CMZ study

What is channel migration and a CMZ?

Channel migration is typically a long-term, 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, flooding and human influences can rapidly affect the speed at which a channel changes course or migrates over time.

A Channel Migration Zone (CMZ) is that area in which a channel is likely to move over a period of time.

What is a CMZ study?

Channel migration zone 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.

Once the CMZ study is fully complete (including a full public process with the affected communities), 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.

Learn more by viewing our project factsheet and frequently asked questions (FAQs).

Example map from a completed CMZ study from the Puyallup, Carbon, and Lower White River. Completed study map shows various zones of river migration.
The above map illustrates what a completed CMZ study map will look like – this example is taken from the Puyallup, Carbon, and Lower White River CMZ study, completed in 2003.
Click to enlarge