C ARVING C ANYONS: A Look at Bedrock River Incision…

1. CARVING CANYONS: A Look at Bedrock River Incision… Will Ouimet Colorado College Lunch Seminar Wed. March 25th, 2008 Yalong River Gorge, China

2. 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 cover…there may be a patchy or thin alluvial veneer that is mobilized during high flows… • Sediment transport capacity (Qc) exceeds sediment supply (Qs) Qc >> Qs • Common in mountain ranges around the world, especially active ones… Jin Jiang River - eastern Tibet, China

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

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

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

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

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

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

9. 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

10. 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…

11. 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…

12. 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…

13. Others US canyons…just 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 world’s river canyons… Grand Canyon of the Yellowstone, WY Crooked River Gorge, OR Cache La Poudre Canyon, CO

14. 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 world’s river canyons… Dadu River, eastern Tibet Yalong River, eastern Tibet Yalong River, eastern Tibet

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

16. 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)

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

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

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

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

21. 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)

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

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

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

25. Bedrock River Incision - Abrasion - Fossil Falls, CA

26. Processes of bedrock incision Erosion per impact • Bedload abrasion… • Erosion proportional to kinetic energy transfer: • M - mass of bedload • U - velocity • Saltating bedload… Sklar and Dietrich (2004) H– Hop height… Ws– Settling velocity…

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

28. Bedload abrasion… Impacts below waterfalls…

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

30. 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…

31. 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.).

32. Both equally efficient at bedrock steps… Coalesced potholes (Ukak River, Alaska) Joint controlled plucking (Rocky Gorge, NH)

33. 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)

34. 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.)

35. 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…

36. 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

37. 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)

38. 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)

39. 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…

40. 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: tc • KR=KR(physical parameters) • KC=KC (climate parameters) • Ktc=Ktc (threshold parameter) Tucker and Bras (2000) Snyder et al. (2003)

41. 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 (Q~Ac) works well for moderate events. • Point-source, large-magnitude discharge events (landslide dam bursts) not adequately captured.

42. 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: • What about debris flow erosion and incision? Bf speaks directly to how landslides and debris flows might influence river incision efficiency…

43. 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…

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