A Methodology for Delineating Planning-Level Channel Migration Zones

Abstract

The Washington State administrative codes that implement the Shoreline Management Act (SMA) require communities to identify the general location of channel migration zones (CMZs), and regulate development within these areas on shoreline streams. Shoreline streams are defined as those with a mean annual flow equal to or greater than 20 cfs. While many channel migration studies and CMZ delineations have been done in Washington State, nearly all have been detailed assessments. These CMZ delineations are more rigorous then required by the state SMA administrative codes, which emphasize planning-level assessments. The rigorous studies are cost-prohibitive to implement for all regulated shoreline streams in the state. The SMA and its administrative codes provide no guidance on planning-level CMZ delineation methods. The Washington Department of Ecology (Ecology) developed a planning-level CMZ delineation (pCMZ) method to support local communities’ updates and implementation of the SMA requirements. Ecology developed the pCMZ method through a process of: (1) initial pCMZ method development; (2) application and refinement of the method over 900 stream miles near the Puget Sound; and, (3) further refinement through comparison of CMZs mapped using the planning-level approach to CMZs mapped using detailed CMZ methods. The pCMZ method uses the nature and extent of valley bottom features to assess past and potential future channel migration, and then define CMZ boundaries. This document describes the pCMZ approach in context of Washington State regulations.