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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
Minimum flow increases in the peaking and bypassed reaches in the wettest water
year types would increase wetted stream perimeter, which may increase habitat area and
habitat diversity in some reaches for fish and aquatic invertebrates. Increased edge habitat
may improve rearing success of YOY fish in wet water years if shallow water or cover is
available to increase their ability to avoid aquatic and terrestrial predation. Higher
minimum flows should provide more area for aquatic macroinvertebrate communities
that make up a large portion of the diet of trout and other fish.
The range of proposed minimum flows in the peaking reach would be from 75 to
450 cfs, whereas the Alternative 1 minimum flows would range from 90 to 450 cfs.
However, many of the Alternative 1 minimum flows would be from 15 to 100 cfs higher
than the comparable proposed minimum flows for comparable water years and time
frames. For the reasons stated in the previous paragraph, this prevailing higher minimum
flow would result in the Alternative 1 regime providing minor habitat enhancements
compared with the proposed flow regime. However, we expect the primary factor that
would limit the quality and quantity of aquatic habitat in the peaking reach to be flow
fluctuations, rather than minimum flows. Daily peaking flow fluctuations would be
maintained under both action alternatives, and effects of the new minimum flow regime
in the peaking reach on fish and other aquatic biota are expected to be negligible because
changes in wetted habitat area would be of very short duration and likely insufficient to
affect behavior (e.g., foraging), food availability, or production of aquatic biota.
Typically, the period of snowmelt and high runoff occurs between April through
mid-June in the basin and the spring pulse flows in the action alternatives reflect this
timing. Restoring flows that mimic the spring snowmelt pulse may provide benefits to
the aquatic community by helping to maintain a variety of seasonal life-history behaviors.
Spring minimum flows would be substantially higher than under existing conditions in all
water-year types under both action alternatives and, when coupled with the pulse flow
releases (see section 3.3.1.2, Geologic and Soils Resources, table 3.3.1-2), would provide
flow changes that simulate those found under unregulated conditions during the seasonal
snowmelt period. Both the proposed and Alternative 1 minimum flows would enhance
flow conditions for spawning by resident rainbow trout during spring (i.e., mid-March to
June), based on the results of PCWA’s instream flow analysis (PCWA, 2010b). Higher
spring flows may provide access to additional spawning or rearing habitat but may also
act to scour redds in some years, both of which occur under natural stream conditions.
Precipitation in the Middle Fork American River watershed has high inter-annual
variation; water-year types tend to be either wet or dry, with few years receiving
“average” precipitation. Under existing conditions, no provision is made for within-year
or between-year variation in flows to reflect local seasonal changes and water year types.
Many fish and aquatic macroinvertebrate species may use changes in flows as cues for
behaviors such as spawning or movements into appropriate winter or summer habitat.
Year-to-year variations in flows may maintain species diversity by benefiting certain
species in wet years and others in dry years. Western streams with intact seasonal flow
variation are believed to be more resistant to invasion by fish from eastern U.S. streams
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