Recovery of aquatic macroinvertebrates from the effects of mine drainage was documented using a 'weight-ofevidence' approach which included measures of physical, chemical, and biological data. Taxa richness; number of taxa in the orders Ephemeroptera, Plecoptera, and Trichoptera; and shredder taxa richness all increased downstream of the point source after water treatment was initiated. Cluster analysis of aquatic macroinvertebrate community data along with abundance of a metals sensitive mayfly (Rhithrogena hageni) also suggested recovery from metals effects. Response to decreased metal inputs was rapid and biological measurements of impacted sites attained levels comparable to upstream reference sites in two years. Our results suggest that aquatic communities impacted by metals, in the absence of degraded habitat and with nearby colonist pools, will recover quickly if low instream concentrations of toxicants are achieved.

Water quality, types, and diversity of aquatic macroinvertebrates show that heavy metal pollution of the upper Arkansas River is presently moderately severe and conditions for aquatic life are generally poor, particularly in the Leadville area. A 2 year study was done on a 30 km section of the upper Arkansas River, Colorado, to determine the effects of heavy metals pollution on the distribution of the aquatic macroinvertebrates. Physical and chemical water parameters were measured, and aquatic macroinvertebrates were collected. The major sources of heavy metal-laden inflows are Leadville Drain, California Gulch, and a number of intermittent flows entering the Arkansas River between Lake Fork and Lake Creek. Important freshening flows are Tennessee Creek, Lake Fork (containing Halfmoon Creek), and Lake Creek.

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Macroinvertebrates from the upper Arkansas River were studied t o i d e n t i f y taxa s e n s i t i v e t o heavy metal ( z i n c ) impacts. F i e l d c o l l e c t i o n s and introduced substrates i n d i c a t e d t h a t Pithrosena haaeni was i n t o l e r a n t o f c o n d i t i o n s caused by mine drainage i n t o t h e Arkansas River. Numbers o f 8. haqeni a t the impacted s i t e increased s i g n i f i c a n t l y a f t e r treatment o f mine drainage water was i n i t i a t e d . Data c o l l e c t e d from t h i s study suggest t h a t the use o f colonized substrates may be useful i n studying metal impacts on macroinvertebrate communities.

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