c arving c anyons: a look at bedrock river incision… colorado college lunch seminar wed. march 25...

CCARVING ARVING CCANYONS: ANYONS: A Look at Bedrock River Incision…A Look at Bedrock River Incision…

Colorado Colorado College Lunch College Lunch

SeminarSeminar

Wed. March Wed. March 2525thth, 2008, 2008

Will OuimetWill Ouimet

Yalong River Yalong River Gorge, ChinaGorge, China

OverviewOverview: Processes and models of river incision in

bedrock channels

• Frequent exposures of intact bedrock Frequent exposures of intact bedrock in channel bed and in channel bed and banks…banks…

• Lack of a thick, coherent alluvial Lack of a thick, coherent alluvial cover…there cover…there may be a patchy or thin may be a patchy or thin alluvial veneer that is mobilized during alluvial veneer that is mobilized during high flows…high flows…

• Sediment transport capacity (QSediment transport capacity (Qcc) )

exceeds sediment supply (Qexceeds sediment supply (Qss) )

QQcc >> Q >> Qss

• Common in mountain ranges around Common in mountain ranges around the world, especially active ones…the world, especially active ones…

Jin Jiang River - eastern Tibet, ChinaJin Jiang River - eastern Tibet, China

Focus on Bedrock Channels:Focus on Bedrock Channels:

Chain of potholes in Navajo Sandstone bedrock, Utah

Bedrock ChannelMorphology

Stepped plane-bed bedrock Stepped plane-bed bedrock channel in mudstone, Waipaoa channel in mudstone, Waipaoa

River, North Island New ZealandRiver, North Island New Zealand

Bedrock Channel MorphologyBedrock Channel Morphology

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

Bedrock ChannelMorphology

Gravel-bedded Gravel-bedded incised bedrock incised bedrock gorge, Taroko gorge, Taroko National Park, National Park,

TaiwanTaiwan

Bedrock Channel MorphologyBedrock Channel Morphology

Coarse boulder lag in incising Coarse boulder lag in incising bedrock gorge, Min River, bedrock gorge, Min River,

ChinaChina

Bedrock ChannelBedrock ChannelMorphologyMorphology

Diana’s Baths, NH

Incising bedrock rivers in Incising bedrock rivers in New England !!!New England !!!

Isolated reaches of bedrock Isolated reaches of bedrock channel incision within old, channel incision within old,

decaying mountains….decaying mountains….

Bulls Bridge, CT

Coos Canyon, ME

Bedrock ChannelBedrock ChannelMorphologyMorphology

Why do we care?Why do we care?

• Bedrock channel networks govern Bedrock channel networks govern the evolution of erosional the evolution of erosional landscapes…landscapes…

• Topographic relief in Topographic relief in mountainous regions is set by mountainous regions is set by bedrock river channel profiles.bedrock river channel profiles.

• Bedrock channel incision sets Bedrock channel incision sets lower boundary condition for lower boundary condition for eroding hillslopes .eroding hillslopes .

• Bedrock channels transmit Bedrock channels transmit changes in boundary changes in boundary conditionsconditions (tectonics, climate, (tectonics, climate, base-level, etc.)base-level, etc.) throughout throughout the landscape AND therefore the landscape AND therefore govern system response time govern system response time to such perturbations.to such perturbations.

Bedrock ChannelsBedrock Channels

3D landscape perspective, Taiwan

Generalized Gorge Cross-sectionGeneralized Gorge Cross-section

River ProfilesRiver Profiles

Why do we care?

Bedrock Channels

Incision and growth of canyons to balance uplift…

Incision initiated by base-level

fall…

• Sustained bedrock river incision (initiated by uplift or base-level fall) forms the world’s river canyons…

Why do we care?

Bedrock ChannelsGrand Canyon, Colorado

River (of course!)


Why do we care?

Black Canyon of the Gunnison, CO

Bedrock Channels


Why do we care?

Others US canyons…just to show a few…

Bedrock Channels

Crooked River Gorge, OR

Grand Canyon of the Yellowstone, WY

Cache La Poudre Canyon, CO

Gate of Ladore, Green River - CO


Why do we care?

Bedrock Channels Eastern margin of the Tibetan Plateau, China

Dadu River, eastern Tibet Yalong River, eastern Tibet Yalong River, eastern Tibet

Cross-section


• Processes and mechanicsProcesses and mechanics

• Plucking/QuarryingPlucking/Quarrying• Abrasion (suspended load & bedload)Abrasion (suspended load & bedload)• WeatheringWeathering• Cavitation (?)Cavitation (?)

• Things to think about:Things to think about:

– How do these processes interact ? Which are dominant?How do these processes interact ? Which are dominant?

– Under what conditions are the different processes dominant?Under what conditions are the different processes dominant?

– What relation do these processes have to the average stress (i.e. What relation do these processes have to the average stress (i.e. force) applied to the channel bottom from flowing water?force) applied to the channel bottom from flowing water?

• Lots of work still to be done…Lots of work still to be done…

– We need We need more field studies of bedrock rivers…more field studies of bedrock rivers…

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

River Incision into Bedrock

Processes of bedrock incision

• Plucking/Quarrying: Hydrodynamic Block Extraction

– Physical/chemical weathering– SedimentSediment wedging– Bedload impact fracture/crack propagation

Whipple et al., 2000 (GSA Bulletin)

Bedrock River Incision

- Plucking -

Ukak River, Alaska

Whipple et al. (2000)

Fall Creek, Ithaca, NYFinger Lakes region

Plucking in action during flood

Snyder et al. (2003)


- Plucking -

Dadu River Tributaries, China


- Plucking -

Bedded sandstone flysch

Same boulder…two field seasons

Waipaoa River, New Zealand


- Plucking -

• Abrasion by suspended loadAbrasion by suspended load– Sculpting Sculpting – PotholingPotholing– FlutingFluting

• Turbulent eddies shed off of roughness elements Turbulent eddies shed off of roughness elements (e.g., boulder obstructions...) drive processes(e.g., boulder obstructions...) drive processes

Processes of bedrock incisionProcesses of bedrock incision

Whipple et al., 2000 (GSA Bulletin)

Banks of the Colorado Banks of the Colorado River (Grand Canyon)River (Grand Canyon)


- Abrasion -

Sculpting and fluting…

Indus River Gorge, PakistanIndus River Gorge, Pakistan

Large coalescing potholes

Rhythmic fine flutes

Sculpted boulders


- Abrasion -

Ukak River, Alaska

Whipple et al. (2000)


- Abrasion -

Fossil Falls, CA


- Abrasion -

Processes of bedrock incisionProcesses of bedrock incision

• Bedload abrasion…Bedload abrasion… Erosion per impactErosion per impact

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

Saltating bedload…Saltating bedload…

2

2

1~ MUE

HH – Hop height…– Hop height…

WWss – Settling velocity…– Settling velocity…

Sklar and Sklar and Dietrich (2004)Dietrich (2004)

• Bedload abrasion…

Sand abrasion from suspended

load

Impact marks from bedload abrasion….

Henry Mountains Slot Henry Mountains Slot Canyon…Canyon…

Impacts below waterfalls…

• Bedload abrasion…

Processes of bedrock incision• Weathering…

Wetting and Drying in clay-rich shales

Mudstone, New Zealand

** Weakens the eroding bed and banks material…making it more susceptible to plucking and abrasion…

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

• Cavitation (??)

** Degree to which this occurs in nature not known…

Processes of bedrock incision

Channels exhibiting plucking-dominance tend to be cut into Channels exhibiting plucking-dominance tend to be cut into well-jointed or bedded rock well-jointed or bedded rock units.units.

Channels exhibiting abrasion-Channels exhibiting abrasion-dominance with flutes, ripples, dominance with flutes, ripples, and potholes tend to be carved and potholes tend to be carved into massive, cohesive rocksinto massive, cohesive rocks(granites, schists, basalts, (granites, schists, basalts, gneiss, etc.).gneiss, etc.).

Process Dominance

Coalesced Coalesced potholes (Ukak potholes (Ukak River, Alaska)River, Alaska)

Joint controlled plucking Joint controlled plucking (Rocky Gorge, NH)(Rocky Gorge, NH)

Both equally efficient at bedrock steps…Both equally efficient at bedrock steps…

Generating a basic bedrock incision model…Generating a basic bedrock incision model…

• Lots of elements to Lots of elements to consider…but what’s consider…but what’s most important?most important?

• Is there a general Is there a general erosion rule that can erosion rule that can capture dynamics that capture dynamics that happen at small happen at small scales? scales?

• Start basic, then build Start basic, then build in complexity to in complexity to captures all these captures all these elements…elements…

Hancock et al. (1998)

• Basic postulate:Basic postulate:

Incision rate (Incision rate (EE) is proportional to shear stress () is proportional to shear stress (bb) on the bed (to a power ) on the bed (to a power ), the sediment load ), the sediment load ff((qqss) (flux, grain-size distribution) carried by the ) (flux, grain-size distribution) carried by the

flow, and the rock mass quality and erosion process (flow, and the rock mass quality and erosion process (kkee).).

• From fluid mechanics, we know that shear stress (From fluid mechanics, we know that shear stress (bb) is proportional to flow discharge () is proportional to flow discharge (QQ) per unit width () per unit width (WW) and channel gradient () and channel gradient (SS):):

• Final piece: Empirical relationships for discharge (Final piece: Empirical relationships for discharge (QQ) and channel width () and channel width (WW) as a function of drainage area () as a function of drainage area (AA) )

• We get:We get:

bbkE seb qfkk

nms SAqKfE KK -- -- Coefficient of erosionCoefficient of erosion

m, n m, n -- Positive constants (erosion -- Positive constants (erosion process, basin hydrology and process, basin hydrology and channel geometry, etc.)channel geometry, etc.)

3232~ SWQb

Generating a basic bedrock incision model…Generating a basic bedrock incision model…

‘‘Stream Power’ bedrock incision model:Stream Power’ bedrock incision model:

• Erosion rate highest where shear stress (i.e., stream power) is highest.Erosion rate highest where shear stress (i.e., stream power) is highest.

• Also called detachment-limited because incision rate set by the ability Also called detachment-limited because incision rate set by the ability of flows (and the sediment tools carried by the flow) to “detach” rock of flows (and the sediment tools carried by the flow) to “detach” rock from the bed.from the bed.

• Change in elevation through time (dz/dt) is function of uplift (U) and Change in elevation through time (dz/dt) is function of uplift (U) and incision rate:incision rate:

nmSKAE

nmSKAUdt

dz

** Channels adjust to ** Channels adjust to incise more rapidly incise more rapidly through changes in through changes in gradient…gradient…

nms SAqKfE

Simpliest form: f(Simpliest form: f(qqss) = 1) = 1

Bedrock River Incision Bedrock River Incision ModelsModels

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

Henry Mountains, Henry Mountains, UTAHUTAH

• CoverCover• ToolsTools

• No CoverNo Cover• No ToolsNo Tools

Building off the basic ‘stream power’ model…Building off the basic ‘stream power’ model…

• The role of sediment - The role of sediment - ff((qqss) - is more dynamic !!!) - is more dynamic !!!

Sediment patches, Sediment patches, grain-size grain-size

distributions and distributions and varying bedrock varying bedrock

exposure…exposure…



Sediment-flux-dependent bedrock erosion modelsSediment-flux-dependent bedrock erosion models

nms SAqKfE

c

sss Q

Q

W

Qqf 1

Sediment supplySediment supply Bed cover termBed cover term

ReferencesReferences -- Sklar (2004); Parker (2004); Gasparini et al. (2006; 2007) -- Sklar (2004); Parker (2004); Gasparini et al. (2006; 2007)

Sediment Starved: QSediment Starved: Qss 0 0

Armored Bed: QArmored Bed: Qss Q Qcc

……both force both force ff((qqss) ) 0 0



Coarse sedimentCoarse sediment

Choke/inhibit inner channels Choke/inhibit inner channels ((negative feedbacknegative feedback))Choke/inhibit inner channels Choke/inhibit inner channels ((negative feedbacknegative feedback))

Focus erosion on sidewallsFocus erosion on sidewalls ((positive feedbackpositive feedback))Focus erosion on sidewallsFocus erosion on sidewalls ((positive feedbackpositive feedback))


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

• Width adjusts in concert with Width adjusts in concert with channel gradients..channel gradients..

• Finnegan et al. (2005)Finnegan et al. (2005) W ~ AW ~ A3/83/8SS-3/16-3/16

• Steeper, narrower channels Steeper, narrower channels related to:related to:

• Increased incision…Increased incision…• Transient evolution…Transient evolution…• Coarse boulders…Coarse boulders…• Lithologic contrasts…Lithologic contrasts…


• Incision thresholds, Incision thresholds, c c (critical shear stress)…(critical shear stress)…

• Flood frequency/magnitudes (climate variability)…Flood frequency/magnitudes (climate variability)…

Incision happens during events Incision happens during events that exceed a threshold:that exceed a threshold:

cc

Stochastic-threshold incision Stochastic-threshold incision modelmodel

• KKRR==KKRR(physical parameters)(physical parameters)

• KKCC==KKCC (climate parameters) (climate parameters)

• KKcc==KKcc (threshold parameter) (threshold parameter)

nmCR SAKKKE

c

Snyder et al. (2003)Snyder et al. (2003)

)( cbbkE

Tucker and Bras (2000)Tucker and Bras (2000)

nmSKAE

Building off the basic ‘stream power’ modelBuilding off the basic ‘stream power’ model• Moving beyond simple, generalized erosion rules (E ~ Moving beyond simple, generalized erosion rules (E ~

bb) and trying to incorporate physical erosion processes. ) and trying to incorporate physical erosion processes.

– This has been done for saltation abrasion (Sklar and This has been done for saltation abrasion (Sklar and Dietrich, 2004) - can we do it for others? Issues will be: Dietrich, 2004) - can we do it for others? Issues will be: interactions of processes, process dominance…interactions of processes, process dominance…

• For simplicity, we tend to think of rivers flowing steadily For simplicity, we tend to think of rivers flowing steadily and uniformly with constant bed roughness through time. and uniformly with constant bed roughness through time. – What about waterfalls and other local accelerations? What about waterfalls and other local accelerations? – What about those coarse boulders that cover bedrock What about those coarse boulders that cover bedrock

channels, creating highly tortuous and turbulent flows? channels, creating highly tortuous and turbulent flows? – What happens if bed roughness can vary as well?What happens if bed roughness can vary as well?

• Basin hydrology (Q~ABasin hydrology (Q~Acc) works well for moderate events.) works well for moderate events.– Point-source, large-magnitude discharge events (landslide Point-source, large-magnitude discharge events (landslide

dam bursts) not adequately captured.dam bursts) not adequately captured.

Building off the basic ‘stream power’ modelBuilding off the basic ‘stream power’ model

• Most channels incising bedrock may behave more as mixed bedrock Most channels incising bedrock may behave more as mixed bedrock alluvial channels…alluvial channels…

• Landslides dams, debris flows…DISCUSSED yesterday…like the tools Landslides dams, debris flows…DISCUSSED yesterday…like the tools effect…ANOTHER coefficient of erosion for normal stream power effect…ANOTHER coefficient of erosion for normal stream power incision:incision:

• What about debris flow erosion and incision?What about debris flow erosion and incision?

BfBf speaks directly to how landslides speaks directly to how landslides and debris flows might influence river and debris flows might influence river incision efficiency…incision efficiency…

nmsf SAQfBKE )()1(

We NEED more field studies of bedrock rivers…We NEED more field studies of bedrock rivers…

We need sites where we can monitor bedrock incision and We need sites where we can monitor bedrock incision and quantify incision rates? Henry mountains, San Gabriel quantify incision rates? Henry mountains, San Gabriel Mountains, etc…Mountains, etc…

We field data to quantify different bedrock channel morphologies We field data to quantify different bedrock channel morphologies and process dominance, and to link these data with measured and process dominance, and to link these data with measured incision rates… incision rates…

Let’s go collect more data !!!Let’s go collect more data !!!

Questions?Questions?

Dadu River Dadu River Gorge, ChinaGorge, China

Photo Credits:Photo Credits:Will OuimetWill Ouimet

Kelin WhippleKelin WhippleEric KirbyEric Kirby

Joel JohnsonJoel JohnsonBen CrosbyBen CrosbyNoah SnyderNoah Snyder

c arving c anyons: a look at bedrock river incision… colorado college lunch seminar wed. march 25...

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