Channel migration hazards
King County, Washington
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 can also 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, for example, where steep rivers flow out of foothills onto flatter floodplains.
Why map channel migration?
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.
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.
Channel migration zone mapping
King County has studied and mapped Channel Migration Zones along the following major rivers:
- South Fork Skykomish from the confluence of the Foss and Tye Rivers to the King-Snohomish county line, River Mile 19.7 to River Mile 6.6 (2017)
- Tolt River Update from River Mile 6 to the river's confluence with the Snoqualmie River, River Mile 6 to River Mile 0 (2017)
- Three Forks of the Snoqualmie River, including the lower South, Middle, and North Fork (1996) (PDF, 10MB)
- Middle Green River, River Mile 46 to River Mile 25 (1993) (PDF, 4MB)
- Tolt River, River Mile 1.7 to River Mile 6, and Raging River, River Mile 8.1 to River Mile 0 (1991) (PDF, 10MB)
King County looked at historical channel locations, geology, basin hydrology, riverbank materials, current channel conditions, abandoned channels and potential avulsion sites, channel migration rates, and existing infrastructure to characterize channel migration zones. Study findings were used to map both severe hazard and moderate hazard areas within the channel migration zone.
Online channel migration hazard maps
iMAP interactive maps
View and print map displays which you can customize using online King County GIS data. Includes a property search tool.
Following the policy direction specified in the 1993 King County Flood Hazard Reduction Plan, the 2006 King County Flood Hazard Management Plan and subsequent flood plan updates, King County has identified and mapped channel migration hazard areas through geomorphic analyses and had adopted and applied land-use regulations in order to restrict unsafe development in these hazard areas.
In 1999, a King County Public Rule on channel migration was adopted to regulate land use within mapped channel migration zones (it was amended in 2014 and 2015). In 2005, the land-use regulations in the public rule were incorporated into King County Code as part of the Critical Areas Ordinance.
As an example of channel migration zone land-use regulations, new development generally is not allowed within the channel migration zone mapped as a severe hazard area, and new development may be restricted within the channel migration zone mapped as a moderate hazard area.
While land use within mapped channel migration zones is regulated by King County Code, the 1999 public rule continues to function as the document that specifies King County channel migration zone mapping methods and is the mechanism by which the County adopts newly completed channel migration zone maps.
In 2012, amendments made to King County Code as part of the King County Shoreline Master Plan update revised the definition of channel migration zones (KCC 21A.06.182) and required that channel migration zone mapping methods be revised (KCC 21A.24.274) in order to comply with State Shoreline Management Act provisions.
Revised King County Channel Migration Zone Public Rule
In 2014, King County adopted revisions to its public rule that specifies how to map channel migration zones. Regulations on land use within channel migration hazard areas were not changed.
- Critical Areas Designation, Classification and Mapping of Channel Migration Zones (amended) (PDF, 141.67 KB)
The public rule was revised to be consistent with the 2012 King County Shoreline Master Plan update, channel migration zone provisions in the Washington Administrative Code (WAC 173-26-221(3)(b)) and Washington State Department of Ecology guidance on channel migration zone mapping. The authority to update the public rule is granted in King County Code 2.98, 21A.02 and 21A.24.
Public rule revision process
In 2013, a draft revised public rule was released for a 45-day public comment period and a public meeting was held on Nov. 20, 2013, during the comment period. King County responded to written and verbal comments and revised the draft public rule, improving both content and clarity. The King County Council adopted the final revised public rule on February 28, 2014, and it went into effect March 31, 2014.
Preparation of channel migration zone maps using updated mapping methods will proceed through 2014 and beyond. King County plans to host a public meeting for each channel migration zone study area to present draft mapping and solicit public comments.
For information about King County’s regulations in channel migration zones, please contact Steve Bottheim, King County Department of Permitting and Environmental Review.