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3.5.2.2 Operations-related effects
Most of the unstocked populations of the Colorado River Cutthroat Trout are restricted to relatively small (< 6 kilometers) and/or unproductive headwaters (above 8,000 feet). They require cool, clear water and well vegetated streambanks for cover and bank stability (BLM 2010). Unstocked populations of the Colorado River Cutthroat Trout are not expected to occur in Seminoe Reservoir or the downstream reach of the North Platte River and therefore, no effects to this species are expected to occur from the Proposed Project.
3.5.2.2 Operations-related effects
Long-term direct effects include the potential fish injury and mortality due to Project operations, and potential revisions to Seminoe Reservoir aquatic habitats due to Project operations.
Potential Fish Injury and Mortality
As stated previously, the potential for fish to become entrained or impinged at hydroelectric facilities is dependent on a variety of factors such as fish life history, size and swimming ability, water quality, operating regimes, inflow, and inlet/outlet and turbine configurations and locations (e.g., shoreline, deep water) (Cada et al. 1997). Furthermore, the risk of impingement is dependent on the presence of debris or fish screening structures at the inlet/outlet structure. Habitat and entrainment analyses suggest that the areas in proximity to the inlet-outlet structure provides limited suitable habitat for the species in Seminoe Reservoir, and none that is rare or unusual as compared to the remaining portions of the reservoir that could be used alternatively. Furthermore, many of the species in Seminoe Reservoir, such as trout species, exhibit localized migrations for spawning in tributaries, away from the inlet/outlet location.
The number of fish estimated to be entrained at the Project is attributed to the relative abundance of specific species, life history characteristics, physical and operational characteristics of the Project, reservoir stratification, and proximity of the structures to feeding and rearing habitat. Based on the habitat in the vicinity of the intake structure, it is unlikely that this is the preferred spawning, rearing or feeding habitat for walleye in Seminoe Reservoir. Observations during the Resident Fish Survey study show that the lentic portion of the lower reservoir (i.e., the intake vicinity) contains relatively ubiquitous and generic habitats and homogenous substrates. Although some walleye and other fish may be potentially entrained at the Project, the burst swimming speeds for juvenile and adult walleye (as well as juvenile and adult species targeted by the Fish Entrainment study report are greater than the calculated maximum intake velocities at the Project. The calculated velocities at the Project intake are low and are expected to decrease and decay substantially within a very short distance from the face of the bar rack. Fish burst speeds coupled with the limited spawning, feeding and rearing habitat preferred by walleye and other resident fish located in the vicinity of the Project intakes would significantly reduce the number of walleye and other fish from becoming entrained by the Project. Peak velocities at the intakes are generally short lived and tend to decrease as the lower reservoir elevations decrease and are confined to the face of the bar racks or screens (Bear Swamp Power Company 2018).
Based on these considerations, and the low estimated approach velocity of the inlet structure and proposed fish exclusion bar rack design, overall entrainment and impingement risk to aquatic species in Seminoe Reservoir is low and these effects are not likely to significantly impact reservoir populations.
Total dissolved gas supersaturation downstream of some hydroelectric dams can cause localized gas bubble trauma or mortality in fish and other aquatic species, primarily as a result of air mixing with water during spillway use or as it is injected into turbines during operations to prevent cavitation. There is no evidence that gas supersaturation adversely affects hatching success of salmonid embryos (AFGD undated). Gas bubble disease can result in low-level fish mortality, however adult and juvenile fish exposed to high total dissolved gases will seek greater depth or cooler waters where the saturation level is lower (AFGD undated). Because pumped storage operations do not involve spillway use and no air injection will be required as part of Project operations, gas supersaturation is not anticipated to occur.
As described above, the Project will create an “open loop” hydraulic arrangement between the existing Seminoe Reservoir and the proposed upper reservoir. The Project will not change the operations, area, or volume of Seminoe Reservoir; thus no injury or mortality to fishery resources from supersaturation within and downstream of Seminoe Reservoir are expected from construction and operation of the Project.
As discussed in Section 2.1.1.2, Project operations are predicated to have a daily surface water fluctuation of 6.4 inches under normal operating headwater elevation of 6,357 feet. Any change in Seminoe Reservoir elevation due to Project operations will be within the historic range of Seminoe Reservoir water levels and, therefore, any impacts to aquatic habitats would occur in areas where Seminoe Reservoir has operated historically. Much of Seminoe Reservoir aquatic habitat consists of gradual to steep drop-offs from the shoreline and shallow sections of Seminoe Reservoir dominated by sandy substrate. Project operations will result in gradual changes in reservoir elevation over the course of pumping and generation events. At Seminoe Reservoir low operating elevation of 6,290 feet, a full 9.7 hour generation event will produce less than a 2 inch per hour increase in reservoir elevation. A full pumping event will occur over an average of 17.05-hour period at a rate of less than 2 inch per hour change in elevation. Project effects on Seminoe Reservoir water level and surface area will occur within a band where wave action has historically altered water levels and impacted aquatic habitat conditions on an intra-day and even intra-hour scale.
