Mitigation for the Construction and Operation of Libby Dam

conditions (Table 5). Other significant results of project construction occurred in the mean
bankfull depth, maximum depth, and bankfull width, respectively, with trends toward a
narrower and deeper stream channel. Cross-sectional area did not differ significantly
between years (p = 0.166; Table 5). None of the 6 channel dimension parameters differed
significantly (p < 0.05) between 2002 and 2003. However, width to depth ratio and mean
bankfull depth were close to being significant (Table 5), with trends toward a slightly deeper
channel.
The 41 stream restoration structures described above, increased channel diversity
within the project area along the longitudinal profile (Figure 7). The existing stream channel
prior to the implementation of this project contained long riffle sections and relatively low
sinuosity (Table 4). This project constructed a stream pattern within this reach of Grave
Creek that decreased the overall stream gradient by increasing stream length (increased
sinuosity; Table 4).
The Grave Creek Phase I Restoration Project also increased the quality and quantity
of rearing habitat for native salmonids by increasing the total number and depth of pools
compared to conditions that existed prior to restoration (Dunnigan et al. 2003). Due to the
importance of pool habitat to rearing native salmonids within lower Grave Creek, we devoted
a substantial effort to monitor pool habitat after project construction to evaluate whether the
pools maintained depth, width and length through time. After project construction in the fall
of 2002, we measured mean width, length and maximum bankfull depth of the 27 pools
constructed in the project area. We repeated these measurements in the summer of 2003
after the project had experienced the first spring freshet (Table 6). We did not perform a
statistical comparison for these data because the pool measurements represented all pools
within the project area (i.e. complete census), making statistical comparisons not necessary.
Pool length had the highest relative change between years. Both total length and mean pool
length increased by 16.2% from 2002 to 2003 (Table 6). Mean maximum bankfull depth
decreased by 14.8% from 2002 to 2003, and although we did not measure mean bankfull
depth of the 27 pools, it is likely that it also decreased between years. Total pool surface area
within the project area increased by 26.2% from 3,206 ft 2 in 2002 to 4,046 ft 2 in 2003.
Therefore, even if mean bankfull pool depth decreased from 2002 to 2003, it is likely that
total pool volume remained similar between years since pool area increased.
In addition to a complete census of all pools within the project area, we also surveyed
7 riffles within the project area in order to evaluate changes in riffle slope through time. The
post-construction mean riffle slope in 2002 was 1.46% (variance = 6.55*10 -5 ), which
decreased by 39.2% in 2003 to a mean riffle slope of 0.89% (variance = 1.79*10 -5 ). This
trend was similar to the one we observed for riffles within the Libby Creek Upper Cleveland
Project (see above). Furthermore, we attribute the overall flattening of the riffles within this
project to the same 2 factors that caused the Libby Creek project riffles to flatten. The top
end of the Grave Creek riffles also incised within the channel (degraded) due in part to the
scour achieved below many of the gradient control cobble structures installed at the tailouts
of many of the pool structures, and the lower portion of many of these same riffles aggraded
with bed materials, which had the overall result of reducing the overall riffle slope.
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