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Collecting sediment core samples on Dillon Reser- voir. (Photograph by Norman Spahr, U,S. Geological Survey.) Mining activities have affected trace-element concentrations and aquatic invertebrates in French Gulch and the Blue River. Sam- ples from different media were used to investigate the effects of mining activities on the water qual- ity and associated biota at 10 sites along the Blue River and French Gulch, a tributary of the Blue River (fig. 12). Sites 2 and 3 (fig. 12) are background sites on French Gulch and are minimally affected by min- ing activities. Zinc concentrations in water and streambed sediment and zinc accumulated by aquatic moss at these background sites were lower than those from sites downstream from the mined areas. The invertebrate community struc- ture (as indicated by mayfly abun- dance and percent midges) was minimally affected at the back- ground sites. Although habitat conditions at site 4 were degraded and zinc con- centrations were greater than those upstream, the invertebrate commu- nity was not found to be signifi- cantly degraded. Farther down- stream, where underground mine seepage contributes to surface run- off (site 6), zinc concentrations in the water, streambed sediment, and aquatic moss were high and the invertebrate communities were degraded. The most affected site along French Gulch was site 7, located near the confluence with the Blue River. Site I, upstream from French Gulch on the Blue River, was char- acterized by lower zinc concentra- tions in water, moss, and sediment than downstream sites, indicating that the French Gulch basin is con- tributing zinc to the Blue River. Downstream from the confluence of French Gulch with the Blue River (site 8), concentrations of zinc in water, moss, and streambed sediment were higher than back- ground levels but slightly lower than in French Gulch. Also at site 8, brown trout livers had the high- est concentrations of zinc of any sites sampled in the Southern Rocky Mountains (Deacon and Stephens, 1998). The invertebrate community in the Blue River downstream from French Gulch was degraded (site 8). The site con- ditions remain moderately affected downstream to Dillon Reservoir (site 10). Mining activities have affected trace-element concentrations in the bottom sediment of Dillon Reservoir. Dillon Reservoir was constructed in 1963 in an area with a long history of mining activity. Sources of trace elements in Dillon Reservoir, a drinking-water supply for the city of Denver, are located in the Blue River, Snake River, and Tenmile Creek Basins. The reser- voir is accumulating some trace elements. For example, loads calculated using streamflow and water concentration indicate that only 37 percent of the zinc that enters Dillon Reservoir leaves the reservoir (fig. 12, top left graph). Sediment cores were collected at several locations in Dillon Reser- voir. Concentrations of lead and zinc throughout the core collected near the dam exceeded the PEL. Cadmium, lead, and zinc concen- trations in sediment cores from the Blue River, Snake River, and Ten- mile Creek arms of the reservoir also were above the PEL (values for zinc are shown in the top of fig. 12). Although bottom sediment con- centrations of some trace elements exceeded the PEL, concentrations in the water column were not high. The concentrations of trace ele- ments in the reservoir water col- umn did not exceed the Colorado surface-water-quality standards. Sediment core from Dillon Reservoir. (Photograph by Norman Spahr, U.S. Geological Survey.) Major Findings 13