CARVING CANYONS:

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CARVING CANYONS A Look at Bedrock River
Incision
Will Ouimet
Colorado College Lunch Seminar Wed. March 25th,
2008
Yalong River Gorge, China
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Overview Processes and models of river incision
in bedrock channels
Focus on Bedrock Channels

• Frequent exposures of intact bedrock in channel
  bed and banks
• Lack of a thick, coherent alluvial coverthere
  may be a patchy or thin alluvial veneer that is
  mobilized during high flows
• Sediment transport capacity (Qc) exceeds sediment
  supply (Qs) Qc gtgt Qs
• Common in mountain ranges around the world,
  especially active ones

Jin Jiang River - eastern Tibet, China
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Bedrock Channel Morphology
Chain of potholes in Navajo Sandstone bedrock,
Utah
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Bedrock Channel Morphology
Stepped plane-bed bedrock channel in mudstone,
Waipaoa River, North Island New Zealand
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Bedrock Channel Morphology
Step-pool bedrock channels in the San Gabriel
mountains, CA
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Bedrock Channel Morphology
Gravel-bedded incised bedrock gorge, Taroko
National Park, Taiwan
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Bedrock Channel Morphology
Coarse boulder lag in incising bedrock gorge, Min
River, China
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Incising bedrock rivers in New England !!!
Bedrock Channel Morphology
Isolated reaches of bedrock channel incision
within old, decaying mountains.
Bulls Bridge, CT
Dianas Baths, NH
Coos Canyon, ME
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Bedrock Channels
Why do we care?

• Bedrock channel networks govern the evolution of
  erosional landscapes
• Topographic relief in mountainous regions is set
  by bedrock river channel profiles.
• Bedrock channel incision sets lower boundary
  condition for eroding hillslopes .
• Bedrock channels transmit changes in boundary
  conditions (tectonics, climate, base-level, etc.)
  throughout the landscape AND therefore govern
  system response time to such perturbations.

Generalized Gorge Cross-section
3D landscape perspective, Taiwan
River Profiles
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Bedrock Channels
Why do we care?

• Sustained bedrock river incision (initiated by
  uplift or base-level fall) forms the worlds
  river canyons

Incision and growth of canyons to balance uplift
Incision initiated by base-level fall
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Grand Canyon, Colorado River (of course!)
Bedrock Channels
Why do we care?

• Sustained bedrock river incision (initiated by
  uplift or base-level fall) forms the worlds
  river canyons

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Black Canyon of the Gunnison, CO
Bedrock Channels
Why do we care?

• Sustained bedrock river incision (initiated by
  uplift or base-level fall) forms the worlds
  river canyons

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Others US canyonsjust to show a few
Bedrock Channels
Why do we care?
Gate of Ladore, Green River - CO

• Sustained bedrock river incision (initiated by
  uplift or base-level fall) forms the worlds
  river canyons

Grand Canyon of the Yellowstone, WY
Crooked River Gorge, OR
Cache La Poudre Canyon, CO
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Eastern margin of the Tibetan Plateau, China
Bedrock Channels
Why do we care?
Cross-section

• Sustained bedrock river incision (initiated by
  uplift or base-level fall) forms the worlds
  river canyons

Dadu River, eastern Tibet
Yalong River, eastern Tibet
Yalong River, eastern Tibet
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River Incision into Bedrock

• Processes and mechanics
• Plucking/Quarrying
• Abrasion (suspended load bedload)
• Weathering
• Cavitation (?)
• Things to think about
• How do these processes interact ? Which are
  dominant?
• Under what conditions are the different processes
  dominant?
• What relation do these processes have to the
  average stress (i.e. force) applied to the
  channel bottom from flowing water?
• Lots of work still to be done
• We need more field studies of bedrock rivers

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Processes of bedrock incision

• Plucking/Quarrying Hydrodynamic Block Extraction
• Physical/chemical weathering
• Sediment wedging
• Bedload impact fracture/crack propagation

Whipple et al., 2000 (GSA Bulletin)
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Bedrock River Incision - Plucking -
Ukak River, Alaska
Whipple et al. (2000)
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Bedrock River Incision - Plucking -
Plucking in action during flood
Fall Creek, Ithaca, NY Finger Lakes region
Snyder et al. (2003)
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Bedrock River Incision - Plucking -
Bedded sandstone flysch
Dadu River Tributaries, China
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Bedrock River Incision - Plucking -
Same bouldertwo field seasons
Waipaoa River, New Zealand
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Processes of bedrock incision

• Abrasion by suspended load
• Sculpting
• Potholing
• Fluting
• Turbulent eddies shed off of roughness elements
  (e.g., boulder obstructions...) drive processes

Whipple et al., 2000 (GSA Bulletin)
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Bedrock River Incision - Abrasion -
Banks of the Colorado River (Grand Canyon)
Sculpting and fluting
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Bedrock River Incision - Abrasion -
Rhythmic fine flutes
Indus River Gorge, Pakistan
Sculpted boulders
Large coalescing potholes
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Bedrock River Incision - Abrasion -
Ukak River, Alaska
Whipple et al. (2000)
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Bedrock River Incision - Abrasion -
Fossil Falls, CA
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Processes of bedrock incision

• Bedload abrasion

Erosion per impact

• Erosion proportional to kinetic energy transfer
• M - mass of bedload
• U - velocity

Saltating bedload
Sklar and Dietrich (2004)
H Hop height Ws Settling velocity
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• Bedload abrasion

Henry Mountains Slot Canyon
Sand abrasion from suspended load
Impact marks from bedload abrasion.
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• Bedload abrasion

Impacts below waterfalls
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Processes of bedrock incision

• Weathering

Mudstone, New Zealand
Wetting and Drying in clay-rich shales
Weakens the eroding bed and banks
materialmaking it more susceptible to plucking
and abrasion
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Processes of bedrock incision

• Cavitation (??)

Occurs when a fluid's operational pressure drops
below it's vapor pressure causing gas pockets and
bubbles to form and collapse.
Indentations and pits on soft aluminum with
increasing exposure
Degree to which this occurs in nature not
known
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Process Dominance
Channels exhibiting plucking-dominance tend to be
cut into well-jointed or bedded rock
units. Channels exhibiting
abrasion- dominance with flutes, ripples, and
potholes tend to be carved into massive,
cohesive rocks (granites, schists, basalts,
gneiss, etc.).
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Both equally efficient at bedrock steps
Coalesced potholes (Ukak River, Alaska)
Joint controlled plucking (Rocky Gorge, NH)
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Generating a basic bedrock incision model

• Lots of elements to considerbut whats most
  important?
• Is there a general erosion rule that can capture
  dynamics that happen at small scales?
• Start basic, then build in complexity to captures
  all these elements

Hancock et al. (1998)
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Generating a basic bedrock incision model

• Basic postulate
• Incision rate (E) is proportional to shear
  stress (tb) on the bed (to a power a), the
  sediment load f(qs) (flux, grain-size
  distribution) carried by the flow, and the rock
  mass quality and erosion process (ke).
• From fluid mechanics, we know that shear stress
  (tb) is proportional to flow discharge (Q) per
  unit width (W) and channel gradient (S)
• Final piece Empirical relationships for
  discharge (Q) and channel width (W) as a function
  of drainage area (A)
• We get

K -- Coefficient of erosion
m, n -- Positive constants (erosion process,
basin hydrology and channel geometry, etc.)
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Bedrock River Incision Models
Simpliest form f(qs) 1

• Stream Power bedrock incision model
• Erosion rate highest where shear stress (i.e.,
  stream power) is highest.
• Also called detachment-limited because incision
  rate set by the ability of flows (and the
  sediment tools carried by the flow) to detach
  rock from the bed.
• Change in elevation through time (dz/dt) is
  function of uplift (U) and incision rate

Channels adjust to incise more rapidly through
changes in gradient
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Building off the basic stream power model

• The role of sediment - f(qs) - is more dynamic !!!

Sediment provides the tools for incision, but it
can also cover and armor the bed from incision.
Henry Mountains, UTAH

• Cover
• Tools

Sediment patches, grain-size distributions and
varying bedrock exposure

• No Cover
• No Tools

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Building off the basic stream power model

• The role of sediment - f(qs) - is more dynamic !!!

Sediment-flux-dependent bedrock erosion models
Sediment Starved Qs ? 0
Armored Bed Qs ? Qc
both force f(qs) ? 0
Sediment supply
Bed cover term
References -- Sklar (2004) Parker (2004)
Gasparini et al. (2006 2007)
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Building off the basic stream power model

• The role of sediment - f(qs) - is more dynamic !!!

Coarse sediment
Choke/inhibit inner channels (negative
feedback)
Focus erosion on sidewalls (positive feedback)
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Building off the basic stream power model

• Channel width is an important free parameter
• - Simple empirical relationships may not be
  realistic

• Width adjusts in concert with channel gradients..
• Finnegan et al. (2005)
• W A3/8S-3/16
• Steeper, narrower channels related to
• Increased incision
• Transient evolution
• Coarse boulders
• Lithologic contrasts

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Building off the basic stream power model

• Incision thresholds, tc (critical shear stress)
• Flood frequency/magnitudes (climate variability)

Stochastic-threshold incision model
Incision happens during events that exceed a
threshold

• KRKR(physical parameters)
• KCKC (climate parameters)
• KtcKtc (threshold parameter)

Tucker and Bras (2000)
Snyder et al. (2003)
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Building off the basic stream power model

• Moving beyond simple, generalized erosion rules
  (E tb) and trying to incorporate physical
  erosion processes.
• This has been done for saltation abrasion (Sklar
  and Dietrich, 2004) - can we do it for others?
  Issues will be interactions of processes,
  process dominance
• For simplicity, we tend to think of rivers
  flowing steadily and uniformly with constant bed
  roughness through time.
• What about waterfalls and other local
  accelerations?
• What about those coarse boulders that cover
  bedrock channels, creating highly tortuous and
  turbulent flows?
• What happens if bed roughness can vary as well?
• Basin hydrology (QAc) works well for moderate
  events.
• Point-source, large-magnitude discharge events
  (landslide dam bursts) not adequately captured.

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Building off the basic stream power model

• Most channels incising bedrock may behave more as
  mixed bedrock alluvial channels
• Landslides dams, debris flowsDISCUSSED
  yesterdaylike the tools effectANOTHER
  coefficient of erosion for normal stream power
  incision
• What about debris flow erosion and incision?

Bf speaks directly to how landslides and debris
flows might influence river incision efficiency
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Lets go collect more data !!!
We NEED more field studies of bedrock rivers We
need sites where we can monitor bedrock incision
and quantify incision rates? Henry mountains, San
Gabriel Mountains, etc We field data to
quantify different bedrock channel morphologies
and process dominance, and to link these data
with measured incision rates
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Questions?
Dadu River Gorge, China
Photo Credits Will Ouimet Kelin Whipple Eric
Kirby Joel Johnson Ben Crosby Noah Snyder