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Puget Sound Stormwater Action Monitoring Basin Characteristics

February 11, 2020

The Stormwater Action Monitoring (SAM) program is a cumulative and regional monitoring effort in the Puget Sound region of Washington State. Under the SAM program, the status and trends of water chemistry, sediment chemistry, habitat and biota in small streams and nearshore marine waters in the Puget Lowlands are monitored and documented. As part of recommendations to the program made in 2018, site selection in 2019 focused on small watersheds ranging in size from 2.5-50 km2 with varying amounts of urbanization. A master list of 19,970 possible sampling sites was digitally snapped to the National Hydrologic Dataset (NHD) high-resolution version flowlines (1:24000), and from this a set of characterized nested basins were automatically delineated. These nested basins were then characterized by their land cover types using the 2016 National Land Cover Dataset (NLCD). To create the 19,970 basins, a custom ArcGIS tool was created to leverage the Water Boundary Dataset (WBD) at the Hydrologic Unit Code 12 level (HUC 12). At the single HUC 12 level, the basin area is derived using the watershed tool in ArcGIS with a hydrologically conditioned 10-meter digital elevation model (DEM). If the basin area extends beyond the boundaries of a single HUC 12, upstream HUC 12s can be recursively found and added to the basin utilizing the 'ToHUC' field, which is a standardized and quality-controlled field available within all WBD datasets. The NLCD-based basin characteristics and upstream area are then computed and added to the finalized basin boundaries to create a filterable table for site selection. Nearshore basins were obtained from the Puget Sound Characterization project and characterized according to assessment unit. Because these units account for issues unique to the nearshore environment, such as tidal influence and longshore currents, and interfluvial zones, they provide the most accurate characterization for in the nearshore environment.