Channel Migration Assessment Steps

1b) Identify streams for channel migration assessment.

Product: Map of streams/reaches where a channel migration assessment is needed or desired.

Information sources: Channel and valley characteristics including channel confinement and gradient, multi-year air photographs, orthophotos, maps, LiDAR (Light Distance and Ranging). Table 1 provides sources for information used in this step.

The following flow chart illustrates the tasks associated with Step 1b. Channel confinement, channel gradient, evidence of channel movement, and channel pattern and are initially used to areas of potential channel migration. The tasks can be done manually. GIS provides organization and simplifies mapping. All examples provided are from publicly available data that can be downloaded from the internet (Table 1, below). The flowchart is interactive, allowing the user to go to specific steps. These steps and associated tasks can also be followed by scrolling through the document.

Are there migrating rivers and/or other geo-hazards such as eroding banks and avulsions in my jurisdiction?

Generally, high rates of channel movement occur where:
In the illustrations below, the blue dotted line outlines an alluvial fan, near Winthrop, WA. As bedload rapidly accumulates where hillslope gradient decreases, the channel aggrades and distributary channels are formed on the surface of the fan. This process creates a fan-like landform composed almost entirely of bed material cut by the distributary channels. In this example, stream flow occurs most of the year across the lower portion of the fan. One of the most dynamic areas within any landscape, alluvial fan CMZs pose substantial risks and costs if developed. The contours illustrate the fan-like landform and extent of bedload aggradation on the valley floor.

 

Topographic maps, air photos, or orthophotos can be used to identify other indicators of current and past migration. The more obvious indicators of channel migration are listed below:

  1. Multiple channels
  2. Gravel bars
  3. Multiple bar forms
  4. Meandering form
  5. Secondary channels
    1. Side channels: persistent secondary channels, typically with vegetated islands or other persistent landforms separating it from the main channel
    2. Relic channels: those channels that may have flow in them only during very high water. They don't have to be connected to main channel but are still in the floodplain.
  6. Young disturbance vegetation
  7. Wood jams

Be aware that this list does not mean that other areas or reaches do not migrate. This is only a starting point.

The aerial photographs below illustrate the above listed channel migration indicators:

If one or more of the indicators are present than it is likely these areas will need some level of channel migration assessment.

If none of the indicators are present, bank erosion hazards may still exist. Proceed to the bank hazard section.

Table 1: Some sources of free information
Confinement Gradient (ft/ft or m/m) Channel type, pattern and movement
Data for 20 WRIAS (link to map) are available from Salmonscape1 interactive mapping web site: If not mapped than use other data. Data for all the WRIAs are available from Salmonscape interactive mapping web site as previous EXAMPLE  
USGS 7 ½’ quadrangle topographic maps
available from USGS Seamless Distribution Data, NASA Whirlwind.
Channel gradient (change in elevation/distance) USGS 7 ½’ quadrangle topographic maps
Current aerial photos or ortho-rectified aerial photos. Every 5-6 years the state has aerial photos flown and ortho-rectified. These are publicly available at: PugetSoundLidar   Current and historic aerial photographs, at a minimum oldest and most recent; recommended—some years in between oldest and most recent
10-meter DEM, PugetSoundLidar2, USGS Seamless Distribution Data Channel gradient can be measured from DEM 10-meter DEM—difficult to identify shallow side channels because of resolution and error
LIDAR in the Puget Lowlands is more available than other areas PugetSoundLidar. Check with federal agencies within your area (e.g., Bureau of Reclamation, USCOE) Elevation points and coordinates can be measured from the LiDAR data Light Distance and Ranging (LIDAR) provides valuable information on past channel migration patterns. It greatly simplifies the task of looking for evidence. Older LIDAR resolution has more errors than more recent. Also, data in heavily vegetated riparian areas has less point coverage.
Google Earth and NASA Whirlwind allows 3-D look at topography, useful for visually determining confinement   Google Earth , NASA Whirlwind, USGS Seamless Distribution Data
 
1 This data can be downloaded as shapefiles for use in GIS
2 The data are free but require registration before downloading

 

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