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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.
|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.|
|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.|
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|
|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.
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