Simple definitions

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Simple definitions

• Steady state No change in average condition over
  graded time (100s to 1000s of years) short-term
  changes occur about some average condition.
  Gilbert and Hack called this dynamic
  equilibrium.
• Dynamic equilibrium Progressive change over
  cyclic time (millions of years) net changes
  result in gradual change in condition

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Timescales associated with equilibrium concepts
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Equilibrium and evolution

• A contrast in landform development
• G.K. Gilbert
• Mutual adjustment between present forms and
  processes
• W.M. Davis
• Progressive changes in the landscape from
  youthful to mature to old age
• Functional versus historical

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Gilberts concept of dynamic equilibrium

• A state of balance or adjustment between the
  processes of erosion and the resistance of rock
• At the basin level, equilibrium is achieved when
  the ratio of erosive action, as dependent on
  declivities, became equal to the ratio of the
  resistances as dependent upon rock character
• Time-independence
• Gilbert recognized evolutionary change, time or
  stage for him had nothing to do with a streams
  ability to transport the eroded materials, or
  with the amount of relief or ruggedness of the
  terrain

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Hacks concept of dynamic equilibrium

• Every slope and every channel in an erosional
  system is adjusted to every otherdifferences and
  characteristics of form are therefore explainable
  in terms of spatial relations in which geologic
  patterns are the primary consideration rather
  than in terms of a particular theoretical
  evolutionary development such as Davis envisaged.

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Hack and time independence

• The forms and processes are in a steady state of
  balance and may be considered as time
  independent
• The theory of dynamic equilibrium explains
  topographic forms and the differences between
  them in a manner that may be said to be
  independent of time. The theory is concerned
  with the relations between rocks and processes as
  they exist in space. The forms can change only
  as the energy applied to the system changes.

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Montgomery (2001) and steady state topography

• Steady state topography
• Local erosion rate everywhere equals the local
  rock uplift rate
• Entails implicit or explicit specification of a
  relevant time frameand requires definition of a
  spatial scale
• Tectonic steady state
• Long-term rock uplift rate is constant
• Erosional steady state
• No net sinks or changes in storage of material
  eroded from the landscape
• When both erosional and tectonic definitions are
  met, the resulting topography has a steady state
  condition of no net change in the surface
  elevation due to a balance between rock uplift
  and erosion

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Chorleys take on equilibrium

• Chorley proposed a General Systems Theory to
  describe landform evolution
• Sought to generalize the concepts of open and
  closed systems from biology and thermodynamics to
  geomorphology
• An open system is maintained by a constant
  supply and removal of material and energy
  (readGilbert/Hack)
• In a closed system, the initial system
  conditions, particularly the energy conditions,
  are sufficient to determine its ultimate
  equilibrium condition. (read Davis)

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Drainages as open systems

• Because natural systems, like drainages, are open
  systems, adjustment to changes in energy and
  resistance tend toward a mean condition of unit
  forms, about which variations may take place over
  periods of time with fluctuations in the energy
  flow
• These constant adjustments to new steady state
  conditions may be superimposed on a general
  tendency for change possibly associated with the
  reduction of relief through time (read dynamic
  equilibrium)
• Open systems distribute their energy such that
  the most probable state (dynamic equilibrium) is
  achieved (Leopold and Langbein, 1962)

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Flaws of Hacks and Gilberts dynamic equilibrium

• The entire drainage is rarely entirely in
  equilibrium, i.e., all elements completely
  adjusted to each other, at the same time
• Climatic and tectonic activity, (not just
  lithology) affect processes and process rates

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Schumm and Lichty (1965) sort it out

• The definition of steady state or dynamic
  equilibrium is dependent on your choice of large
  or small areas and long or short time spans.

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References

• Abrahams, A. Distinguishing between the concepts
  of steady state and dynamic equilibrium in
  geomorphology, Earth Science Journal, vol. 2,
  no. 2, 1968.
• Chorley, R.J. Geomorphology and general systems
  theory, USGS Professional Paper 500B, 1962.
• Hack, J.T. Interpretation of erosional
  topography in humid temperate region, American
  Journal of Science, 258A, 1960.
• Leopold, L. and Langbein, W., The concept of
  entropy in landscape evolution, USGS
  Professional Paper 500A, 1962.
• Montgomery, D.R. Slope distributions, threshold
  hillslopes, and steady-state topography,
  American Journal of Science, vol. 301, 2001.
• Schumm, S. and Lichty, R., Time, space, and
  causality in geomorphology, American Journal of
  Science, 263, 1965.