Channel Migration Assessment

Optional task: Identify channel geomorphic processes

This optional task may be more useful later in the assessment Channel process supplements information on stability, possible hazard, and level of effort discussed in Step 2b. Many processes can be identified from one set of aerial photographs or maps where information is not available on processes. Channel widening or narrowing usually requires at least two sets of aerial photographs. The time interval between the photo sets should be at least 10 years. Refer to Ecology Publication [A Framework for Delineating Channel Migration Zones: Appendix A] for more information.

  • Translation migration - the down valley movement of meander bends which occurs when bank erosion in meander bends is concentrated at the outer bank between the bend apex and the downstream crossing. Scroll bars are often an indicator of down valley movement (Figure 5).
  • Lateral migration - lateral growth increases the meander bend amplitude. Irregular erosion occurs where variability in the erosion resistance of the banks disrupts the regular pattern of lateral activity. Oxbow lakes or meander cutoffs indicate previous lateral erosion.
  • Avulsion - the rapid abandonment of the river’s present course in favor of a new one. Avulsions can occur into a historic channel, side channel or create a new channel. Other indicators of potential avulsions can be determined from aerial photographs. Avulsions can occur in any channel type or pattern (Figures 7 and 9c).
  • Channel widening occurs when channel banks expand in response to geotechnical failure of the banks or from rapid aggradation that has led to a decrease in flow capacity. Indicators include:
  • Increase in bar size
  • Increase in channel width
  • Bank erosion or slumps
  • Tree fall
  • Channel narrowing occurs with a decrease in the top width of a stream via channel incision or from an increase in bank resistance from vegetation (Figure 4 and Figure 7). A decrease in bar size is one indicator.
  • Bank erosion is more prevalent where bank materials are loose or composed of easily eroded material, e.g., glacial outwash (see Figure 9), ash, loose alluvium. The exposed soil (scarp) of the eroding bank may also be observable in the photos. Bank erosion can be episodic and strongly correlated with flood frequency.
  • Geologic control slows channel erosion or migration. Incision does occur through all materials but may take centuries or longer to detect. For the CMZ delineation, bedrock and other resistant materials are considered to prevent channel migration and bank erosion. Often geologic control can be detected on migrating rivers. Reaches that have bends but no detectable migration may have geologic control. Bends that are sharp rather than curved may also indicate control (e.g. Figure 2a).
Coding reaches by pattern and processes on the maps or orthophotos also provide information for determining reach breaks and those reaches where channel movement occurs.
Pattern Code Process Code
Meandered M Translational T
Braided B Lateral L
Island braided A Avulsion AV
Wandering W Widening or aggradation CW or CA
Delta or alluvial fan DA Narrowing or incision CN
Straight and entrenched SE Bank erosion EH
Straight and narrow SN Geologic control (bedrock or other erosion resistant material) GC

 

Figure 7: The topographic map is from 1976 and the USGS orthophoto is from 1998. Both the map and air photo show that the downstream reach is straight and possibly entrenched while the upstream reach is wandering with Island braided and meandering patterns. Lateral erosion and avulsions appear to be the dominant migration process. An avulsion (AV) appears to have occurred between 1978 and 1998. The main channel appears to have moved from river right to river left.

Are banks eroding?

Evidence of bank erosion on larger streams or smaller unvegetated streams can usually be observed on recent aerial photographs or orthophotos. However, bank erosion on streams in well-vegetated reaches and smaller streams is not always evident on air photos or maps.

Some indicators are clearings adjacent to stream and meanders abutting steep slopes (see Figure 8 below). Geology or soils can indicate areas that are more likely to erode (see Table 3 below). Geology maps are available through the Washington Department of Natural Resources. The USGS may have sources of geologic information for your area also. Wetland maps may provide additional information on extent of riparian wetlands, which are often associated with channel migration.

Figure 8: (a) A clearing along the left bank of Buttermilk Creek indicates a potential bank failure as verified by field observation (b). The geology is glacial moraine deposits covered with older ash deposits indicating rocks not-resistant to erosion. The clearing in the riparian area combined with topographic evidence of the stream flowing against the bank (c). Also refer to Figure 9.

 

Figure 9: Bank erosion on the Elwha River provides an example of identifying potential erosion and avulsion hazards from topographic maps and aerial photos. The yellow arrows identify where the river is eroding a high glacial terrace which will affect an existing residence on the left bank of the river (left side of photo a)). The topographic map (b) and air photo (c) show that the river is flowing at the base of the bank. The topographic map is from 1978 and the right air photo is from 1994. Between those years some avulsions occurred in general locations shown by red arrows. Upstream avulsion switched the main channel to river right. Downstream avulsion sent river back to 1978 channel.

 

Table 3: Erosion potential of common rocks found in Washington
Erosion potential Geology
Resistant Fine-grained granites, strongly cemented sandstone, gabbros, diabases, rhyolites, slates, gneiss, quartzite
Moderate Coarse-grained granites, poorly cemented sandstone, basalts, soft sedimentary, schists, volcanic tuff and ash deposits, pyroclastic flows
High Alluvial fan deposits, alluvium, glacial drift, glacial outwash, some glacial tills, lahars, mass-wasting deposits, outburst flood deposits, peat, glacio and fluvial terrace deposits

While Table 3 provides general information, local knowledge is preferred over general relationships.

County soil survey documents and maps also provide information on erosion. County soil data is available on the Natural Resources Conservation Service Soil Survey web page. Most counties or agricultural extension offices have hard copies of the County Soil Surveys.

 

Is there evidence of bank erosion?

Click to go to step 4. 
If no, a CMZ or bank erosion hazard assessment is not necessary
at this time but maybe in the future.
 

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