Mitigation for the Construction and Operation of Libby Dam

Discussion
The Grave Creek Phase I and Libby Creek Upper Cleveland Restoration Projects
maintained the designed channel dimensions after each stream experienced the spring
freshet after project construction. Streams with C3 channel types should have
width/depth ratios >12 and typically range from 10-37 (Rosgen 1996). Both the Libby
and Grave Creek restoration projects were designed within this criterion and continued to
meet it after the channel forming flows that occurred during the first spring freshet
following project construction. The stream channel dimensions measured at the crosssection
surveys on both the Libby Creek Upper Cleveland and Grave Creek Phase I
Restoration Projects did not significantly (p > 0.05) change after the first spring freshet.
Mean bankfull width, depth, maximum depth, width to depth ratio and cross sectional
area all remained similar between years. Pool dimensions within both projects were
similar between years. However, mean and maximum depth decreased after each project
was subjected to bankfull channel shaping flows during the spring freshets. Although we
did observe a decrease in pool depths after project construction, pool depth, quantity and
quality still exceeded conditions that existed prior to project construction. We believe
the observed changes in the pools that occurred as a result of the spring freshet on both
Libby and Grave Creeks had relatively minor effects on the quantity and quality of
juvenile salmonid rearing habitat, and that overall the stream channels are existing at a
state near dynamic equilibrium. In addition to the minor changes in pool depth, we did
observe an overall decrease in riffle slope after the spring freshets that were common for
both the Grave and Libby Creek projects. The changes were primarily associated with
the cobble gradient control structures that were placed in the pool tail out areas. These
were the first two stream restoration projects that the cobble gradient control structures
were utilized, and help point out that the science of natural channel design is still
evolving, with improvements in structure design occurring in this relatively new science.
We believe that these particular structures can serve a useful purpose, but that design
modification could improve their function. We currently recommend designing these
structures between 1.25 to 1.5 times the mean bankfull riffle width. We further
recommend the run width below the cobble gradient control structures be designed at 0.8
to 1.0 times the mean bankfull riffle width. The cobble gradient control structures were
constructed with material that was between D90 to D100 sized substrate within each
particular stream. We will continue to monitor the performance of the cobble gradient
control structures and modify their design criteria accordingly.
We monitored the benthic macroinvertebrate and fish communities at the Libby
Creek Upper Cleveland Project in order to evaluate the ecological response the
restoration activities (see Chapter 1). Our results were somewhat counter intuitive. Of
the seven metrics selected before the study, 3 had values in 2003 that were significantly
different from the values attained from the 2000 sampling. However, they were all in the
opposite direction expected. That is, we expected the samples to reflect an improvement
after restoration, but the directions of change for the three significantly different metrics
were consistent with disturbance. Similarly, redband trout abundance decreased by an
average of 33.5% the first year after project completion. However, bull trout abundance
within the project area increased by 80% compared to data collected three years prior to
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