AbstractBenthic macroinvertebrate assemblages, environmental variables, and associated mine density were evaluated during the summer of 2000 at 18 reference and test sites in the Coeur d’Alene and St. Regis River basins, northwestern USA as part of the US Geological Survey’s National Water-Quality Assessment Program. Concentrations of Cd, Pb, and Zn in water and (or) streambed sediment at test sites in basins where production mine density was ≥0.2 mines/km2 (in a 500-m stream buffer) were significantly higher than concentrations at reference sites. Zn and Pb were identified as the primary contaminants in water and streambed sediment, respectively. These metal concentrations often exceeded acute Ambient Water Quality Criteria for aquatic life and the National Oceanic and Atmospheric Administration Probable Effect Level for streambed sediment. Regression analysis identified significant correlations between production mine density in each basin and Zn concentrations in water and Pb in streambed sediment (r2 ...