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mitigated major damaging effects to the aquatic macrofauna. The Bureau of Reclamation has scheduled two actions that may affect the upper Arkansas River. They are: (ll plugging the Leadville Drainage Tunnel to eliminate a serious heavy metal pOllution source, and (2) diverting much of the flow of Lake Fork and Halfmoon Creek to the forebay of the Mt. Elbert Pumped-Storage Powerplant. However. according to recent studies by the Bureau of Reclamation (1979) 1191 and Turk and Taylor (1979) 11511. plug. ging the Leadville Drainage Tunnel would have little effect on the water quality of the local aquifer or the quality of the water eventually entering the upper Arkansas River via Evans or California Gulches. In contrast, the diversion of Lake Fork and Halfmoon Creek will reduce the dilution in the river, and heavy metal con- centrations will likely increase. Based on a recent study, Moran and Wentz (1974) [1131 wrote: "Further degradation of water quality in the Arkansas River could result if planned diver- sions of water from Halfmoon Creek and the Lake Fork of the Arkansas River are implemented by the U.S. Bureau of Reclamation. Such measures would greatly reduce the flow of these streams, which presently help to dilute the metals and acid from California Gulch." The Bureau has modified project operations and agreed to provide minimum bypasses which will mitigate damaging heavy metal concentrations that may be present in the upper Arkansas River. The major purpose of this study. a follow-up of previous work done by LaBounty et al. (1975) 1941. was to determine current water quality and macroinvertebrate abundance data, and to evaluate the effects of the water quality on the distribution of the aquatic macrofauna. This in- formation will be useful in determining the im- pacts of the proposed actions on the ecology of the upper sections of the river. APPLICATION The results of this study will be of interest to those involved in the assessment of water pollu- tion in mining areas, and of particular interest to those concerned with the problem of heavy metals contamination of streams in the Colorado "Mineral Belt." CONCLUSIONS , . General water quality conditions, including physical parameters and major ion concentra- tions, but excluding heavy metal concentra- tions, were acceptable for aquatic life throughout the study area. 2. Nitrogen and phosphorus levels were apparently high enough to promote some primary production. 3. Most metal concentrations at stations EF-2 and AR-3, immediately downstream of the Lead- ville Drainage Tunnel and California Gulch, respectively, are highly correlated with concen- trations from these sources. 4. Concentrations of all metals studied. and particularly iron and lead. are highly dependent on flow and tend to increase during high spring runoff. This would suggest that these metals are abundant in sediments and are suspended during high flows. 5. Concentrations of zinc are consistently above documented toxic levels for fish at all sta- tions except EF-' . 6. Concentrations of heavy metals decrease below the confluence of Lake Fork. except during the high runoff period. This indicates that during the high runoff period metal-laden sediments are scoured from the area of the California Gulch inflow and moved downstream. 7. Freshening inflows are essential in maintain- ing satisfactory water quality for the production of aquatic macrofauna. 8. High spring runoff may be essential in improving the substrate for aquatic macroinvertebrates by removing metal-laden sediments from the upstream areas. 9. Mean diversity indices of aquatic macroin- vertebrates were lower below the Leadville Drainage Tunnel, California Gulch, Iowa Gulch, some small intermittent inflows. and above the Lake Creek inflow. The smaller values above the Lake Creek inflow were probably due to a difference in substrate. , O. Diversity indices of aquatic macroin- vertebrates were higher below the inflows of Tennessee Creek, Lake Fork, Box Creek, and Lake Creek. 4