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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
facilities or downstream sediment transport. Similarly, at sedimentation augmentation
areas downstream of Middle Fork interbay dam, riparian vegetation management would
occur to ensure that riparian vegetation does not impede downstream transport of
sediment from the augmentation areas. However, under existing conditions, there is
minimal riparian vegetation at these augmentation areas because of steep side slopes,
coarse substrate, and periodic high flows from spills that scour vegetation and prevent
established plant populations.
For routine maintenance activities and non-routine recreation maintenance
activities there may be minor disturbances to existing vegetation. However, vegetation
likely would reestablish through pioneering of plant species from adjacent areas, growth
of plants from the existing seed bank, and restoration of native vegetation as provided for
in the previously discussed VIPMPs.
Effects of Proposed Changes in Hell Hole Reservoir Operations on Vegetation
Daily and seasonal fluctuations in reservoir water surface elevations at Middle
Fork interbay and Ralston afterbay are not expected to change under the proposed action
compared with existing conditions; therefore, riparian resources would be maintained
around these reservoirs. Under the proposed action, average water surface elevation at
French Meadows reservoir in April through August would typically be slightly lower
compared with existing and conditions, and would not likely have a substantive effect on
the associated vegetative communities. However, proposed changes to the operation of
Hell Hole reservoir have the potential to affect plant communities along the shoreline.
Currently, Stebbins’ phacelia grows along the shoreline of Hell Hole reservoir,
including portions of the shoreline that are inundated for part of the growing season
(April through August) in wet, above normal, and below normal water year types (see
table 3.3.1-3). Under the proposed action, in years when French Meadows dam and Hell
Hole dam would spill, operation of Hell Hole reservoir with the proposed modified
spillway crest gates would allow storage of up to an additional 7,600 acre-feet of water.
This operation of the spillway crest gates would result in an increase in the existing
maximum normal operating water surface elevation from 4,630 feet to 4,636 feet in the
early summer. This proposed change in project operations has the potential to affect
Stebbins’ phacelia populations. The Alternative 1 reservoir operating regime, for the
purposes of this analysis, would be essentially the same as the proposed action.
Our Analysis
Changes to spillway crest gate operations could affect Stebbins’ phacelia in Areas
1, 2a, and 2b (table 3.3.3-6). In Area 1, model results for the proposed action indicate
that number of years in which inundation occurs would be similar to existing conditions.
In Area 2a, the overall frequency of inundation during the growing season for Stebbins’
phacelia for all water year types combined would increase from 11 (existing conditions)
to 14 years (proposed operation) over the period of record (33 years modeled). This
increase in inundation frequency would occur in above normal and below normal water
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