Characterization of trace elements in water, bed sediment, and biota is essential for planning effective and cost-efficient remediation in watersheds affected by historical mining and for providing baseline data to document the effectiveness of watershed-restoration efforts. To aid cleanup efforts on Federal lands, abandoned-mine-land investigations were conducted in the Boulder River watershed in southwestern Montana. Characterization of ore-related trace elements in water and bed sediment in this watershed provided information to (1) delineate stream reaches having elevated trace-element concentrations, (2) determine the sources of material contaminated with trace elements, (3) understand the transport of dissolved and particulate trace elements, (4) estimate pre-mining concentrations of trace elements, and (5) evaluate the potential for trace-element toxicity to biota. Biological assessment in the watershed identified trace-element exposure and bioaccumulation as measured by concentrations in biofilm (abiotic and biotic material on rock surfaces), invertebrates, and fish. This metals exposure likely explains the impairment of aquatic health documented by tissue damage in livers, smaller fish size, and reduced fish populations observed in one stream reach and the elimination of fish in several reaches. On the basis of relative enrichment of dissolved concentrations above aquatic-life standards, zinc appears to be the most elevated and, presumably, the most toxic ore-related trace element in water. In contrast, relative enrichment of leachable solid-phase concentrations above screening-level criteria indicates that copper, lead, and arsenic likely are the most toxic ore-related trace elements in bed sediment.