Channel Migration Zone (CMZ)
Channel migration is a natural geologic process which describes how a stream or river channel moves over time. Streams and rivers may change course, or migrate, through a variety of factors such as erosion and deposition of sediments, which alter geology, stream/river boundaries, shape, and functionality. As streams and rivers change course, their potential hazards also change.
While these processes normally occur over long periods of time, quick avulsions (movements) in a single storm, flooding, or human influences can rapidly affect the speed at which a channel changes course or migrates over time.
A graphic example of channel migration can be seen below. Click to enlarge
Channel Migration Zone (CMZ):
An area in which a channel is likely to move over a period of time.
CMZ studies identify areas at a severe, moderate, or low risk of erosion. The county adopts the study and the severe risk areas are then regulated under Pierce County’s floodway codes. The moderate and low risk areas are not regulated but can be helpful information when planning a project.
Channel Migration Zone (CMZ) studies and maps provide the baseline information necessary to understand the effects of potential river migration on hazards in river valleys. Identifying channel migration zones can help communities establish management practices that guide development away from the most dangerous channel migration zones and reduce flood hazards. The study considers a wide variety of factors, and does not focus on flooding-only erosion.
How is a Study Completed?
A channel migration zone study is a geotechnical report that requires the expert knowledge of a licensed geologist, trained in fluvial morphology to complete. The study will look at many limiting factors to erosion including the presence of bedrock or hard material versus loose gravels from recent flood events. The study also looks at the track record of the river. A photographic record is compiled searching as far back as the history of flight in the County. River movement is quantified over the period of photographic record and the reaches of the river are grouped by common factors (slope, width, volume of flow, erosive banks, vegetation etc.) by the geologist. The geologist sorts all these together, using state approved practices, to generate three risk zones: severe, moderate and low.
Once CMZ studies are fully complete, and the channel migration and erosion hazard is mapped as severe, moderate, or low, the county adopts the study and the severe risk areas are then regulated under Pierce County’s floodway codes.
How Does Pierce County Use CMZ Studies?
CMZ studies enable Pierce County to develop zoning maps based on science and guide future development away from high-risk areas. The county has completed channel migration studies on the following rivers: Puyallup, Carbon and lower White (2003), Upper Nisqually (2007), and the Greenwater (2017), and South Prairie Creek (2005). You can find these studies on the links to the right.
The Upper White River and Greenwater River CMZ studies are the only remaining CMZ study to be adopted in the county. Once they are formally adopted by the Pierce County Council, they will placed on this website. Pierce County is committed to practices (like the CMZ study) that reduce risks to residents, businesses, and infrastructure, while protecting and improving fish and wildlife habitat that rely on our river systems.
For more information on the methodology behind identifying CMZs, please visit Washington State Department of Ecology’s CMZ webpage.
For more information on the different types of channel migration processes and patterns in Washington State, view the Department of Ecology’s Synthesis for Floodplain Management and Restoration, August 2014.
A channel migration zone floodway occurs only in the areas determined to be a severe channel migration zone risk. Pierce County conducts studies on a wide variety of factors that can influence a river to migrate through erosion. When completed, a channel migration report shows areas at severe, moderate, or low risk. A severe channel migration hazard is determined to be areas where erosion can occur in the near future and an area can become part of the river channel. For this reason, severe channel migration areas are regulated under Pierce County’s floodway codes. Unlike other floodways, this floodway allows for homes that are beyond the reach of the deep and/or fast flowing waters or FEMA floodways which become damaged or destroyed by anything but a flood or channel migration to be repaired or rebuilt. No new structures are allowed within a severe channel migration zone once the study has been adopted.