What is channel migration?
Channel migration is the process by which a stream moves over time. Channel migration zones are areas where a stream channel can be expected to move over time due to natural hydrological and related processes. These natural processes are a response to gravity and topography that allows the stream to release energy and distribute its sediment load. Channel migration can occur on streams ranging in size from big rivers to small creeks.
Benefits and risks
The dynamic process of channel migration provides new habitats and ecological diversity that support critical fish and wildlife habitat. Streams that migrate have the highest diversity of aquatic habitats.
Channel migration can also damage or destroy homes, septic systems, roads, and other infrastructure. Impacts from a moving stream may take lives. In Washington, migrating streams have washed homes downriver, undercut banks, destroyed roads, and sent trees toppling onto homes. This is why local governments are required to identify channel migration zones and limit development within these areas.
Understanding channel migration
Channel migration is a geologic process, so significant migration may not have occurred within historic time periods. Local knowledge, map and imagery records, as well as old growth trees or homes built 70-100 years ago may not be good indicators of future channel migration. Homes built more than 100 years ago that seemed safe from channel migration have been damaged in recent years or may be at risk.
Changing conditions such as land development can alter peak stream flows, increasing the frequency of high magnitude floods. Climate change may result in different flooding patterns in the future and further affect channel migration.
Streams are wild and unpredictable. A stream’s active channel may build up through sediment deposits, deepen, widen, rapidly create a new channel, or migrate across its valley. The rate of change can vary depending on gradient, geology, sediment supply, stream flow, vegetation, natural instability or human development. Channel migration can occur gradually as a stream erodes one bank and deposits sediment along the opposite bank. During during flood events, migration can happen quickly.
Identifying channel migration zones
Most streams in Washington do not have channel migration zone maps even though these streams may migrate. Flood Insurance Rate Maps have not been updated in some areas, and older maps may not accurately show the current floodplain or where streams have migrated beyond the map boundaries. Channel migration zones may be within the 100-year (1 percent) floodplain, roughly equal to the floodplain, or larger than the floodplain.
Identifying channel migration zones can help your community establish management measures to guide development away from channel migration zones and reduce flood hazards. Limiting development within channel migration zones can reduce the costs of repairing or replacing infrastructure that might be threatened or damaged by channel migration and also prevent adverse effects on floodplain habitat.
Barriers to channel migration
Some natural and built structures may act as barriers to channel migration. Bedrock along streams is generally considered a geologic control on channel migration processes. Artificial structures built above the 100-year (1 percent) flood elevation or able to withstand the hydraulic stresses of these large floods may be barriers to channel migration.
These include publicly maintained flood control structures, public roads, highway overpasses and ramps, diversion structures, and dams. A maintained road below the 100-year flood elevation is not likely to be a barrier.
Channel migration assessment
Conducting a channel migration assessment is a complex technical job. In Washington, assessing channel migration equates to practicing geology and requires a state geology license. We also recommend the analyst be a fluvial geomorphologist with proven experience evaluating channel response and channel migration.
A brief list of recommended steps for conducting an assessment is listed below. You can find a systematic procedure for delineating channel migration zones in A Framework for Delineating Channel Migration Zones.
Determine if or where a channel migration assessment is needed
Some areas along streams are disconnected from the channel migration zone by the barriers mentioned above. An assessment is not needed for these areas.
An assessment may be required if one or more of the following indicators of migration are present:
- Bank erosion
- Bar formation
- Changes in the meander
- Oxbows and side channels
- Meander cutoffs
- Young disturbance vegetation
- Wood jams
- Channel widening
- Increased aggradation
High rates of channel movement occur where:
- Steep streams flow from foothills onto flatter floodplains
- Stream reaches have high rates of sediment deposition
- Stream reaches have large woody material or other objects that deflect flow
- Alluvial fans and deltas are present
- Eroded banks are present
Tasks involved in determining whether an assessment is needed include mapping channel gradient and channel confinement, and identifying dominant channel patterns.
Determine minimum standards of practice and level of effort
The assessment analyst should determine the approach for conducting the assessment based on the assessment area size and channel pattern. The level of effort is related to the local community’s objectives regarding protecting and restoring ecological values and evaluating hazards to people and critical infrastructure.
The Shoreline Master Program Guidelines have minimum standards regarding the interference with channel migration processes and loss of ecological functions.
Determine channel reach breaks for analysis
The five main stream channel patterns include straight (not common in nature), meandering, braided, island braided, and wandering.
Change in channel confinement, gradient, pattern, geologic constraints, human-made constraints, stream confluences, land use and zoning, and other factors are used to determine reach breaks.
Analyze migration, avulsion, and erosion components and map all hazards
Generally, three types of analyses are used in Washington to determine historic migration zone boundaries and migration rates:
- Manual measurement
- Polygon analysis
- Transect measurements
Avulsion hazards are delineated in order to anticipate possible shifts in channel location that may threaten infrastructure. An avulsion is the sudden switching of the stream channel to a different channel. The erosion hazard area is outside the historic migration and avulsion hazard areas. It may be susceptible to bank erosion from stream flow and mass wasting.
Develop hazard rating zones
This optional step involves assessing whether the migration and erosion hazard is severe, high, moderate, or low. The risk may vary within the mapped migration zone. Hazard ratings often rely on best professional judgment and migration zone assessment information. This includes rates of channel migration, avulsion and erosion hazards, and locations of armored banks.