Channel Migration Hazards

What is Channel Migration?

River channels can move, or migrate, laterally across their floodplains. Channel migration can occur gradually, as a river erodes one bank and deposits sediment along the other. Channel migration also can occur as an abrupt shift of the channel to a new location, called an avulsion, which may happen during a single flood event. There are several areas in King County with migrating river systems that represent hazards to permanent structures. The highest rates of channel migration occur in zones of rapid sediment deposition, e.g., where steep rivers flow out of foothills onto flatter floodplains. Channel migration represents a different type of flood hazard than getting inundated by overbank flow, and can endanger properties located outside of the regulatory floodplain.


Channel migration can endanger properties outside of the floodplain.

The erosion caused by channel migration can undermine houses, roads, and infrastructure, wash away property, and threaten lives. It may be the least recognized and yet most destructive type of damage that results from flooding. Channel migration poses a risk to public health and safety in the affected parts of King County.


Undermined house along the Raging River

Channel Migration Zone Mapping

King County has studied and mapped Channel Migration Zones (CMZs) along four of its major rivers. The County looked at historic channel locations, geology, basin hydrology, riverbank materials, current channel conditions, abandoned channels and potential avulsion sites, channel migration rates, and existing infrastructure to characterize CMZs. Study findings were used to map both severe hazard and moderate hazard areas within the CMZ. To date, CMZ studies and maps have been completed for the Tolt River, River Mile (RM) 1.7 to RM 6; the Three Forks of the Snoqualmie River, including the lower South, Middle, and North Forks; the Raging River, RM 1.5 to RM 9; and the middle Green River RM 25 to RM 46. Mapping of CMZs will occur along other major rivers of King County in the near future.

Example of CMZ mapping along the middle Green River

Online Channel Migration Hazard Maps

iMAP (high speed Internet connection required)
View and print map displays which you can customize using online King County GIS data. Includes a property search tool.

Channel Migration Zone Regulations

Following the policy direction specified in the 1993 King County Flood Hazard Reduction Plan, channel migration hazard areas have been identified and mapped through geomorphic analyses, and land use regulations have been adopted and applied in order to preclude unsafe development in these areas. A King County Public Rule (KCPR) on channel migration was enacted in June 1999 to regulate land use where CMZ maps have been completed along the four river segments listed above. An example of the provisions in the KCPR is that new development generally is not allowed within the CMZ mapped as severe hazard, and new development may be restricted within the CMZ mapped as moderate hazard.

Full text of the June 1999 channel migration public rule (Adobe Acrobat)

Information on purchasing a King County channel migration study

Where the original driver for mapping and regulating land use within CMZs was to keep people and structures out of harm's way, allowing a river channel to migrate naturally also can benefit riparian and aquatic habitat. Conservation of fish and wildlife habitat, including salmonid habitat, is one of the reasons that the County is currently preparing a comprehensive Critical Areas Ordinance (CAO). The draft CAO also calls for delineation and mapping of CMZs. Therefore, once the CAO is finalized it will incorporate the provisions and requirements of the KCPR. Land use within all mapped CMZs will be regulated according to the CAO. To that end, channel migration zones along other major rivers of King County will be mapped in the near future, in addition to the four rivers listed above.

For more information on how CMZ regulations may affect development proposals, please refer to Department of Development and Environmental Services web site or call Steve Botthiem, Supervising Engineer 206-296-7144.

For questions about Channel Migration Zone Mapping, please contact Terry Butler, Earth Scientist, River and Floodplain Management Unit.