The U.S. Geological Survey’s StreamStats program is a publicly-accessible web application (https://streamstats.usgs.gov) that can be used to delineate drainage areas, compute basin characteristics, and estimate flow statistics for user-selected locations on streams. StreamStats services are typically implemented at the statewide or watershed scale (referred to as state or basin applications), and although the three core functionalities remain consistent, many states have implemented custom tools to address specific water-resources planning and management needs. In Massachusetts, a watershed-scale application for the Mystic River Basin was developed to support stakeholder efforts to address stormwater challenges in this highly urbanized basin. The Mystic River Basin stormwater functionality was developed by incorporating 1-meter resolution lidar-derived elevation data and municipal storm drain data to accurately represent urban topography and stormwater flow (that is, subsurface piped flow). In the Mystic River Basin application, users can view the network of stormwater pipes and inlets, delineate drainage areas derived from lidar topography and stormwater infrastructure, and compute land-use/land-cover basin characteristics. This data release contains the 1-meter resolution digital elevation model (DEM; dem.tif) and two datasets derived from the DEM that support on-the-fly watershed delineation in the StreamStats web application. The flow direction raster (fdr.tif) is a raster dataset that indicates the direction of flow out of each cell; if the cell contains a stormwater inlet, it is represented as a sink in the flow direction raster. The catchment raster (cat.tif) represents the drainage areas to stormwater inlets and to surface-water flowpaths within the basin. The flow direction and catchment rasters are used in conjunction with the stormwater network to determine the drainage area to a point of interest selected by the user in StreamStats. This point must lie on the stormwater network, at either an inlet, on a pipe, or on a surface-water flowpath. The delineation produced in StreamStats is the accumulation of all catchments draining to the point of interest. To describe the processing steps used to produce the DEM, fdr, and cat rasters published in this data release, the overall approach to developing the Mystic River Basin stormwater functionality is given in the associated metadata. Please note that the stormwater network, comprised of stormwater inlets, pipes, culverts, and surface flow, produced for this study is not available for publication due to sensitivity concerns. Inquiries about these data may be made to the point of contact provided in the metadata.