The Bureau of Reclamation began collecting samples from a variety of sites in the Dolores River basin of Colorado in 1989. Sampling began after elevated mercury levels observed in fish in Reclamation's McPhee Reservoir prompted the Colorado Department of Health to post warnings against the consumption of large amounts of fish from the reservoir. The purpose of the sampling of water and sediment was to identify the source of the mercury. The results of the sampling effort are to be reported here. Samples were first collected for the Dolores River Mercury Study in September 1989. Both water and sediment samples were collected. Water samples were analyzed by Casa del Sol Laboratories in Durango, Colorado. The sediment samples and two splits of the water samples were analyzed by the USGS Geochemistry Laboratory in Denver. Subsequently only water samples were collected. Field determinations included discharge, water temperature, electrical conductivity, and hydrogen-ion concentration (pH). Samples were analyzed for total dissolved solids (TDS), major cations, and anions and a variety of heavy metals and other trace elements. In addition to mercury, analytes (potential contaminants) included: Silver, aluminum, arsenic, boron, barium, beryllium, cadmium, chromium, copper, iron, manganese, nickel, lead, selenium, thallium, vanadium and zinc. On occasion, water sample analyses also included cobalt, lithium, silicon, tin, and uranium Sediment samples were analyzed for major cations and the trace elements above.