Channel Migration Zone Delineation: The Convergence of Flood Protection and Restoration

The delineation of channel migration zones (CMZs) in Washington State has become an important element of long-term flood hazard reduction and salmon recovery planning.  Channel migration is a natural process in many alluvial rivers that is critical to the recruitment of woody debris, habitat formation, sediment budgets, and structuring riparian forests.   Development within a CMZ is subject to serious erosion hazards and possible flooding.   Delineating CMZs provides the public with valuable information of potential hazards and of potential restoration opportunities where levees and infrastructure can be set back.  Establishing CMZs can help to preserve corridors poorly suited for development, but ideal for fish and wildlife.  Channel migration can occur gradually or catastrophically, streams can move across their valleys or jump into side channels, and streams can erode valley hillslopes that trigger landslide that impact areas well above the 100 yr flood.  Changes in sediment, flow, wood or bank strength (e.g. vegetation removal) can all drive changes in channel geometry and dynamics.  All these factors can be directly influenced by watershed development, channel alterations, and climate, thus accounted for in the delineation.  Post-flood recovery efforts can utilize CMZs to better establish safe and environmentally sustainable corridors to prevent future disasters.