Mitigation for the Construction and Operation of Libby Dam

We believe that Libby Reservoir during the previous 12-15 years has stabilized in
terms of biological production. Total fish abundance, as indexed by trends in gill net catch
rates have stabilized since 1988. Fish and zooplankton species composition and abundance
have also experienced similar trends. Mountain whitefish, rainbow trout and westslope
cutthroat trout abundance all exhibited dramatic decreases in abundance (Figure 28)
following the first ten years after reservoir filling, but have stabilized at much lower
abundances than the pre-dam period. Fish species composition also shifted during the first
10 years after reservoir construction, but has also stabilized. Zooplankton abundance,
species composition, and size distribution have also all been similar during the second half
of the reservoir’s history. We attribute these trends toward trophic equilibrium to the aging
process of the reservoir (Kimmel and Groeger 1986) and the operational history of Libby
Dam during the past 15 years.
Although all of the statistically significant trends observed for our
macroinvertebrate monitoring on the Libby Creek Upper Cleveland Project are consistent
with declining ecological integrity, the declines were relatively minor. The direction of
change and taxa involved were consistent with changes caused by mild sedimentation,
but were probably due to adjustments from an exceptional year at the beginning of the
study and a return to more natural conditions. That is, the exceptionally low abundance of
collectors in 2000, may have be responsible for some of the observed changes. The site
will be monitored to ensure that sedimentation is not an issue. However, before we
disregard the potential impairments, we should note that the restoration was relatively
recent and there may be some associated ecosystem disturbance. Nonetheless, the
evidence in this study suggests that these effects, if present are relatively minor.
Two of the most tangible macroinvertebrate metrics that can be used to assess
ecological integrity are Taxa Richness and EPT Richness because they summarize how
many taxa (species, genera, families) inhabit the site. Both of these measures showed
subtle improvements that were not statistically significant. We ran a post hoc power
analysis and found that we had 81.2% power to detect a difference between the taxa
richness of the 2 years at the Libby Creek site. Similarly, we found that the study had
27% Power to detect differences in EPT Richness. For future reference, we ran an
analysis to show how replication could have improved the ability to detect differences in
these two metrics (Figures 46 through 48). Based on these results, we will further
evaluate the utility of modifying future sampling to improve our ability to detect
differences between years.
The rapid bioassessment classification we used for the Libby and Grave Creek
classified both sites as moderately deviant from mountain reference conditions. It is
important that these results are interpreted conservatively because they were not collected
using the same procedures used to calibrate Marshall and Kerans (2003) biocriteria. In
particular, the method used by FWP may increase the collection of midges (Diptera,
Collector-gatherers), and be partially responsible for moderately deviant classification
rating. However, it is also likely that the results could be indicative of sediment problems
upstream in the watershed.
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