A synoptic sampling campaign was conducted on the Animas River near Silverton, Colorado, under low flow conditions in September 2021. The sampling campaign was designed to quantify constituent loading and identify sources of contamination along a 3.8-kilometer study reach. The study reach began approximately 170 meters upstream of Arrastra Creek and extended downstream to U.S. Geological Survey (USGS) gage 09358000 within the city of Silverton, Colorado. A continuous, instream injection of a sodium bromide tracer was initiated at the head of the study reach three days prior to the start of the sampling campaign and maintained until the completion of main stem sampling. Bromide concentrations were subsequently used to determine streamflow using the tracer-dilution method. Water quality samples were collected at 23 sites along the Animas River main stem, and 28 inflow sites including springs, seeps, small tributaries, and ponded water. Main stem sites were sampled using three sampling approaches. Under the first approach, a subset of 8 main stem sites were sampled "simultaneously" (in less than 20 minutes) to assess the effects of diel variation in constituent concentration. Under the second approach, all main stem sites were sampled with the sampling team working in the downstream-to-upstream direction, a protocol typically used during synoptic sampling. A subset of 5 main stem sites were also sampled using an Equal Discharge Increment approach that was designed to indicate which side of the stream was responsible for the observed constituent loads. This data release includes field parameters (water temperature, pH, and specific conductivity), concentration data (inorganic cations and anions), estimated streamflow, and calculated loads for the sampling campaign. Calculated loads may be used to identify and rank sources of contamination to the Animas River. The data release consists of a kmz file showing site locations and the following 3 tables: Table 1, Locations of sampling sites Table 2, Synoptic sampling results, September 20-21, 2021 Table 3, Spatial profiles of streamflow and constituent load