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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
frequency of high flows (number of days and years) that would scour and initiate gravel
motion in the small bypassed streams (Duncan, North Fork Long Canyon, South Fork
Long Canyon, and Long Canyon Creeks) is less under the proposed action compared with
historical conditions, but is greater than would occur in the future under existing
conditions with expected increased demands on available water supplies.
Most of the bypassed channel reaches (65 to 70 percent of the total length) are
relatively stable and have a relatively low potential to adjust in response to alterations in
flow and sediment supply or transport. Low response potential is due in part to exposure
of bedrock and boulders in the channel bed and banks, channel entrenchment within
resistant valley walls and valley bottom material, and high unregulated transport capacity
relative to unregulated sediment supply. The exceptions are South Fork Long Canyon
Creek, which has a moderate response potential along about 61 percent of the reach, and
North Fork Long Canyon Creek, which has a high response potential along about 84
percent of the reach. For all of the bypassed reaches, the most likely channel responses to
changes in flow and sediment regimes is a coarsening of bed surface particle size and a
reduction in the frequency and size of mobile coarse sediment deposits (e.g., spawning
gravel patches).
The peaking reach is predominately alluvial and exhibits more potential for
channel adjustment in response to sediment augmentation and altered flows than the
bypassed reaches. About 95 percent of the reach is highly responsive due to the presence
of finer-grained alluvial bedforms. Lateral shifts in channel planform occur infrequently.
Other types of adjustments that could occur include changes in width, depth, and slope,
and sediment storage (channel bars and mobile coarse sediment patches).
The proposed action would restore sediment supply to the reaches downstream of
the small diversions and improve sediment supply to the reaches downstream of the
medium dams. The increase in sediment supply under the proposed action would provide
long-term channel geomorphology and aquatic and riparian ecosystem benefits to the
small bypassed streams. Proposed spring pulse flows prescribed for all bypassed reaches
in May of wet and above normal water years, combined with more natural recession rates
would restore natural dynamics of riparian vegetation recruitment similar to what might
occur in a similarly sized stream under unregulated conditions. The frequency of gravel
bed mobilization and scour under the proposed action is sufficient to maintain the
channel geometry and minimize fine sediment accumulation in pools and spawning
gravels.
Alternative 1 specifies that, during wet water years, pulse flows would begin on
May 15 instead of May 1. This later initiation of pulse flows would provide more time
for rainbow trout fry to emerge from the gravel prior to a planned high flow event. The
additional details regarding the time of day when pulse flows would be released would
provide more user friendly whitewater boating opportunities than releasing flows at
unspecified times. The Alternative 1 provision to conduct a feasibility study regarding
the maximum pulse flow that can safely be released from Hell Hole dam would provide a
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