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Geomorphic Patterns, Processes, and Perspectives in Aquatic Assessment
and refinement of the dynamic equilibrium concept
(Hack, 1960). In an attempt to synthesize the seemingly divergent concepts of cyclic and dynamic theories, Schumm (1974) proposes different temporal
scales of equilibrium adjustment. This hierarchical
structure of stream dynamics recognizes the scale
dependency of variables in cause-effect relations
(Schumm and Lichty, 1965).
Dynamic equilibrium describes the mutual adjustment of hillslope and fluvial form in the
process-response adjustment of the stream to erosional processes, hydrodynamics, and sediment delivery and transport. Equilibrium and stability
(grade or steady state) are attractive concepts, because they explain variability as a result of channel self-adjustment to complex interaction and
process-response behavior of stream and watershed energetics at different scales (Mackin, 1948;
Schumm, 1974), although differences exist on how
to view equilibrium (Ahnert, 1994; Lane and
Richards, 1997). To explain the complexity of geomorphic change, Chorley (1962) considers a system as always trying to track or attain a steady-state
equilibrium condition due to changing conditions,
but that it seldom attains it and is best viewed as
disturbance or change at various spatial scales in a
hierarchy (Frissell et aI., 1986) in that "those parts
of the system whose time scale is shorter than the
disturbance react to produce a new steady state"
(Hack and Goodlett, 1960).
As with most theoretical perspectives, there are
limits to applying the equilibrium-steady-state concept, which has led to use of alternative approaches.
The variable nature of channel response to climatic,
hydrologic, and erosional processes, especially as
expressed in mountainous and arid climates of the
western United States, is often considered stochastic in nature and more amenable to probabilistic approaches in landscape analysis (Megahan, 1996;
Benda and Dunne, 1997). This is reflected in Graf's
(1988) concept of the "probabilistic river." Some
investigators use the concept of critical power
(stream power related to resisting power) in place
of equilibrium. Critical power can be used to explain threshold stream response to varying sediment conditions at various temporal and spatial
scales (Bull, 1979), as well as for stream response
to climate change (Bull, 1991). Thresholds represent an important quantitative and qualitative concept for watershed management and assessment
(Megahan, 1996) and for understanding geomorphic change (Bull, 1979) and the relative impact of
processes on the landscape. Thresholds are characterized by sudden interruptions of progressive landscape change where process-response mechanisms
are triggered that lead to significant landscape alteration (Schumm, 1974).
The concept of magnitude-frequency of processes
is essential for understanding process-landform relations. Climatic and geomorphic processes differ
in frequency of occurrence, magnitude (size), and
ecosystem function, such as for habitat formation
and maintenance. The relative magnitude of events,
their relative effectiveness or impact, and causeeffect relations depend on the temporal and spatial
scale at which a system is viewed and the general
biophysical setting. The effectiveness of processes
at altering the landscape and the relative rate of recovery following disturbance depend on the climatic variability, sensitivity of the geologic setting,
and history of past events (Wolman and Gerson,
1978; Kochel, 1988). In general, the greater the climatic and hydrologic variability is, the larger the
relative influence that large-magnitude events such
as floods have on amount of sediment transported,
channel morphology, and recovery time (Wolman
and Gerson, 1978). For example, 100-year floods
have a greater and longer lasting impact on channel conditions in arid regions than in humid temperate regions of the northeast United States (Graf,
1988). Magnitude-frequency relations of channelforming processes also change with valley setting
or basin size (Kochel, 1988; Grant and Swanson,
1995).
There also are limitations with the quantitative
process-based approach that dominates contemporary geomorphology and landscape ecology.
Knowledge of hillslope and fluvial processes is
necessary to understand system behavior, causeeffect relations, and the impacts of management actions and application of recovery prescriptions.
However, we need to consider processes and extrapolation of models in the spatial context of environmental controlling factors and driving variables, which is uncommon. Also, the emphasis on
prediction modeling and quantifying processes has
overshadowed the benefits of perspectives that emphasize interpretation and explanation (Baker,
1988). Climatic geomorphology (Derbyshire, 1976)
and geologic evolution (Tricart, 1974) are examples of underutilized perspectives that provide an
appropriate scale of inquiry for coarse-scale regional assessments.
Another concern is whether the emphasis on surface processes can adequately explain landscape
evolution (Douglas, 1982; Sugden et aI., 1997),
which is a similar concern with landscape ecology
in linking disturbance processes and evolution. A
more evolutionary-based perspective is partially
addressed by the habitat template concept, which
Geomorphic Patterns, Processes, and Perspectives in Aquatic Assessment
and refinement of the dynamic equilibrium concept
(Hack, 1960). In an attempt to synthesize the seemingly divergent concepts of cyclic and dynamic theories, Schumm (1974) proposes different temporal
scales of equilibrium adjustment. This hierarchical
structure of stream dynamics recognizes the scale
dependency of variables in cause-effect relations
(Schumm and Lichty, 1965).
Dynamic equilibrium describes the mutual adjustment of hillslope and fluvial form in the
process-response adjustment of the stream to erosional processes, hydrodynamics, and sediment delivery and transport. Equilibrium and stability
(grade or steady state) are attractive concepts, because they explain variability as a result of channel self-adjustment to complex interaction and
process-response behavior of stream and watershed energetics at different scales (Mackin, 1948;
Schumm, 1974), although differences exist on how
to view equilibrium (Ahnert, 1994; Lane and
Richards, 1997). To explain the complexity of geomorphic change, Chorley (1962) considers a system as always trying to track or attain a steady-state
equilibrium condition due to changing conditions,
but that it seldom attains it and is best viewed as
disturbance or change at various spatial scales in a
hierarchy (Frissell et aI., 1986) in that "those parts
of the system whose time scale is shorter than the
disturbance react to produce a new steady state"
(Hack and Goodlett, 1960).
As with most theoretical perspectives, there are
limits to applying the equilibrium-steady-state concept, which has led to use of alternative approaches.
The variable nature of channel response to climatic,
hydrologic, and erosional processes, especially as
expressed in mountainous and arid climates of the
western United States, is often considered stochastic in nature and more amenable to probabilistic approaches in landscape analysis (Megahan, 1996;
Benda and Dunne, 1997). This is reflected in Graf's
(1988) concept of the "probabilistic river." Some
investigators use the concept of critical power
(stream power related to resisting power) in place
of equilibrium. Critical power can be used to explain threshold stream response to varying sediment conditions at various temporal and spatial
scales (Bull, 1979), as well as for stream response
to climate change (Bull, 1991). Thresholds represent an important quantitative and qualitative concept for watershed management and assessment
(Megahan, 1996) and for understanding geomorphic change (Bull, 1979) and the relative impact of
processes on the landscape. Thresholds are characterized by sudden interruptions of progressive landscape change where process-response mechanisms
are triggered that lead to significant landscape alteration (Schumm, 1974).
The concept of magnitude-frequency of processes
is essential for understanding process-landform relations. Climatic and geomorphic processes differ
in frequency of occurrence, magnitude (size), and
ecosystem function, such as for habitat formation
and maintenance. The relative magnitude of events,
their relative effectiveness or impact, and causeeffect relations depend on the temporal and spatial
scale at which a system is viewed and the general
biophysical setting. The effectiveness of processes
at altering the landscape and the relative rate of recovery following disturbance depend on the climatic variability, sensitivity of the geologic setting,
and history of past events (Wolman and Gerson,
1978; Kochel, 1988). In general, the greater the climatic and hydrologic variability is, the larger the
relative influence that large-magnitude events such
as floods have on amount of sediment transported,
channel morphology, and recovery time (Wolman
and Gerson, 1978). For example, 100-year floods
have a greater and longer lasting impact on channel conditions in arid regions than in humid temperate regions of the northeast United States (Graf,
1988). Magnitude-frequency relations of channelforming processes also change with valley setting
or basin size (Kochel, 1988; Grant and Swanson,
1995).
There also are limitations with the quantitative
process-based approach that dominates contemporary geomorphology and landscape ecology.
Knowledge of hillslope and fluvial processes is
necessary to understand system behavior, causeeffect relations, and the impacts of management actions and application of recovery prescriptions.
However, we need to consider processes and extrapolation of models in the spatial context of environmental controlling factors and driving variables, which is uncommon. Also, the emphasis on
prediction modeling and quantifying processes has
overshadowed the benefits of perspectives that emphasize interpretation and explanation (Baker,
1988). Climatic geomorphology (Derbyshire, 1976)
and geologic evolution (Tricart, 1974) are examples of underutilized perspectives that provide an
appropriate scale of inquiry for coarse-scale regional assessments.
Another concern is whether the emphasis on surface processes can adequately explain landscape
evolution (Douglas, 1982; Sugden et aI., 1997),
which is a similar concern with landscape ecology
in linking disturbance processes and evolution. A
more evolutionary-based perspective is partially
addressed by the habitat template concept, which