Waterbodies, and Wetlands

The most recent high flow event occurred during June 2010. During the event from June 7 to June 19, 2010, the highest daily mean total discharge leaving Kortes Reservoir into the Miracle Mile was 14,600 cfs, recorded on June 15, 2010. During this period the highest daily mean total discharge exceeded 10,500 cfs for 6 days in a row, between June 11 and June 16 (Reclamation 2019b).

Based on the historic flows, both Kortes Reservoir and the Miracle Mile have successfully managed sustained flows more than 11,500 cfs during three different periods (1983, 1984, and 2010). The Miracle Mile has experienced 18 days between 1945 and 2019 of daily mean total discharge values greater than 11,500 cfs (Reclamation 2019b). No major failures were reported, and infrastructure was able to manage the high flows in each of the three high flow events. Therefore, Black Canyon does not anticipate significant adverse effects to the operation of Kortes Dam and Reservoir or the North Platte River downstream of the Project due to inadvertent discharge of 10,500 cfs from the upper reservoir.

Emergency Action Plan

Black Canyon will develop, prior to Project operation, an Emergency Action Plan as part of the Dam Safety requirements of the FERC license. As appropriate, this plan would include remediation requirements to address impacts in the highly unlikely event of over pumping. Black Canyon has consulted with Reclamation and will address/has addressed any concerns or recommendations raised by Reclamation.

3.4.2.3 Potential Impacts on Other Aquatic Habitat, Surface Waterbodies, and Wetlands

The potential effects of construction and operation upon surface water quality will be minimized by implementation of BMPs and Black Canyon’s compliance with relevant permits prior to Project construction. During final design, Black Canyon will design the transmission facilities to avoid surface-disturbing activity in identified 100-year floodplains, within 500 feet of perennial waters and wetland/riparian areas, and 100 feet from the inner gorge of ephemeral channels, as specified in the BLM Rawlins Field Office RMP. If transmission structures cannot be located outside the buffers, Black Canyon will coordinate with BLM on steps to mitigate impacts to water features.

Most of the streams within the study area for the Aquatic Resources Delineation Study are ephemeral stream channels (HDR 2022a). According to USACE (undated-b), an ephemeral stream has flowing water only during, and for a short duration after, precipitation events in a typical year. Most of the identified ephemeral streams within the Footprint of Potential Disturbance are episodic stream channels that appear to convey flows only during and immediately after precipitation events. Many of the ephemeral channels in the Footprint of Potential Disturbance are typical of arid fluvial systems, including sparsely vegetated washes with sand or sand and gravel beds with varying densities of scattered stones of variable sizes. Historic agricultural activities within portions of the Project vicinity have disturbed natural hydrology through some ephemeral washes.

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 little effect on erosion and sedimentation. Transmission structures will be sited to avoid streams and floodplain areas to the extent practical, thus minimizing the potential for affecting watercourses.

The transmission line will span about 24 streams or stream segments according to the Aquatic Resources Delineation Study along the proposed transmission line alignment (HDR 2022a; USGS 2019). It is anticipated that transmission line structures will be placed outside of streams. New temporary or permanent access roads may be constructed to provide access to the proposed transmission line. Some of those 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 will be ameliorated with the implementation of an Erosion and Sediment Control Plan that incorporates BMPs designed to minimize the potential for erosion and sedimentation to stream and wetland areas.

The proposed transmission line for the Project will be sited adjacent to an existing transmission line, thus minimizing effects on undisturbed habitats. Table 3.4-13 below lists surface waterbodies that will be crossed by the proposed transmission line for the Project. During final design, Black Canyon will design the transmission facilities to avoid surface disturbing activity in identified 100-year floodplains, within 500 feet of perennial waters and wetland/riparian areas and 100 feet from the inner gorge of ephemeral channels, as specified in the BLM Rawlins Field Office RMP. If transmission structures cannot be located outside the buffers, Black Canyon will coordinate with BLM on steps to mitigate impacts to water features.

Table 3.4-13. Ephemeral, Intermittent, and Perennial Streams Crossed by the Project Transmission Line

Resource ID Type1 Hydrologic Regime2 FGDC Code3 Average OHWM4 Width (feet)5 Length (feet)5

Area (acres)5

S-4 NRPW E R6 2 498 0.023 S-8 NRPW E R6 2 506 0.023

S-9 NRPW E R6 12 977 0.271 S-10 NRPW E R6 5 448 0.052

S-11 NRPW E R6 2 926 0.042 S-13 NRPW E R6 6 297 0.041

S-15 NRPW E R6 5 140 0.016 S-16 NRPW E R6 5 580 0.067

S-17 NRPW E R6 6 493 0.068 S-18 NRPW E R6 5 463 0.053

Resource ID Type1 Hydrologic Regime2 FGDC Code3 Average OHWM4 Width (feet)5 Length (feet)5

Area (acres)5

S-20 NRPW E R6 3 522 0.036 S-6A RPW I R4UB3 4 669 0.061

S-6B RPW I R4UB3 3 3,160 0.201 S-6C RPW I R4UB3 2 200 0.009

S-6D RPW I R4UB3 4 95 0.009 S-6E RPW I R4UB3 2 1,161 0.053 S-14 RPW I R4UB3 6 221 0.03 S-5A RPW P R2UB1 4 1,730 0.159 S-5B RPW P R2UB1 4 201 0.019 S-5C RPW P R2UB1 4 330 0.03

S-5D RPW P R2UB1 4 62 0.006 S-7 RPW P R2UB1 18 1,496 0.663 S-12 RPW P R2UB2 14 879 0.284 S-19 RPW P R2UB1 7 649 0.097

Totals 16,703 2.313

Source: HDR 2022a. 1. NRPW = non-relatively permanent water; RPW = relatively permanent water 2. E = ephemeral; I = intermittent; P = perennial 3. FGDC 2013 4. Ordinary High-Water Mark 5. As measured/calculated in ArcMap™ version 10.7.1 within the study area

To facilitate access to the main access tunnel entrance for the Project, a bridge will be constructed across the North Platte River downstream of Seminoe Dam. Existing roadways will be used to access the bridge and main access tunnel. The bridge structure is anticipated to be comprised of precast beams and a cast-concrete deck and will consist of six spans supported on sets of three columns with capping beams at each end of the bridge. The approach slab will be supported by an embankment confined by concrete and reinforced-earth walls. BMPs and relevant permitting, including USACE in-water permitting as appropriate, will minimize effects of this structure on aquatic and wetland habitats.

As previously described, 13 emergent wetlands within the Aquatic Resources Delineation study area were identified through secondary data source review and field investigations conducted by Black Canyon on June 16 to 18, 2021, August 24 to 26, 2021, and on August 3, 2022 (HDR 2022a). The wetlands totaled approximately 9.1 acres. Some of these wetlands abut channels within the study area, some were associated with springs and seeps, and others occurred as slope and depressional wetlands.

Potential impacts to wetlands will be avoided and minimized to the maximum extent practicable. Any impacts to wetlands occur during construction of roads or other activities will be minimized. Temporary impacts are anticipated to be mitigated by returning disturbed areas to preconstruction elevations, stabilizing soils with erosion control measures, and revegetating disturbed areas with appropriate species for long-term soil stability and decreased erosion potential. Construction of the proposed upper reservoir will permanently impact approximately 1.5 acres of palustrine emergent wetland (W-2)12. This wetland is an isolated depressional wetland, and cattle activity and grazing are prevalent in the area (HDR 2022a). As described above, in wet years, this area is known to fill with water but dries up by late spring or early summer. Some of the water may seep downward to become the partial supply for the springs discharging to local gullies. Construction of the upper reservoir will, therefore, result in the reservoir capturing and retaining a limited amount of rainfall that would have otherwise reached these springs. If this area does act as a recharge area for the springs, then placing an impermeable membrane on the proposed upper reservoir floor could effectively seal off the flow/partial flow of water to the springs and dry them up. These springs are the primary source of any water flowing down gulches in the area (Number One Gulch and Number Two Gulch) and Dry Lake Creek (Bennett and Aalto 1982). Black Canyon proposes to conduct pre-construction and postconstruction monitoring of water flow in Number One Gulch, Number Two Gulch, and Dry Lake Creek. In the event that reduced streamflows are identified post-construction, Black Canyon will work with the BLM to identify mitigation measures.

The proposed upper reservoir will occupy about 114 acres at normal maximum operating pool, about 0.02 percent of Pathfinder-Seminoe Reservoirs watershed (HUC 10180003), which is about 1,030 square miles (USGS 2020). Therefore, impact to springs, runoff, and stream flow within the watershed will be negligible.

Precipitation on the upper reservoir water surface will represent the only natural upper reservoir recharge. The upper reservoir will be lined so that the reservoir will not leak; therefore, any losses would be attributable to evaporation. Since the upper reservoir will be enclosed on all sides by an embankment, surface water runoff will not enter or be intercepted by the upper reservoir. The presence of the upper reservoir will preclude captured water from flowing downstream into the North Platte River watershed. Interception of rainfall by the upper reservoir is expected to be minimal on a watershed level.

Construction could increase turbidity in area streams and in Seminoe Reservoir through two primary pathways: 1) increased surface erosion; and 2) in-water construction activities. Construction activities could contribute to known turbidity issues in Seminoe Reservoir. However, these short-term construction impacts on surface water resources are anticipated to be minimal as a result of implementation of Black Canyon’s Erosion and Sediment Control Plan. Construction of the proposed upper reservoir will necessitate the

12 The wetland is an isolated feature, not adjacent to nor draining to a relatively permanent water or traditional navigable water. Therefore, W-2 is a potential non-jurisdictional aquatic resource (HDR 2022a).