Channel Migration Assessment

Channel gradient influences channel type and migration and bank erosion potential:
 
<2%--low gradient Migration, flooding
2-4%--moderate gradient Limited migration & flooding, bank erosion
>4%--high gradient Bank erosion, slope failure

Step 1b, Task 1: Map the channel gradient

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

Is gradient ≤4%?

Click to go to Task 2.  Click to look for evidence of channel change.

 

  • Confined: ratio of valley width to active
    channel width <2.
         - Confinement restricts channel movement
  • Moderate confinement: ratio of valley width to active channel width ≥2 and <4.
         - Channel movement can occur
  • Unconfined: ratio valley width to active
    channel width >4.
         - Channel can move rapidly

Figure 2 below illustrates channel confinement.

Step 1b, Task 2: Is the channel confined?

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.

Is channel confinement moderate to unconfined or is the valley floor obviously more than twice as wide as the active channel?

Click to Step1B, Task 3.  Click to look for evidence of channel change.

 

Additional information on channel gradient and confinement

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
4-8% Moderate-confined Step-pool Straight, sinuous Low-Moderate
8-20% Moderate-confined Cascade Straight, sinuous Low
1-12% Unconfined-moderate Alluvial fans, deltas Multiple channel, distributary channels Moderate-High

Previous Page l Next Page