes in
  precipitation/evaporation/snowmelt, etc changes
  in Q
• Base level changes
• Sea level changes (related to climate)
• Sea level or other base level changes associated
  with tectonics
• Complex response combination of aggradation and
  degradation
• Stream capture causes an increase in discharge

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Stream capture
Stage 1 - Beaverdam Creek, Gap Run, and Goose
Creek flow eastward through the Blue Ridge and
enter the Potomac. (from Judson and Kauffman)
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Stream capture
Stage 2 Capture by the Shenandoah!! Snicker's
Gap, Ashby Gap, and Manassas Gap are left as wind
gaps. As the land on either side of the ridge is
eroded down together with the ridge summit, the
relative elevation of the wind gaps becomes
higher and higher. (from Judson and Kauffman)
Stage 1 - Beaverdam Creek, Gap Run, and Goose
Creek flow eastward through the Blue Ridge and
enter the Potomac. (from Judson and Kauffman)
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• What can cause changes in discharge or sediment
  supply?
• Climate changes
• Glaciation usually increases sediment supply
• Climate change can cause changes in
  precipitation/evaporation/snowmelt, etc changes
  in Q
• Base level changes
• Sea level changes (related to climate)
• Sea level or other base level changes associated
  with tectonics
• Complex response combination of aggradation and
  degradation
• Stream capture causes an increase in discharge
• Landuse/human activities

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Landuse/human activities Hydraulic mining
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Yuba River, CA Bed of river rises over 30 feet
from additional sediment
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Ok Tedi Mine tailings Papua New Guinea
Natural Channel in area
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Instream gravel mining
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Dam Construction What are the downstream effects
of dams?
Oxbow dam on the Snake River, Idaho
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Effects of Hells Canyon dams on discharge and
sediment supply Snake River
(From USGS circular 1126)
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Elephant Butte dam on the Rio Grande
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What are the downstream effects of dams?
Dams change downstream sediment supply Dams
change the timing and magnitude of discharge Dams
can change stream temperatures and migration
routes for fish Dams can change local channel
elevations and access to floodplains The actual
effects vary widely from system to system
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The characteristics of a channel reflect the flow
conditions
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Crevasse splays
Crevasse splay of the Columbia River, Canada
(left, picture by H.J.A. Berendsen) and the
Mississippi River (right, picture by T.E.
Törnqvist).