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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
3.3.2 Aquatic Resources
3.3.2.1 Affected Environment
Water Quantity
Water Storage
The project includes two large reservoirs for water storage (Hell Hole and French
Meadows), two medium-size reservoirs (Middle Fork interbay and Ralston afterbay), and
three small diversion pools (Duncan Creek, North Fork Long Canyon, and South Fork
Long Canyon Creek) (see figures 1 and 2). Table 3.3.2-1 contains information on water
storage characteristics of the project impoundments. French Meadow and Hell Hole
reservoirs and Duncan Creek diversion pool have minimum pool requirements under the
existing license (table 3.3.2-2).
French Meadows reservoir has a gross storage capacity of 134,993 acre-feet and
an active storage capacity of 127,358 acre-feet. Hell Hole reservoir has 207,590 acre-feet
of gross storage and an active storage capacity of 205,057 acre-feet. PCWA manages
water storage in French Meadows and Hell Hole reservoirs to be at the lowest by the
early winter to provide adequate carryover storage and to manage spring runoff (figures
3.3.2-1 and 3.3.2-2). During spring runoff, operating flows are adjusted to store as much
water as possible without spilling the reservoirs. After the reservoirs have reached their
maximum capacity in late spring or summer, water is managed first to meet instream
flow requirements, then to meet consumptive water supply requirements, and finally to
optimize power generation.
Middle Fork Interbay dam is a 70.5-foot-high, 233-foot-long concrete gravity
structure with a crest elevation of 2,536 feet. The dam impounds the Middle Fork
American River forming Middle Fork interbay. The reservoir has a maximum operating
surface area of about 7 acres, and has 175 acre-feet of gross storage and 173 acre-feet of
active storage capacity. Middle Fork interbay is typically held at full capacity. Ralston
afterbay dam is an 89-foot-high, 560-foot-long concrete gravity structure with a crest
elevation of 1,189 feet. The dam is located just below the confluence of the Middle Fork
American and the Rubicon Rivers. The dam impounds the Middle Fork American River
and forms the Ralston afterbay. Ralston afterbay has 2,782 acre-feet of gross storage
capacity and an active storage capacity of 1,804 acre-feet (at 1,179 feet water surface
elevation).
PCWA uses Ralston afterbay and Middle Fork interbay primarily as powerhouse
forebays and afterbays, and not as storage reservoirs. Middle Fork interbay water surface
elevations typically remain near full pool. Ralston afterbay water surface elevations
typically remain within 8 feet of full pool. Ralston afterbay water surface elevation
typically fluctuates daily in the summer and early fall due to peaking operations of the
project and whitewater boating releases. Both Middle Fork interbay and Ralston afterbay
are lowered annually during the fall, for about 3 to 6 weeks, for maintenance.
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