WWRC 88-05
Impact of Sedimentation on the Aquatic Macroinvertebrates
of the North Fork of the Little Snake River
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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