This study measured selenium concentrations and loadings in the Gunnison River, Colorado River, and tributaries in Mesa County, Colorado. Samples were collected from the rivers using a Van Dorn bottle and from tributaries by wading and collecting along transects. Samples were analyzed for selenium using ICP-AES. All tributary samples exceeded the state standard of 4.6 μg/L, with concentrations up to 137 μg/L. The rivers were below the standard due to dilution from tributary inflows. High tributary selenium originates from contact with Mancos Shale soils and enters waterways during irrigation season, posing a risk to aquatic life.