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”]:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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 movement.

 

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 floodplain islands.

Are channel reaches straight or narrow?

Click 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?

Click to go to step 2.  Click to look for evidence of bank erosion.

 

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