Introduction The North Fork of the Little Snake River is a steep, rough, regulated headwater stream located in the Colorado River basin in southwest and south-central Wyoming. A water development project, the Cheyenne Stage II Diversion Project began in 1983 in an effort to collect 23,000 acre feet of water annually from 30 tributaries of the North Fork of the Little Snake River for the city of Cheyenne, Wyoming. The North Fork and its tributaries support the largest known, essentially pure population of the Colorado River cutthroat trout, (Salmo clarki pleuriticus Cope) (Binns, 1977). This species is classified as "Endangered" (Utah), "Threatened" (Colorado) and "Sensitive" (Wyoming), and has been listed in the "Special Concern"category of the American Fisheries Society.
Earlier work on the effects of Stage I of the Cheyenne Diversion Project demonstrated that the Colorado River cutthroat standing crop had been reduced to 37% of its former levels (Jespersen 1981). While the factors resulting in this impact were not independently analyzed, Jespersen (1981) recommended that monitoring the impact of construction of roads, pipelines, and diversion structures should emphasize the potential damage caused by increased sediment on fish and macroinvertebrate populations. This recommendation appears to be well-founded based on the work by Brusven and Prather (1974), Leudtke et al. (1976), and others which has demonstrated that aquatic insects are sensitive and reliable indicators of sedimentary pollution and stream quality.
During the late summer of 1984, intensive rainfall in the construction area resulted in the deposition of a broad size range of sediments in a section of stream where flushing flow recommendations (removal of sediments by high discharge) had been made (Wesche et al. 1985). The introduction of this sediment into the North Fork presented an excellent opportunity to study the impacts of sedimentation and flushing flows on the aquatic macroinvertebrates in the North Fork of the Little Snake River as recommended by Jespersen (1981).
Beginning in July of 1985 and ending in September of 1987 collections of the aquatic macroinvertebrate community were taken to assess the impact of the addition of this sediment on the aquatic macroinvertebrate community. To best ascertain the degree of impact on the North Fork of the Little Snake River the objectives of the study were to: 1) describe the changes in selected biological indices (diversity, evenness, richness and abundance) between impacted and non-impacted sites; 2) determine the preferences of the aquatic macroinvertebrate taxa for selected substrate sizes; 3) determine the preferences of the aquatic macroinvertebrate taxa for mean water velocity; 4) determine the preference of the aquatic macroinvertebrate taxa for water depth.
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