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The Dunn Creek site is at the same location as Station 3 in the pre- impoundment study (Bonde and Bush 1975). Six benthic invertebrate samples were taken at each station each month (except December and February) for a one-year period. The sampling was done when releases from Libby Dam were at or near the operational minimum flow of 4,000 cfs. Water depths and mean current velocity measured at the 15.2 cm depth were taken just upstream from each benthic sample. During the first year (October 1979 - September 1980), 252 quantitative benthic samples were collected and analyzed numerically and volumetrically. An additional 36 quantitative benthic samples were collected during October, 1980 and January, March and May, 1981. Three samples were taken at each station and three different samplers were used to reduce biomass. The modified Knapp-Waters sampler (Waters and Knapp 1961) which was used in preimpoundment studies (Bonde and Bush 1975) was used at the Dunn Creek, Elkhorn and Fisher River sites. It could not be effectively used at the Pipe Creek station due to the very large substrate size at that site. The other two samplers employed, a modified kick net and a circular depletion sampler (Carle 1976) were designed for use in large substrates. Both of these samplers enclosed a sample area of 0.33 m2 and have a mesh size of 150 ~m, compared with a sample area of 0.093 m2 and a mesh size of 471 ~m for the Knapp-Waters sampler. The circular sampler was used at the Pipe Creek site in place of the Knapp-Waters sampler and the kick net was used at all four sites. After the sampler was placed over the substrate, rocks within the enclosed area were lifted and brushed free of insects by hand. After all rocks were cleaned and removed, the remaining substrate was disturbed by kicking for 15 seconds. Insects were collected in the cod end of each sampler. These insects were removed and placed in jars containing 10 percent formalin and Rose Bengal stain. Laboratory analysis of each sample included placing the sample in a white porcelain tray and sorting out all insects larger than 2 mm in length. These larger insects were identified to order and preserved in separate vials. The remaining sample was divided into eight subsamples and all insects removed from one subsample. These insects were sorted to order and preserved in vials. Occasionally, a larger or smaller sub- sample was taken depending upon the mass of insects in the total sample. All insects except dipterans were identified to the lowest taxonomic group possible and enumerated using a laboratory counter. Dipterans were sorted into Chironomidae and other taxonomic groups and counted. Biomass was measured by volume displacement, with any volume less than 0.1 ml assigned a trace value of 0.05 ml. Volumetric measurements were made with the use of a 50 milliliter burette and a graduated centrifuge tube. Adult insects were collected at all four sample stations from rocks and vegetation with sweep nets or by hand. Pit traps (buried cans con- taining formalin covered with a thin film of diesel fuel to prevent -3-