Will Ouimet
Colorado
College Lunch
Seminar
Wed. March
25 th , 2008
C
C
Yalong River
Gorge, China

Focus on Bedrock Channels:
• Frequent exposures of intact bedrock in channel bed and banks…
• Lack of a thick, coherent alluvial cover…there may be a patchy or thin alluvial veneer that is mobilized during high flows…
• Sediment transport capacity (Q c
) exceeds sediment supply (Q s
)
Q c
>> Q s
• Common in mountain ranges around the world, especially active ones…
Jin Jiang River - eastern Tibet, China

Bedrock Channel
Morphology
Chain of potholes in Navajo
Sandstone bedrock, Utah

Bedrock Channel Morphology
Stepped plane-bed bedrock channel in mudstone, Waipaoa
River, North Island New Zealand

Bedrock Channel
Morphology
Step-pool bedrock channels in the San Gabriel mountains, CA

Bedrock Channel Morphology
Gravel-bedded incised bedrock gorge, Taroko
National Park,
Taiwan

Bedrock Channel
Morphology
Coarse boulder lag in incising bedrock gorge, Min River,
China

Bedrock Channel
Morphology
Bulls Bridge, CT
Diana’s Baths, NH
Incising bedrock rivers in
New England !!!
Isolated reaches of bedrock channel incision within old, decaying mountains….
Coos Canyon, ME

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

Bedrock Channels
Why do we care?
• Sustained bedrock river incision
(initiated by uplift or base-level fall) forms the world’s river canyons…
Incision and growth of canyons to balance uplift…
Incision initiated by base-level fall…

Bedrock Channels
Why do we care?
• Sustained bedrock river incision
(initiated by uplift or base-level fall) forms the world’s river canyons…
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 world’s river canyons…
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 world’s river canyons…
Others US canyons…just to show a few…
Gate of Ladore,
Green River - CO
Grand Canyon of the
Yellowstone, WY
Crooked River
Gorge, OR
Cache La Poudre
Canyon, CO

Bedrock Channels
Why do we care?
• Sustained bedrock river incision
(initiated by uplift or base-level fall) forms the world’s river canyons…
Eastern margin of the
Tibetan Plateau, China
Cross-section
Dadu River, eastern Tibet Yalong River, eastern Tibet Yalong River, eastern Tibet

• 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…
– We need field data to quantify different bedrock channel morphologies and process dominance.

Plucking/Quarrying:
Hydrodynamic Block Extraction
– Physical/chemical weathering
– Sediment wedging
– Bedload impact fracture/crack propagation
Whipple et al., 2000 (GSA Bulletin)

Ukak River,
Alaska
Whipple et al. (2000)
Bedrock River
Incision
- Plucking -

Fall Creek, Ithaca, NY
Finger Lakes region
Snyder et al. (2003)
Bedrock River
Incision
- Plucking -
Plucking in action during flood

Dadu River Tributaries,
China
Bedrock River
Incision
- Plucking -
Bedded sandstone flysch

Waipaoa River,
New Zealand
Bedrock River Incision
- Plucking -
Same boulder…two field seasons

• 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)

Bedrock River Incision
- Abrasion -
Banks of the Colorado
River (Grand Canyon)
Sculpting and fluting…

Bedrock River Incision
- Abrasion -
Indus River Gorge, Pakistan
Sculpted boulders
Large coalescing potholes
Rhythmic fine flutes

Bedrock River Incision
- Abrasion -
Ukak River,
Alaska
Whipple et al. (2000)

Bedrock River Incision
- Abrasion Fossil Falls, CA

• Bedload abrasion…
Erosion per impact
• Erosion proportional to kinetic energy transfer:
• M - mass of bedload
• U - velocity
E
1
~ MU
2
2
Saltating bedload…
Sklar and
Dietrich
(2004)
H – Hop height…
W s
– Settling velocity…

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

• Bedload abrasion…
Impacts below waterfalls…

• Weathering…
Mudstone, New Zealand
Wetting and Drying in clay-rich shales
** Weakens the eroding bed and banks material…making it more susceptible to plucking and abrasion…

• 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…

Channels exhibiting plucking-dominance tend to be cut into well-jointed or bedded rock units.
Channels exhibiting abrasiondominance with flutes, ripples, and potholes tend to be carved into massive, cohesive rocks
(granites, schists, basalts, gneiss, etc.).

Coalesced potholes (Ukak
River, Alaska)
Joint controlled plucking
(Rocky Gorge, NH)

Generating a basic bedrock incision model…
• Lots of elements to consider…but what’s 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)

Generating a basic bedrock incision model…
• Basic postulate: E
 k b t b a k
 b k e f
  s
Incision rate ( E ) is proportional to shear stress ( t b
) on the bed (to a power a
), the sediment load f (q s
) (flux, grain-size distribution) carried by the flow, and the rock mass quality and erosion process ( k e
).
• From fluid mechanics, we know that shear stress ( t b
) is proportional to flow discharge ( Q ) per unit width ( W ) and channel gradient ( S ): t b
~

Q W

2 3
S
2 3
• Final piece: Empirical relationships for discharge ( Q ) and channel width ( W ) as a function of drainage area ( A )
• We get: E

Kf
  s
A m
S n
K -Coefficient of erosion m, n -- Positive constants (erosion process, basin hydrology and channel geometry, etc.)

E

Kf
  s
A m
S n
Bedrock River Incision
Models
Simpliest form: f(q s
) = 1
E

KA m
S n
‘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 dz dt

U

KA m
S n incise more rapidly through changes in gradient…

Building off the basic ‘stream power’ model…
• The role of sediment - f(q s
) - is more dynamic !!!
Sediment provides the tools for incision, but it can also cover and armor the bed from incision.
Henry Mountains,
UTAH
•
• Cover
Sediment patches,
Tools distributions and varying bedrock exposure…
• No Cover
• No Tools

Building off the basic ‘stream power’ model…
• The role of sediment - f(q s
) - is more dynamic !!!
Sediment-flux-dependent bedrock erosion models
E

Kf
  s
A m
S n f
  s

Q s
W

 1

Q
Q c s


Sediment Starved: Q s
Armored Bed: Q s

 Q c
0
…both force f (q s
)  0
Sediment supply Bed cover term
References -- Sklar (2004); Parker (2004); Gasparini et al. (2006; 2007)

Building off the basic ‘stream power’ model…
• The role of sediment - f(q s
) - is more dynamic !!!
Coarse sediment
Choke/inhibit inner channels
( negative feedback )
Focus erosion on sidewalls
( positive feedback )

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 ~ A 3/8 S -3/16
• Steeper, narrower channels related to:
• Increased incision…
• Transient evolution…
• Coarse boulders…
• Lithologic contrasts…

Building off the basic ‘stream power’ model…
• Incision thresholds, t c
(c ritical shear stress)…
• Flood frequency/magnitudes (climate variability)…
Incision happens during events that exceed a threshold:
E
 k b
( t b
 t c
) a
Stochastic-threshold incision model
E

KA m
S n t c
Snyder et al. (2003)
E

K
R
K
C
K t c
A m
S n
• K
R
= K
R
(physical parameters)
• K
C
= K
C
• K t c
= K t c
(climate parameters)
(threshold parameter)
Tucker and Bras (2000)

Building off the basic ‘stream power’ model
• Moving beyond simple, generalized erosion rules
(E ~ t b
) 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 (Q~A c ) works well for moderate events.
– Point-source, large-magnitude discharge events
(landslide dam bursts) not adequately captured.

Building off the basic ‘stream power’ model
• Most channels incising bedrock may behave more as mixed bedrock alluvial channels…
• Landslides dams, debris flows…DISCUSSED yesterday…like the tools effect…ANOTHER coefficient of erosion for normal stream power incision:
E

K ( 1

B f
) f ( Q s
) A m
S n
Bf speaks directly to how landslides and debris flows might influence river incision efficiency…
• What about debris flow erosion and incision?

Let’s 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…

Photo Credits:
Will Ouimet
Kelin Whipple
Eric Kirby
Joel Johnson
Ben Crosby
Noah Snyder
Dadu River
Gorge, China