Channel Migration Assessment
||Figure 4: Channel narrowing and decrease in migration or bar
building indicates possible channel incision.
Task 3: Identify channel patterns
There are five main channel patterns [Appendix A of “A Framework for
Delineating Channel Migration Zones”]:
- Straight streams or channels are not common in nature. Straight
streams that are entrenched and controlled by bedrock or other less erodible
material are considered stable. Streams that are straight due to
recent incision or downcutting caused by an increase in the streams ability
to erode the bed and banks and may be only temporarily straight. Also, the stream
may be less stable through bank erosion or removal of the cause of incision
(e.g., hydromodifications) (Figure 4, Figure 7). Straight streams also occur
in confined higher-gradient reaches.
- Meandering streams are generally single-thread channels with
meander bends. Lateral and translational migration is typical of meandered
channels (Figure 5, Figure 6). Meandering streams have intermediate
stability. Entrenched meanders are more stable.
- Braided streams or channels are multi-thread channels with two or
more low-flow channels divided by bars that become inundated at bankfull
stage and are subject to frequent shifts in channel position. Bars are
generally not vegetated or sparsely vegetated. Braided channels in moderate
to unconfined reaches are instable (Figure 6). In confined braided reaches,
the active channel is not stable but the areas landward of the ordinary high
water mark are more stable.
- Island braided stream pattern consists of two or more channels
that are evident in low and high flows (Figure 6, Figure 7). The islands
separating the channels are well vegetated. This channel pattern generally
only occurs in low gradient reaches. This channel pattern is vertically
instable and be prone to avulsions. The channels tend to be laterally
stable. There are two major subcategories within this pattern: anabranching
and anastomosing channels.
- Wandering streams or channels consist of a mix of other channel
patterns. Within one reach, straight, meandered, island braided and braided
channels can occur (Figure 6, Figure 7). The channel stability for this
pattern is the most unpredictable.
|Figure 5: The Okanogan River near Oroville illustrates
meandering pattern and migration associated with one type of meandering
stream. Scroll bars and oxbow lakes indicate lateral and translational
|Figure 6: This wandering stream has three channel patterns
within a single reach: (1) meandering with scroll bars; (2) braiding
with multiple low flow channels; and, (3) Island braided with forested
Are channel reaches straight or narrow?
to look for evidence of channel change. Click
to go to step 2.
Look for evidence of channel change
Evidence that channel migration is occurring now or has occurred in the past
can be observed by comparing topographic maps to the most recent air or
orthophotos. GIS can simplify this task by overlaying on the most recent
orthophotos the Salmonscape 24K hydrography (stream) layer, which was digitized
from the USGS 7.5’ topographic maps.
A comparison between streamline traced on USGS 7.5 minute quadrangle (1994)
and a 2003 orthophoto indicates that there has been lateral and translational
channel movement between 1994 and 2003. Direction of flow is from top to bottom
of figures. Blue circles show areas of obvious channel movement.
Is there evidence of channel change?
to go to step 2. Click
to look for evidence of bank erosion.
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