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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
during large storm events when the diversion pools are nearly filled with sediment.
PCWA has routinely excavated sediment from the three small diversion pools. An
average of 416 cubic yards per year, 622 cubic yards per year, and 374 cubic yards per
year were removed from the Duncan Creek, South Fork Long Canyon Creek, and North
Fork Long Canyon Creek diversion pools, respectively. In the Duncan Creek diversion
pool, about 36 percent of the total volume removed was medium and coarse gravel (148
cubic yards per year). In South Fork Long Canyon diversion pool, about 62 percent of
the total volume removed was medium and coarse gravel (386 cubic yards per year). The
proportion of gravel removed from North Fork Long Canyon diversion pool was about 20
percent (75 cubic yards per year).
Shoreline Erosion in Project Reservoirs
Project reservoirs are surrounded predominantly by bedrock slopes that are
resistant to erosion. The banks of French Meadows and Hell Hole reservoirs consist of
predominantly granitic and volcanic rock, and the soils derived from weathering of these
rocks. Hell Hole reservoir is located almost entirely within the Sierra Nevada batholith,
which is dominated by massive and fractured bedrock with little soil development.
About 8.5 miles of the 11-mile-long shoreline around Hell Hole reservoir is bedrock or
boulder material that is highly resistant to erosion. Material subject to erosion along the
remaining 2.5 miles of shoreline is predominantly decomposed granite composed of
coarse sand. Shorelines around French Meadows reservoir, Ralston afterbay, and Middle
Fork interbay are also primarily composed of bedrock and boulder substrates that are
resistant to erosion.
Large Woody Debris Capture and Management in Project Reservoirs and
Diversion Pools
LWD occurs in Hell Hole and French Meadows reservoirs, Duncan Creek
diversion pool, North Fork Long Canyon diversion pool, and Middle Fork interbay.
Recruitment of LWD into project reservoirs and diversion pools comes from upstream
sources transported downstream in the channel and from steep vegetated hillslopes
surrounding the impoundment. Only a small amount of LWD (one to six pieces) has
been observed in Duncan Creek and North Fork Long Canyon diversion pools and
Middle Fork interbay. Larger amounts of LWD have been observed in Hell Hole
reservoir (40 to 50 pieces) and French Meadows reservoir (100 to 150 pieces). LWD
observed in these reservoirs was stored along the high water mark and along the back of
the dam. No LWD was observed in the South Fork Long Canyon diversion pool or
Ralston afterbay.
PCWA has conducted LWD management on an as-needed basis (typically every 5
years) at all reservoirs and diversion pools except French Meadows reservoir.
Maintenance activities focus on removal of debris surrounding intake structures and
along log booms to ensure proper functioning of the spillway and diversion inlets. The
LWD removed from Hell Hole reservoir is typically burned on site. Woody debris in
Middle Fork interbay and Ralston afterbay is flushed through the spillway gates. The
47