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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
The peaking reach from Oxbow powerhouse downstream to the high-water mark
of Folsom reservoir includes two river segments: (1) the Middle Fork American River
from Oxbow powerhouse to the confluence of the North Fork American River and (2) the
North Fork American River from the confluence of the Middle Fork American River to
the high-water mark of Folsom reservoir. Oxbow powerhouse is typically operated to
follow daily power demand and provide whitewater boating flows and is not operated 24
hours per day (except in the wettest of water years and/or seasons of the year) leading to
inter- and intra-daily flow fluctuations in the reach. Except during high-flow times,
releases from the Oxbow powerhouse cause daily fluctuations in flows in the peaking
reach of up to about 900 cfs (about 75 to 1,025 cfs). During winter and spring, flows in
the peaking reach can exceed 3,000 cfs due to natural runoff (see table 3.3.2-3).
Considerable accretion occurs along the peaking reach, particularly during the winter,
from the North Fork of the Middle Fork American River, Volcano Creek, Otter Creek,
Canyon Creek, and North Fork American River watersheds. In summer, flows in the
peaking reach often consist of a daily peaking event starting from a low nighttime base
flow (e.g., 100 to 200 cfs) released from the Oxbow powerhouse followed by a morning
up-ramp (about 250 to 450 cfs/hour) to a high peak flow of about 1,000 cfs and then an
evening down-ramp (about 250 to 450 cfs/hour) back to the base flow. Each ramping
period lasts for about 2 hours. Flows in the upper end of the peaking reach are held
relatively steady (close to the 75 cfs minimum instream flow requirement) during a 3- to
6-week period in the fall during maintenance of the Oxbow powerhouse. The total river
stage change for typical summer peaking events ranges from about 1 to 2.3 feet (average
1.8 feet), depending on location. The duration of the peak is generally shortened, and the
downramps are slower with increasing distance downstream due to channel storage.
Powerhouse Flows
During the filling period (winter and spring), flows through the project
powerhouses are highly dependent on projected and actual runoff conditions. During the
release period (from the end of the spring runoff period until storm runoff begins again in
the late fall or winter), after the reservoirs have reached their maximum storage capacity
(the timing of which can vary greatly based on the type of water year), monthly releases
for generation are largely predictable (table 3.3.2-5). Daily and hourly releases for
generation, which respond to demand for electricity and electrical grid reliability, remain
highly variable.
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