ScholarWorks at University of Montana

The Aquatic Invertebrates of the Upper Clark Fork River, 1972-1984

Benthic invertebrate communities of the upper Clark Fork River were sampled yearly from 1972 to 1984. Invertebrates were collected in fall using a modified Hess sampler at seven stations on the upper Clark Fork River, Mill and Willow Creek and Silver Bow Creek. The upstream control station on Mill and Willow Creek (CFRO) exhibited a balanced, diverse invertebrate community throughout the study. However, during the early part of the study no invertebrates were collected at the other headwater station on Silver Bow Creek (SBC5). Following improved mine wastewater treatment upstream of SBC5 in 1972, insects were eventually collected in 1975 and benthic densities and number of taxa have generally increased through 1984. Nonetheless, over the last 4 years, the densities (3708/m2) and number of taxa (26) in SBC5 have been considerably lower than those at CFRO (12 356/m2 and 57, respectively), The stations below the outflow of the Warm Springs Ponds have exhibited a noticeable response to the outflow of plankton from the ponds with benthic densities exceedil1g 60000/m2. These communities are composed primarily of filter feeding insects, both hydropsychid caddis flies and black flies. At the downstream stations, densities and number of taxa approach those originally observed at CFRO. However, the species composition at these lower stations 18 more typical of a large, low-gradient river rather than the headwater stream community found upstream.