Stream channel migration zones

Channel migration zones are areas in a floodplain where a stream or river channel can be expected to move naturally over time in response to gravity and topography. Water bodies such as rivers and streams gain or release energy as they flow, carrying away or spreading out sediments, building new areas, and supporting a variety of fish, wildlife, and vegetation. Rivers with room to migrate have the highest diversity of aquatic habitats.

For existing communities near rivers and streams, it is important to know where channel migration zones exist and plan accordingly. Communities can manage these higher risk areas by guiding development away from channel migration zones. This strategy helps reduce flood and erosion hazards and costly repairs while preventing the loss of crucial floodplain habitat.

What is channel migration?

Channel migration is the natural process that describes how a stream or river channel moves over time. Channel migration can occur gradually, such as when a stream erodes away one bank and deposits sediment along the opposite side. It can also occur quite quickly during floods or high water events.The rate of change depends on an array of factors, such as gradient, geology, sediment supply, stream flow, vegetation, natural instability, and human development. 

While channel migration provides important habitats and natural diversity, this process can also damage or destroy homes and infrastructure located within these ever-changing zones. A moving stream may take lives. In Washington, migrating streams have washed homes down river, undercut banks, destroyed roads, and sent trees toppling onto homes. This is why local governments are required to identify and limit development within channel migration zones.

Since channel migration is a long-term geologic process, noticeable migration may not occur during recent periods. It also means local knowledge, maps, images, locations of old growth trees, or even where homes have been built in the last 70 to 100 years are not necessarily the best indicators of future channel migration. For instance, homes built more than a century ago that seemed safe from channel migration can be damaged or at risk now or in the near future.

Changing conditions such as land development can alter peak stream flows, thereby increasing the frequency of high-magnitude floods. Climate change may result in different flooding patterns in the future, further affecting channel migration.

Identifying channel migration zones

Identifying channel migration zones can help communities establish management practices that guide development away from these zones and reduce flood hazards. Using this knowledge in planning future development can reduce the costs of repairing or replacing damaged infrastructure, lower exposure to flood and erosion hazards, and prevent adverse effects on floodplain habitat.

Information and images on Washington's stream channel migration.