In terms of channel stability, it is important to recognize that three geomorphic responses or
processes can result from changes in dominant channel flow and sediment conditions. These
are channel widening, channel deepening, and changing planform (a change in sinuosity or
meander pattern). All of these responses will cause some level of streambank erosion.
Channel widening is evidenced through an increase in channel width, with or without an
increase in channel depth. An increase in flow or sediment discharge results in a tendency
toward channel widening. When both sediment discharge and flow increase, however, the
channel section can be expected to increase its depth as well as its width. When only
sediment load increases, width increases but the depth may decrease. In this case the
channel is said to be aggrading, implying that the channel has filled in because of an excess of
sediments.
Channel deepening is a process of channel degradation that increases the depth of the
channel. Channel degradation can cause bank instability by producing a steeper bank angle.
Whether or not instability actually occurs is a function of the properties of the bank materials
and the original bank geometry. Channel deepening results from increased flow without an
appreciable increase in sediment discharge. Increased flow rates can result from an overall
increase in the volume of water moving through the channel or an increase in channel slope.
Changing channel planform includes changes in channel pattern and position as viewed from
above. Changes in planform are most often exhibited through the downstream migration of
meandering bends and changes in the sinuosity of meander bends. Other examples include
the shifting of channels and the cutting off of meander bends. Generally, these changes are
manifested by an adjustment of channel slope to conform with changes in flow or sediment
discharge.
A reduction in sediment discharge or an increase in water discharge will result in a reduction of
the channel slope. These slope reductions result from increased channel sinuosity and/or
channel-bed degradation; both of which lead to a tendency toward increased bank erosion.
Also, a reduction in sediment discharge will result in an increase in channel sinuosity, again,
leading to increased bank erosion.
It is also important to recognize that the three geomorphic processes just discussed (channel
widening, channel deepening, and changing planform) are often interrelated and can occur
simultaneously or in sequence. For example, adjustments in channel slope through
degradation often are accompanied by increases in channel sinuosity and bank caving or
channel widening. Also, the initiation of a given process at a particular site may initiate another
process either upstream or downstream. For example, an aggrading channel reach can cause
an increase in sinuosity in a downstream reach.
5.5.3 River Pattern Thresholds and Response
The work of Lane (1957) and Leopold and Wolman (1960) as summarized in Section 5.4.5
indicates that there is a gradient or discharge threshold above which rivers tend to be
braided (Figure 5.18). The experimental work reported by Schumm and Khan (1972) shows
that for a given discharge, as valley-floor slope is progressively increased, a straight river
becomes sinuous and then eventually braided at high values of stream power and sediment
transport (Figure 5.3). Rivers that are situated close to the meandering-braided threshold
should have a history which is characterized by transitions in morphology from braided to
meandering and vice versa.
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