3.3.2.6 Aboveground Project Features

3.3.2.6 Aboveground Project Features

For purposes of this discussion, aboveground Project features include the proposed upper reservoir, upper reservoir embankment, lower reservoir, switchyard, proposed access roads, bridge across the North Platte River below Seminoe Dam, and transmission line. The proposed upper reservoir will be constructed in granite (slopes cut in the granite should be stable [Bennett and Aalto 1982]) that has been intruded by younger granite and gabbro dikes; the existing rock is moderately to intensely jointed.

During construction, soil disturbances associated with clearing, access road and bridge construction, construction of the upper reservoir, and other activities can cause erosion and sedimentation and increase windblown dust. Construction of the Project may, therefore, affect the rates of erosion and sedimentation, resulting in degraded water quality. Because of the inherent nature of overhead transmission systems (lines suspended above the ground surface), the construction of most of the proposed transmission line is anticipated to produce relatively little effect on erosion and sedimentation. Construction of access roads, however, is expected to increase the potential for erosion and sedimentation. Some access roads might involve stream crossings and could include culverts, bridges, or low-water crossings. Open streams may also be channeled through culverts over the course of construction. Construction of these roads and stream crossings could cause or contribute to erosion. These impacts are potentially significant but would be mitigable to a less-than-significant level with the implementation of an Erosion and Sediment Control Plan that incorporates BMPs designed to minimize the potential for erosion, and sedimentation and windblown dust, and to control fugitive dust emissions. The Erosion and Sediment Control Plan will specify requisite erosion control measures to ensure that excess sediment does not enter Number 1 Gulch and Number 2 Gulch.

Water levels in Seminoe Reservoir are highly dependent upon the runoff each year. Typically, the lowest surface elevation occurs in April, immediately preceding the runoff period. Reservoir levels typically rise rapidly during the runoff and peak around the end of June. Elevations can drop rapidly in July and August depending on irrigation demand and slowly decrease from September through April as slightly more water is released than flows in. Since 2007, minimum surface elevation has varied from a low of 6,289 feet in 2008, to a high of 6,346 feet in 2012. Maximum water elevation has varied from a low of 6,321 feet in 2007 to a high of 6,356 feet in 2010. Average within-year fluctuation since 2007 is 21 feet (SD = 14.5) (WGFD 2021a).

Pumped storage electrical generation operations will involve the cycling of water between Seminoe Reservoir and a new upper reservoir, generating power with release of water from the upper reservoir to Seminoe Reservoir, and returning water to the upper reservoir for storage. Black Canyon analyzed the potential effect of Project operations on water level fluctuations in Seminoe Reservoir with an assumption that the Project will have the flexibility to operate whenever Seminoe Reservoir is within Reclamation’s normal operating range (surface water elevation ranging from 6,290 feet to 6,357 feet).