Draft EIS_072312.pdf - Middle Fork American River Project ...

20120723-4002 FERC PDF (Unofficial) 07/23/2012
diversion dams because sediment would collect on the inclined screens, aggrading the
channel upstream and creating more riverine channels that would replace the former pool
habitat. We do not expect this loss of pool habitat at the three small diversion dams to
have negative effects on trout spawning—pools are abundant throughout these streams.
Passage of wood at the modified diversion dams would likely increase in the
bypassed reaches of Duncan Creek, South Fork Long Canyon Creek, and North Fork
Canyon Creek. This may have a beneficial effect on trout spawning because large wood
can add habitat diversity to the channel by promoting pool scour and trapping and storing
patches of sediment that may be used for spawning by trout. Large and small woody
debris also provide valuable cover from predators for fry, juvenile, and adult trout and
velocity refuge from which to most efficiently forage on macroinvertebrates in the drift.
Combined with pulse flows (see section 3.3.1.2, Geologic and Soils Resources),
the modified small diversion dams should act to maintain or enhance existing channel
morphology downstream of these structures and sustain the low levels of fine sediment
currently found in pools and spawning gravels. Effects on trout populations from these
modifications would be beneficial due to increased coarse sediment deposition and the
resulting increase in spawning gravel availability in the affected reaches.
PCWA’s proposed placement of spawning-sized gravel downstream of Middle
Fork interbay dam and downstream of Ralston afterbay at Indian Bar and Junction Bar
would increase spawning gravels in downstream reaches compared with existing
conditions. Additional benefits to trout spawning success and juvenile recruitment in the
Middle Fork American River downstream of Ralston afterbay could likely be realized by
developing and implementing the spawning habitat improvement plan specified in Forest
Service condition no. 26. In response to condition no. 26, PCWA clarified in its
September 6, 2011, filing with the Commission that the spawning habitat improvement
plan would pertain to the 0.48 mile reach immediately downstream of Ralston afterbay
dam, consistent with pre-filing consultation. This suggests that this plan was considered
as a potential environmental measure, even though it is not included in either the
proposed action or Alternative 1. There is evidence in the record that implementation of
a spawning habitat improvement plan may be included in PCWA’s Sediment
Management Plan. Table 4 of the Sediment Management Plan indicates that about 56
cubic yards of “material” would be installed in the streambed downstream of Middle
Fork interbay, whereas about 389 cubic yards of material would be installed in the
channel downstream of Ralston afterbay. The large difference in installed material may
relate to the placement of spawning gravel in the bypassed reach downstream of Ralston
afterbay dam.
Pre-application fisheries surveys indicate that recruitment of trout in the peaking
reach is well below what would be expected in un-regulated streams. This is likely
because the quality of available spawning habitat is compromised by daily flow
fluctuations. Spawning gravel augmentation in the bypassed reach downstream of
Ralston afterbay dam would provide especially favorable conditions for trout spawning,
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