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Geochemical studies of minus-80-mesh size fraction of streamsediment samples collected from the upper Arkansas River drainage basin in Lake County, Co. , show that the effect of mining activity, both past and present, on heavy-metal concentrations in the river can be readily documented. Application of geochemical leaching procedures often used in mineral exploration enhances the effect of acid-weathering caused by the oxidation of pyrite. Lead-isotope measurements of leaches of irregularly spaced stream-sediment samples clearly delimit the extent of elevated heavy-metal concentrations in the upper Arkansas River caused by mineralization at Climax, Leadville, and to a lessor extent, at St. Kevin Gulch. All leaches of the stream-sediment samples collected from either tributaries of Arkansas River, or from the Arkansas River itself within the boundaries of the Leadville mining district, have leadisotopic compositions that match those of published lead-isotope data from the ores at Leadville. In contrast, leaches of all stream-sediment samples collected outside the district within the upper Arkansas River drainage basin do not match the lead-isotopic signature of ores from the Leadville mining district. Detailed studies of the stratigraphy of four cores taken in the Arkansas River flood plain near the confluence with California Gulch in the Leadville mining district document high metal concentrations in an area immediately south of the confluence where heavy-metal-laden tailings from the Leadville mining district have been deposited. 210Pb geochronological studies of these cores show that the tailings in the Arkansas River were deposited over a twenty-year period. Further work is required, however, before definite dates can be assigned to the age of deposition of these tailings. The lead-isotope data and the 210Pb studies allow determination of the heavy-metal concentrations present in the Arkansas River drainage prior to mining at Leadville. Pre-mining concentration of lead in minus-80-mesh stream sediments was about 39 ppm, in sandy sediments about 29 ppm, and in soil samples collected from beneath the tailings deposit, about 38 ppm. Premining concentrations of zinc were 158 ppm, 64 ppm, and about 200 ppm in these sample media. Pre-mining concentrations of copper were about 14 ppm, 10 ppm, and 17 ppm, respectively. Enrichment of various heavy metals in the stream sediments from the Leadville mining district range from 2 to 215 times normal crustal abundance with lead being the most enriched. Core samples of sediments in the Arkansas River drainage derived from tailings from the Leadville mining district have enrichments of heavy metals ranging from 63 to 944 times crustal abundance. The field and analytical methods used in this study demonstrate methods applicable to environmental geochemical perturbations caused by mining and industrial activity worldwide.