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The Tolt River CMZ study and map, completed in June 2017, and the January 16, 2018 Reassessment of the King County Channel Migration Zone Mapping near the Holberg Levee, Tolt River (Tolt River CMZ Reassessment Technical Memo) were incorporated into the final July 2018 Tolt River Channel Migration Zone (CMZ) map per 21A-24-279A(B)(3).

Following are links to the final map, the 2018 reassessment, the 2017 study and map, and the amended channel migration public rule which became effective July 13, 2018

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Paper copies are available for review at the following two locations:


Channel migration is the process in which a river moves across its floodplain either by steady progressive erosion or by suddenly switching to a new course. Channel migration is a type of flood hazard, different from inundation hazards. (Visit the channel migration page for more information)

The Tolt River Channel Migration Zone (CMZ) Study and Map and the Tolt River CMZ Reassessment Technical Memo, adopted June 13, 2018, applies to the Tolt River from River Mile 6 to the river's confluence with the Snoqualmie River in the City of Carnation (River Mile 0).  The combined July 2018 Tolt CMZ map will be used to regulate land use within affected areasidentify hazard areas, and help property owners make informed land use decisions.

New development generally is not allowed within the severe hazard area of the channel migration zone and new development may be restricted within the moderate hazard area of the channel migration zone.

CMZ map preparation and adoption process

The Tolt CMZ study and map and Tolt River CMZ Reassessment Technical Memo were prepared using CMZ mapping methods specified in the King County Channel Migration Zone Public Rule, consistent with policies in the King County Flood Hazard Management Plan, Washington State Shoreline Management Act (external link), and the Washington State Department of Ecology Channel Migration Assessment (external link).

Public involvement

CMZ maps are adopted by amendment to the King County channel migration public rule under authority of the authority of the King County Code 2.98, 21A.02, and 21A.24. Channel migration zone mapping is required by the Washington State Shoreline Management Act.

The draft Tolt CMZ study and map underwent public review in 2017. Public comments were considered in finalizing the CMZ study and map. 

  • May 8, 2017: King County hosted a public meeting in Carnation; 15 people attended the meeting. King County staff presented the draft Tolt River CMZ study and maps and described the channel migration public rule amendment process by which the CMZ map is adopted. There was a question and answer period and the CMZ map was on display. Tolt River Channel Migration Zone Draft Study and Map Presentation (PDF, 9.28MB)
  • June 14, 2017: The public comment period closed. King County Department of Natural Resources and Parks (DNRP) responded to all comments and made minor revisions to the draft CMZ study and map.
  • January 16, 2018: King County DNRP, in consultation with the King County Department of Permitting and Environmental Review (DPER), conducted an independent review of the Tolt River CMZ hazard mapping near River Mile 1.75 (Holberg Levee area) that resulted in modification to the hazard designations in the study area.
  • June 13, 2018: Final Tolt River combined 2017 and 2018 reassessment CMZ mapping was adopted by the King County DPER by amending the existing channel migration public rule.
  • July 13, 2018: Final Tolt River CMZ map became effective.


For more information about the Tolt CMZ study and map or channel migration mapping methods, please contact Jeanne Stypula, Managing Engineer, River and Floodplain Management Section, Water and Land Resources Division, Department of Natural Resources and Parks.

For more information about King County’s existing land use regulations in channel migration zones, please contact Department of Local Services, Permitting Division.