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

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52 Figure 3.3.1-1. Ralston afterbay sediment augmentation (Source: PCWA, 2011a).
20120723-4002 FERC PDF (Unofficial) 07/23/2012 Our Analysis PCWA’s proposed Sediment Management Plan would improve project reliability, reduce project facility maintenance, increase natural delivery and transport of sediment downstream of project diversions, and enhance aquatic and riparian habitat downstream of project diversions and medium size reservoirs. Sediment removal in small and medium-size reservoirs would require partial or complete dewatering of the reservoir impoundments, which could affect aquatic organisms and their habitat as well as terrestrial species that rely on these reservoirs for foraging. Section 3.3.2, Aquatic Resources, contains a discussion of the potential effects of sediment management on aquatic species, and section 3.3.3, Terrestrial Resources, describes potential effects on terrestrial species. Sediment removal and augmentation activities also can release methylmercury produced under anoxic conditions in reservoirs. Section 3.3.2.2, Aquatic Resources, discusses potential effects of methylmercury. The proposed action would improve coarse sediment supply to reaches downstream of diversions and within the peaking reach, as discussed in more detail in the following section. The Sediment Management Plan includes specifications for sediment removal, disposal, and in-channel placement necessary to protect water quality and aquatic and riparian habitat. The Sediment Management Plan also includes the necessary monitoring of water quality, sediment transport, channel geomorphology, and fisheries to evaluate the effectiveness of the measures. Channel Responses to Altered Flow Conditions Peak flows in natural rivers provide important geomorphic and ecological functions, such as bed mobilization, floodplain inundation, and fish migration. Project dams and reservoirs trap coarse sediment delivered from upstream sources, limiting available gravel and coarse sediment that would otherwise support and enhance aquatic habitat in the Middle Fork American River. A reduction in coarse sediment supply due to sediment trapping in reservoirs with little coincident decrease in scouring flows can reduce mobile sediment storage and coarsen the channel bed surface, resulting in fewer and less suitable deposits for salmonid spawning. Conversely, continued sediment loading from tributary inputs in the absence of infrequent high flows that scour the channel bed can result in fine sediment accumulation in pools and spawning gravel deposits. Sediment accumulation may degrade aquatic and riparian habitat. Smaller cross-sectional areas and aggradation (berm development) can also result in riparian encroachment. The existing license does not specify pulse flows although high spring flows equal to or greater than the proposed pulse flows historically occurred as a result of reservoir spills and tributary accretion. 53
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Page 71 and 72: 41 Table 3.3.1-1. Average number of
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Page 99 and 100: 69 Table 3.3.2-3. Hydrology for sel
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105 Location American River below M
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135 Species Name Status a Butte Cou
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145 Table 3.3.3-5. Special-status w
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147 Species Name Status Habitat Occ
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195 Figure 3.3.5-1. French Meadows
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197 Figure 3.3.5-3. Long Canyon, re
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253 Table 3.3.6-1. Archaeological a
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255 Resource Number Type a Descript
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275 Table 4-2. Summary of annual co
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279 Enhancement/Mitigation Measures
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303 c Base 2010 costs were provided
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307 Resource No-Action Alternative
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319 Table 5-2. Staff-recommended mi
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321 Location Middle Fork American R
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