28
high flows to erode the banks, widening the channel and resulting in deposition on
bars within the widened channel of some of the newly eroded sediment. The bars
deflect current toward the stream banks, resulting in additional bank erosion and bar
formation (Trimble and Mendel 1995; Erskine et al. 2012).
Negative or self-arresting feedback occurs when the response of the river corridor dampens an initial change. A tributary fan that laterally impinges on the main
channel constricts flow during high discharges. This causes peak flows to transition
to critical or supercritical conditions, resulting in high erosive forces that erode the
toe of the fan, widening the channel until peak flow returns to subcritical conditions
and the channel again becomes stable (Kieffer 1989).
Some river ecosystems exhibit first positive and then negative feedback, as illustrated by ephemeral streams with shallow anabranches in the semiarid tropical portion of northern Australia (Larsen et al. 2016). Wet monsoon forests are present
along some alluvial valleys and springs within a landscape dominated by eucalypt
savanna. Periods of enhanced stream flow can cause headcuts to form and migrate
upstream and this, combined with a highly transmissive shallow aquifer, causes the
instream water level and riparian water table to drop. Declining water levels cause
the anabranches and formerly saturated, peaty floodplain soil to desiccate. High
frequency, low intensity wildfires, combined with soil drying, remove monsoon forest vegetation, which remains intact upstream from headcuts. Increasing stream
flows thus create a positive feedback that alters riparian vegetation. However, dieoff of monsoon forest trees recruits large wood to the channel. The wood accumulates in logjams that create a local base level, allowing a wet floodplain to re-form
and promoting return of wet monsoon forest vegetation in an example of negative
feedback (Larsen et al. 2016).
Equilibrium does not imply anything about the rate at which change will occur.
A small increase in sediment inputs may result in negligible initial response within
the river corridor, but sustained increases in sediment may progressively fill available sediment storage areas in the channel bed, channel bars, levees, and overbank
areas, until the channel finally undergoes a substantial change. In this example, a lag
time occurred between the initial changes in controlling variables and the response
of the river corridor.
Fig. 2.6 In this schematic illustration of steady-state equilibrium, the sinuosity of the channel varies as individual meanders grow and are cut off (blue line), but the average sinuosity (dashed black
line) does not vary over the time period represented in the plot
2 Rivers as Ecosystems
high flows to erode the banks, widening the channel and resulting in deposition on
bars within the widened channel of some of the newly eroded sediment. The bars
deflect current toward the stream banks, resulting in additional bank erosion and bar
formation (Trimble and Mendel 1995; Erskine et al. 2012).
Negative or self-arresting feedback occurs when the response of the river corridor dampens an initial change. A tributary fan that laterally impinges on the main
channel constricts flow during high discharges. This causes peak flows to transition
to critical or supercritical conditions, resulting in high erosive forces that erode the
toe of the fan, widening the channel until peak flow returns to subcritical conditions
and the channel again becomes stable (Kieffer 1989).
Some river ecosystems exhibit first positive and then negative feedback, as illustrated by ephemeral streams with shallow anabranches in the semiarid tropical portion of northern Australia (Larsen et al. 2016). Wet monsoon forests are present
along some alluvial valleys and springs within a landscape dominated by eucalypt
savanna. Periods of enhanced stream flow can cause headcuts to form and migrate
upstream and this, combined with a highly transmissive shallow aquifer, causes the
instream water level and riparian water table to drop. Declining water levels cause
the anabranches and formerly saturated, peaty floodplain soil to desiccate. High
frequency, low intensity wildfires, combined with soil drying, remove monsoon forest vegetation, which remains intact upstream from headcuts. Increasing stream
flows thus create a positive feedback that alters riparian vegetation. However, dieoff of monsoon forest trees recruits large wood to the channel. The wood accumulates in logjams that create a local base level, allowing a wet floodplain to re-form
and promoting return of wet monsoon forest vegetation in an example of negative
feedback (Larsen et al. 2016).
Equilibrium does not imply anything about the rate at which change will occur.
A small increase in sediment inputs may result in negligible initial response within
the river corridor, but sustained increases in sediment may progressively fill available sediment storage areas in the channel bed, channel bars, levees, and overbank
areas, until the channel finally undergoes a substantial change. In this example, a lag
time occurred between the initial changes in controlling variables and the response
of the river corridor.
Fig. 2.6 In this schematic illustration of steady-state equilibrium, the sinuosity of the channel varies as individual meanders grow and are cut off (blue line), but the average sinuosity (dashed black
line) does not vary over the time period represented in the plot
2 Rivers as Ecosystems