Stormwater Action Monitoring (SAM) is a collaborative monitoring program between western Washington municipal stormwater permittees, state and federal agencies. SAM’s role is to use the results of regional monitoring and focused studies to inform policy decisions and identify effective strategies to improve stormwater management in the Puget Sound region. The SAM program includes status and trends monitoring of water quality, stream biota (macroinvertebrates, algae), and stream habitat to measure whether conditions are getting better or worse and identify patterns in healthy and impaired Puget Lowland streams. To meet this objective, a framework of fundamental geospatial data was required to develop physical and anthropogenic characteristics of the study region, sampled sites and corresponding watersheds, and riparian zones. The riparian-zone boundaries were created from stream centerlines digitized from imagery (hereinafter the "digitized riparian reach") that were buffered by 50 meters on each side of the stream centerline. The length of the digitized riparian reach was calculated as the distance in kilometers equal to the base-10 logarithm of the geospatially-derived watershed area, in kilometers squared (Johnson and Zelt, 2005). This dataset represents the riparian zone boundaries from 105 sites sampled for the SAM small stream study, and is one of the four fundamental geospatial data layers that were developed for this study. In addition, riparian zone boundaries for 16 reference sites in the Puget lowlands sampled by the Washington State Department of Ecology’s Ambient Biological Monitoring program from 2010 to 2015 were also digitized for this analysis to provide a regional context for the SAM study. In total, riparian zone boundaries for 121 total sites are provided in this data release.