We report the results of a synoptic (“snapshot”) sampling of inorganic and isotopic geochemistry of surface water and groundwater during base flow in Red Canyon Creek watershed (Wyoming, USA) to evaluate how mixing of waters and geochemical processes modify stream-water chemistry. Our synoptic approach of studying the geochemistry of a stream mimics and has the same strengths of that widely used to characterize geochemical processes in groundwater systems. Gypsum dissolution, carbonate precipitation, and the influx of tributary and groundwater all affect Red Canyon Creek stream-water chemistry. Identical ranges of hydrogen and oxygen isotopes show good hydraulic connection between surface water and shallow groundwater. In contrast, δ34SSO4 values of groundwater (15.2‰ ± 1.6‰) and surface water (11.1‰ ± 1.6‰) reflect sulfate mostly contributed by gypsum dissolution. Calcium contributed by gypsum dissolution causes calcite to rapidly precipitate in crusts observed in the streambed. This process occurs even though residence time of water in the stream is orders of magnitude less than that found in groundwater systems showing the same kind of common ion effect. Although microbes oxidize organic matter by sulfate reduction behind beaver dams and in the hyporheic zone, too little of this water reaches the stream to measurably affect the sulfur isotopic signature of its water. The results of this study suggest that calcareous accumulations in the fluvial sedimentary record, in association with gypsum, may possibly be paleohydrologic proxies for environments where mixtures of surface water with different chemical composition occurred.