25
uplands. The disturbance regime is the spatial pattern and statistical distribution of
disturbances with respect to magnitude, frequency, and duration of associated
changes in the physical environment (Montgomery 1999). River corridors dominated by snowmelt runoff, for example, have a disturbance regime characterized by
a single, relatively long duration seasonal flood. River corridors dominated by rainfall runoff, even if the rainfall is strongly seasonal, are likely to have more rapidly
fluctuating peak flows.
Disturbance regime in river corridors reflects sources of water, sediment, nutrients, and other materials that originate outside of the river corridor, as well as disturbances within the river corridor. The characteristics of a disturbance and the
response of the river corridor to the disturbance also vary within the river network.
Headwater channels, for example, commonly lack extensive floodplains and are
closely coupled to adjacent uplands. Increased runoff that causes flooding may thus
be more likely to change channel form in a headwater channel segment than in a
downstream channel segment. All of the excess water during a flood remains within
the active channel of a headwater stream, thus creating substantial increases in erosive energy. In contrast, flood waters in a downstream channel segment can spread
across the floodplain and dissipate energy in overcoming frictional resistance.
The effect of a disturbance also depends on valley and channel geometry and
substrate erosional resistance. Two valley segments in the middle portion of a drainage network can respond very differently to a flood, for example, if one segment has
a relatively steep, narrow geometry that concentrates flow energy, whereas the other
segment has a lower gradient, wider valley that dissipates flow energy among secondary channels and across a floodplain.
Consideration of disturbance regimes is vital to effective river management
because even the most highly engineered and altered river corridor experiences
some disturbances. Indeed, physical disturbances such as an annual flood commonly maintain habitat and connectivity that are critical to the survival of plants
and animals in the river ecosystem (Junk et al. 1989; Bayley 1991). Traditional
river management has sought to minimize natural disturbances and to constrain a
river’s ability to adjust its form in response to disturbance. This has negatively
affected river biotic communities to the point that river management in many
cases now seeks to restore disturbances. The three key aspects of disturbance
regime for river management involve understanding (1) the type, magnitude, and
frequency of disturbance within a particular segment of the river corridor, (2) how
disturbances create and maintain specific features such as channel geometry or
channel- floodplain exchanges within the river corridor, and (3) how disturbances
influence biotic communities in river ecosystems. Where human alterations have
minimized disturbances or constrained the ability of the river ecosystem to adjust
following disturbance, river restoration can be targeted at restoring a more natural
disturbance regime as in the case of experimental flood releases from dams, or
providing space for a river to adjust, as in setting back levees to allow some channel
movement during floods.
2.3 Rivers as Ecosystems
uplands. The disturbance regime is the spatial pattern and statistical distribution of
disturbances with respect to magnitude, frequency, and duration of associated
changes in the physical environment (Montgomery 1999). River corridors dominated by snowmelt runoff, for example, have a disturbance regime characterized by
a single, relatively long duration seasonal flood. River corridors dominated by rainfall runoff, even if the rainfall is strongly seasonal, are likely to have more rapidly
fluctuating peak flows.
Disturbance regime in river corridors reflects sources of water, sediment, nutrients, and other materials that originate outside of the river corridor, as well as disturbances within the river corridor. The characteristics of a disturbance and the
response of the river corridor to the disturbance also vary within the river network.
Headwater channels, for example, commonly lack extensive floodplains and are
closely coupled to adjacent uplands. Increased runoff that causes flooding may thus
be more likely to change channel form in a headwater channel segment than in a
downstream channel segment. All of the excess water during a flood remains within
the active channel of a headwater stream, thus creating substantial increases in erosive energy. In contrast, flood waters in a downstream channel segment can spread
across the floodplain and dissipate energy in overcoming frictional resistance.
The effect of a disturbance also depends on valley and channel geometry and
substrate erosional resistance. Two valley segments in the middle portion of a drainage network can respond very differently to a flood, for example, if one segment has
a relatively steep, narrow geometry that concentrates flow energy, whereas the other
segment has a lower gradient, wider valley that dissipates flow energy among secondary channels and across a floodplain.
Consideration of disturbance regimes is vital to effective river management
because even the most highly engineered and altered river corridor experiences
some disturbances. Indeed, physical disturbances such as an annual flood commonly maintain habitat and connectivity that are critical to the survival of plants
and animals in the river ecosystem (Junk et al. 1989; Bayley 1991). Traditional
river management has sought to minimize natural disturbances and to constrain a
river’s ability to adjust its form in response to disturbance. This has negatively
affected river biotic communities to the point that river management in many
cases now seeks to restore disturbances. The three key aspects of disturbance
regime for river management involve understanding (1) the type, magnitude, and
frequency of disturbance within a particular segment of the river corridor, (2) how
disturbances create and maintain specific features such as channel geometry or
channel- floodplain exchanges within the river corridor, and (3) how disturbances
influence biotic communities in river ecosystems. Where human alterations have
minimized disturbances or constrained the ability of the river ecosystem to adjust
following disturbance, river restoration can be targeted at restoring a more natural
disturbance regime as in the case of experimental flood releases from dams, or
providing space for a river to adjust, as in setting back levees to allow some channel
movement during floods.
2.3 Rivers as Ecosystems