35
the resistance of the river corridor to change and the resilience or characteristic form
time of the river corridor following change. Geomorphology and ecology explicitly
recognize the existence of thresholds and feedbacks, and the potential for a very
different, persistent configuration of the river corridor following change (river metamorphosis, alternate state). The vocabulary and the illustrative examples differ
between geomorphology and ecology, but the conceptual frameworks exhibit striking similarities that can be used to guide understanding and management of river
health.
2.5 The 4Cs of River Health
What I refer to as the 4Cs of river health—connectivity, complexity, change, and
context—is not an established phrase, but rather a useful way to remember some of
the key factors that promote physical and ecological integrity. Physical integrity for
rivers refers to a set of active river processes and landforms such that the river corridor adjusts to changes in water and sediment inputs within limits of change defined
by societal values (Graf 2001). Ecological integrity describes the ability of the river
corridor to support and maintain a community of organisms with species composition, diversity, and functional organization similar to those within natural habitats in
the same region (Parrish et al. 2003). The definition of physical integrity focuses on
a river corridor’s ability to adjust to changing inputs. The definition of ecological
integrity focuses on the river ecosystem’s ability to sustain a relatively natural biotic
community, which partly depends on the physical integrity of the river corridor.
River integrity combines these definitions to describe the ability of the river ecosystem to adjust to changing water and sediment inputs (without constraints imposed
by human manipulation such as dams or levees) and through these adjustments to
maintain the habitat, disturbance regime, and connectivity necessary to sustain
native biotic communities.
As noted briefly in the first chapter, determining natural conditions, whether
physical or biotic, can be very difficult in a region with a long history of human
occupation and resource use. Natural conditions are most commonly described in
terms of reference conditions and natural range of variability. Reference conditions
describe the characteristics of a natural system prior to intensive human alteration
of that system. Because natural systems continually adjust through time, the range
of adjustments is referred to as the natural (or sometimes the historical) range of
variability (Keane et al. 2009). For a river corridor, the natural range of flow variability, for example, describes the upper and lower magnitude limits of discharge, as
well as the range of duration, frequency, rates of change, and seasonal timing of
various components of the hydrograph in the absence of human alteration of the
watershed or manipulation of the flow (Poff et al. 1997) (Fig. 2.10).
Two particularly common forms of information on reference conditions and natural range of variability are historical data from the site of interest and contemporary data from reference sites that are chosen as appropriate analogs for the site of
2.5 The 4Cs of River Health
the resistance of the river corridor to change and the resilience or characteristic form
time of the river corridor following change. Geomorphology and ecology explicitly
recognize the existence of thresholds and feedbacks, and the potential for a very
different, persistent configuration of the river corridor following change (river metamorphosis, alternate state). The vocabulary and the illustrative examples differ
between geomorphology and ecology, but the conceptual frameworks exhibit striking similarities that can be used to guide understanding and management of river
health.
2.5 The 4Cs of River Health
What I refer to as the 4Cs of river health—connectivity, complexity, change, and
context—is not an established phrase, but rather a useful way to remember some of
the key factors that promote physical and ecological integrity. Physical integrity for
rivers refers to a set of active river processes and landforms such that the river corridor adjusts to changes in water and sediment inputs within limits of change defined
by societal values (Graf 2001). Ecological integrity describes the ability of the river
corridor to support and maintain a community of organisms with species composition, diversity, and functional organization similar to those within natural habitats in
the same region (Parrish et al. 2003). The definition of physical integrity focuses on
a river corridor’s ability to adjust to changing inputs. The definition of ecological
integrity focuses on the river ecosystem’s ability to sustain a relatively natural biotic
community, which partly depends on the physical integrity of the river corridor.
River integrity combines these definitions to describe the ability of the river ecosystem to adjust to changing water and sediment inputs (without constraints imposed
by human manipulation such as dams or levees) and through these adjustments to
maintain the habitat, disturbance regime, and connectivity necessary to sustain
native biotic communities.
As noted briefly in the first chapter, determining natural conditions, whether
physical or biotic, can be very difficult in a region with a long history of human
occupation and resource use. Natural conditions are most commonly described in
terms of reference conditions and natural range of variability. Reference conditions
describe the characteristics of a natural system prior to intensive human alteration
of that system. Because natural systems continually adjust through time, the range
of adjustments is referred to as the natural (or sometimes the historical) range of
variability (Keane et al. 2009). For a river corridor, the natural range of flow variability, for example, describes the upper and lower magnitude limits of discharge, as
well as the range of duration, frequency, rates of change, and seasonal timing of
various components of the hydrograph in the absence of human alteration of the
watershed or manipulation of the flow (Poff et al. 1997) (Fig. 2.10).
Two particularly common forms of information on reference conditions and natural range of variability are historical data from the site of interest and contemporary data from reference sites that are chosen as appropriate analogs for the site of
2.5 The 4Cs of River Health