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. This sampled sites dataset represents the most downstream location 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 to the SAM sites, 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 included in this analysis to provide a regional context for the SAM study. In total, downstream locations for 121 sites are provided in this data release. SAM sample sites were selected from the Washington State Master Sample Site List (http://www.ecy.wa.gov/programs/eap/stsmf/mss.html) generated for Puget Lowland ecrogregion streams that drain to Puget Sound. Sites were specified for one of two location categories: inside the Urban Growth Area (UGA) and outside the UGA boundaries. The goals of site selection were to have 50 sites within each of these two categories and to select small to medium size streams (zero-third order) with perennial freshwater flow in natural substrate channels (no canals, lined or concrete ditches).