|Channel gradient influences channel type and migration and bank
The Washington Department of Fish and Wildlife (WDFW) interactive mapping site, Salmonscape, provides channel gradient maps for all Washington streams. The stream gradient attribute data can be downloaded by County or smaller area as GIS Shapefiles (Figure 1 below).
|Figure 1: Channel gradient for all streams in the state are on the SalmonScape interactive mapping site|
Figure 2 below illustrates channel confinement.
Channel confinement is the ratio of valley width to active channel width. Typically, channel confinement is an indicator of how much a channel can move within its valley before it is stopped by a hill slope, terrace, or other hard point.
Valley width is measured from toe of hillslope on both sides of the river.
Holocene terraces, formed post-glaciation, are included in the valley width
measurements because channels can still erode the terraces.
Evaluate channel confinement using existing data sources (see Table 1). Where the channel confinement attribute is not available on SalmonScape, channel confinement can be identified from topographic maps as illustrated in Figure 2.
Figure 2: Confinement can be determined from topographic maps. The aerial photographs illustrate the examples on the map. Sharp bend angles in the photo a) indicate that there is geologic control on meander formation.
Channel gradient and confinement influence a stream’s ability to migrate (Figure 3, Table 2 below). Mapping channel gradient and confinement provides an indicator of the relative potential for a stream to migrate.
Figure 3: Channel confinement and gradient can be an indicator of migration potential.
|Table 2: relationship between gradient, confinement, channel
type and patterns. (See Figure 3 above)
|Gradient||Confinement||Channel type||Associated patterns||Potential to migrate|
|0-2%||Unconfined, moderate||Pool-riffle||Meander, island braided, wandering||High|
|1-2%||Moderate-confined||Pool-riffle, plane-bed||Meander, straight, island braided, wandering||Moderate-High|
|2-4%||Unconfined-confined||Plane-bed, step pool, forced pool-riffle||Braided, wandering, straight, alluvial fans||Moderate-High|
|1-12%||Unconfined-moderate||Alluvial fans, deltas||Multiple channel, distributary channels||Moderate-High|
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