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Mineralogical and chemical analyses of sediment in the Snake River arm of the Dillon Reservoir provided insight into the transport and fate of trace metals in this system, which provides potable water to the city of Denver in Colorado. The sediments contain metal hydroxide precipitates and sorbed trace metals that formed upstream in the Snake River and in Deer Creek, Peru Creek and other tributaries. Iron and trace metal concentrations in the sediments reach a maximum about halfway down the 3.3 km arm, with their distribution probably controlled by physical processes acting on the settling of precipitates entering the arm from the mouth of the Snake River. The concentration of molybdenum is at background concentration near the mouth of the Snake River, increasing towards the centre of the reservoir, where values are 10 times greater, owing to contamination from the Ten Mile Creek and Blue River arms of the reservoir. The results of a sediment-leaching experiment demonstrated that the relative ease of extraction with a decreasing pH value in the sediments is zinc > cobalt > copper > nickel > lead. Although most of the trace metals are low in concentration, or are strongly sorbed in the sediments, the relatively high concentration of zinc and its relatively high mobility make it a metal of potential concern in the reservoir. [ABSTRACT FROM AUTHOR]