William Morris Davis

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William Morris Davis

• Landform Process Time Geology
• Most significant variable - Time

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Youth
Mature
From Plummer and McGeary, 1996
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Old Age
From Plummer and McGeary, 1996
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Form Chernicoff et al., 1997
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Schumm and Licthy (1963)
Figure from Ritter et al., 2002
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Number of Cigarettes Smoked
Number of Babies
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Systems Approach

• Emphasizes the intimate relationship between
  process and form
• It stresses the multivariate nature of
  geomorphology and the changes in energy that can
  occur through time
• Geomorphic systems are open systems.
• Reveals that some forms may not be in balance
  because they owe their character to relict
  conditions.

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Conceptual Models

• Process-Form change in process magnitudes or
  rates will cause a change in form
• Process-Response Model Change in process, the
  system will respond in order to develop a new
  form.
• Process action involved when a force induces a
  change, either chemical or physical, in the
  materials or forms at the Earths surface.

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Thresholds(after Schumm, 1973)

• Extrinsic limits of equilibrium exceeded due to
  an external factor (e.g., climatic change or
  tectonism).
• Intrinsic limits are exceeded due to an internal
  change in the system.
• Geomorphic Threshold originally special kind of
  intrinsic, but later suggested it could also
  include extrinsic change in landform morphology
  results in a period of disequilibrium.
• Suggests that normal landscape development may
  lead to instability and changes do not
  necessarily require a change in the external
  environmental controls

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Threshold RelationshipNorthwest Colorado
From Patton and Schumm, 1975
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Climate Change
Response variable
Response Variable
Reaction time
Relaxation time
Adjusted to new condition
Time
After Bull, 1991
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Axial Channel Aggradation - Degradation
Climate Change
Decrease Increase
Hillslope plant cover
Hillslope denudation rate
Time
After Bull, 1991
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Increase in Energy Required for Adjustment
(Modified from Knighton, 1998)
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C-Scale Arroyos
Inset fills/fans
Maximum Incision
B-Scale Arroyos
Inset fills/fans
Incision Lateral Cutting
A-Scale Arroyos
Point Bars
(From Balling and Wells, 1990)
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From Schumm, 1973
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Characteristics of Threshold Crossing Events

• Lasting, non-reversible changes in process and
  form
• If recurrence interval is longer than the
  response time, then a threshold has not been
  crossed if next disturbance occurs before the
  system has recovered, then a threshold has been
  crossed
• System must tend towards a new equilibrium
  condition adjusted to the characteristics of the
  altered controlling factors
• Thresholds are time-dependent phenomena the
  graded time interval is most conducive to the
  threshold crossing concept
• Thresholds are identified by parameters that
  characterized processes and landforms.

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Complex Response
Time 1
Time 0
Incision
Deposition
Depositional Zones
Time 1
Cross- Section
Cross- Section
Re-incision
Basin Outlet
Time 2
(Adapted from Schumm, 1973, 1977)
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From Yubanet.com
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From Gilbert, 1917
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After Graves and Eliab, 1977
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From Miller, 1997
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Six Mile
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Canyon Fan
14
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9
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Mineral
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Canyon
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Gaging Station
Gold Canyon
7B
7
6
Dayton
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SIERRA NEVADA
Nevada
California
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Light colored materials are Hg contaminated mine
tailings
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Temperature - Precipitation
Geology
Vegetation
Hydrology/Discharge Sediment Discharge Sediment
Size Load Type

• Profile Change
• Slope Adjustment
• Scour and Fill
• Terrace Formation

• Channel Form Adjustment
• Width
• Depth
• Width/Depth Ratio
• Roughness

• Systems
• Pattern Change
• Sinuosity
• Meander Wavelength

Modified from Ritter et al., 1995