27
with multiple stressors, such as competition from introduced species, degraded
water quality, and loss of habitat, all of which occur together in many contemporary
river corridors. Restoring and maintaining a natural disturbance regime and a river
corridor with natural levels of connectivity is the most effective way to enhance the
resistance and resilience of a river ecosystem to human-induced alterations.
2.4 Conceptual Models
The influences on and characteristics of river corridors and river ecosystems discussed in this chapter have been conceptualized in qualitative models of rivers. This
section discusses conceptual models of river process and form, which have primarily been developed by physical scientists, and conceptual models of river ecological
processes and communities that have mostly been proposed by ecologists. There is
substantial overlap between these two categories of conceptual models, as discussed
in section 2.4.3. Incorporating these conceptual models into river management can
facilitate explicit recognition of and emphasis on key factors such as longitudinal or
lateral connectivity.
2.4.1 Conceptual Models of Physical Process and Form
One group of geomorphic conceptual models of rivers focuses on how river corridors change through time, particularly in response to disturbances. These models
include feedbacks, lag times, thresholds, complex response, characteristic form
time, and river metamorphosis, each of which is explained in subsequent paragraphs. The assumption of equilibrium underlies most of these models. In the
absence of major external perturbations, the characteristics of the river corridor
(e.g., channel width/depth ratio, floodplain surface area, channel sinuosity) exhibit
relatively consistent mean values, although there are likely to be continual fluctuations about the mean. This is referred to as steady-state equilibrium (Fig. 2.6).
Equilibrium implies that a change to inputs or controlling parameters will result
in a proportional change in the river corridor. If discharge doubles, for example,
channel dimensions should increase in a manner that allows conveyance of this
larger volume of water downstream. The details of river response can still be difficult to predict precisely: how much of the increased discharge will be accommodated by a larger channel cross-sectional area versus changes in hydraulic
resistance?
Feedbacks among components in a river corridor can also complicate prediction
of river response to external changes. Positive or self-enhancing feedback occurs
when an initial change creates a cascade of subsequent changes that amplify the
initial change. Overgrazing reduces riparian vegetation, for example, leading to
decreased hydraulic resistance and root reinforcement of stream banks. This allows
2.4 Conceptual Models
with multiple stressors, such as competition from introduced species, degraded
water quality, and loss of habitat, all of which occur together in many contemporary
river corridors. Restoring and maintaining a natural disturbance regime and a river
corridor with natural levels of connectivity is the most effective way to enhance the
resistance and resilience of a river ecosystem to human-induced alterations.
2.4 Conceptual Models
The influences on and characteristics of river corridors and river ecosystems discussed in this chapter have been conceptualized in qualitative models of rivers. This
section discusses conceptual models of river process and form, which have primarily been developed by physical scientists, and conceptual models of river ecological
processes and communities that have mostly been proposed by ecologists. There is
substantial overlap between these two categories of conceptual models, as discussed
in section 2.4.3. Incorporating these conceptual models into river management can
facilitate explicit recognition of and emphasis on key factors such as longitudinal or
lateral connectivity.
2.4.1 Conceptual Models of Physical Process and Form
One group of geomorphic conceptual models of rivers focuses on how river corridors change through time, particularly in response to disturbances. These models
include feedbacks, lag times, thresholds, complex response, characteristic form
time, and river metamorphosis, each of which is explained in subsequent paragraphs. The assumption of equilibrium underlies most of these models. In the
absence of major external perturbations, the characteristics of the river corridor
(e.g., channel width/depth ratio, floodplain surface area, channel sinuosity) exhibit
relatively consistent mean values, although there are likely to be continual fluctuations about the mean. This is referred to as steady-state equilibrium (Fig. 2.6).
Equilibrium implies that a change to inputs or controlling parameters will result
in a proportional change in the river corridor. If discharge doubles, for example,
channel dimensions should increase in a manner that allows conveyance of this
larger volume of water downstream. The details of river response can still be difficult to predict precisely: how much of the increased discharge will be accommodated by a larger channel cross-sectional area versus changes in hydraulic
resistance?
Feedbacks among components in a river corridor can also complicate prediction
of river response to external changes. Positive or self-enhancing feedback occurs
when an initial change creates a cascade of subsequent changes that amplify the
initial change. Overgrazing reduces riparian vegetation, for example, leading to
decreased hydraulic resistance and root reinforcement of stream banks. This allows
2.4 Conceptual Models