2025 Riparian Management Zones Map: A User Guide for Local Governments from WDFW’s Priority Habitats and Species Program

Riparian areas occupy the transition zone that begins at the stream’s edge and extends upland. These areas provide habitat to an estimated 85% of Washington's terrestrial vertebrate species, and riparian vegetation provides critical ecosystem functions that support and protect instream health. Protection of these areas is especially important for salmon recovery efforts, and other decision-making affecting threatened or endangered species. For these reasons, WDFW identifies riparian habitat as a Priority Habitat and recommends that local governments designate and protect Riparian Management Zones (RMZs) as a Fish and Wildlife Habitat Conservation Area (FWHCA), a type of critical area under the Growth Management Act.

After extensive review of available evidence, WDFW identified a method to define the upland RMZ boundary according to the lateral distance over which essential riparian functions are provided. This Site Potential Tree Height at 200 years (SPTH_200yr) is the estimated average height attained by dominant trees in that location after 200 years of growth. In addition to serving directly as shelter and sustenance to many species, trees contribute shade, leaf litter, and large woody debris to streams; they also reduce pollutants, attenuate destructive overbank flooding, and enhance many forms of recreation along watercourses. In addition to these functions, in-tact RMZs also provide direct benefits to surrounding communities by filtering pollutants, stabilizing streambanks, reducing flooding, and supporting overall watershed resilience. Tree height therefore provides a generalized yet flexible means to estimate the distance over which riparian functions are provided.

The SPTH_200yr estimate is obtained by identifying the tallest species of tree growing in the area and the soil type. The RMZ’s width is a distance of one SPTH_200yr measured from the stream’s bankfull width, active floodplain, or outer edge of the channel migration zone (CMZ), whichever is wider. In places where tree height is less than 100 feet, the RMZ width is set to a minimum distance of 100 feet to account for the pollutant removal function provided by RMZs. As described in Riparian Ecosystems Volumes 1 and 2, a 100-foot RMZ typically provides 95% pollution removal for phosphorus, sediment, most pesticides, and approximately 85% for surface nitrogen. Because we found no science that justified lesser protections for non-fish-bearing streams, we recommend that RMZ protections be provided to the entire stream network – this is a departure from widespread practice of protecting a narrower RMZ for non-fish-bearing streams.

The PHS RMZ map is intended to provide local jurisdictions with estimated locations of RMZs delineated based on a SPTH_200yr standard to support the designation and protection of critical areas. The map was created using best available scientific data estimating stream locations, likely tree species, and the maximum tree height at 200 years for streams across all of Washington. The map represents estimated locations only and site-scale delineation of an RMZ critical area requires field verification.