3.5.1.6 Aquatic Environment in the Project Vicinity

Table 3.5-12. Seasonal and Mean Annual Entrainment Rates for Target Species at the Proposed Maximum Pumping Capacity (12,000 cfs)

Target Species

Mean Monthly Entrainment Rate (fish/hr) by Season* Mean Annual Entrainment Winter Spring Summer Fall Rate (fish/hr)*

Bigmouth Shiner 0.71 1.16 0.46 0.21 0.64

Brook Trout 8.06 2.46 0.38 2.36 3.32

Brook Stickleback 3.74 9.99 2.62 1.42 4.44

Brown Trout 0.55 0.81 0.08 1.13 0.64

Common Carp 0.38 0.29 0.70 0.24 0.40 Emerald Shiner 3.27 10.67 10.88 15.25 10.01

Fathead Minnow 0.45 10.06 7.10 4.48 5.52

Iowa Darter 0.00 0.48 0.08 0.93 0.37

Johnny Darter 1.64 2.03 1.44 0.62 1.43 Lake Chub 0.00 0.04 0.00 0.00 0.01

Lake Trout 8.06 2.46 0.38 2.36 3.32

Longnose Dace 0.12 1.61 1.63 0.76 0.10

Longnose Sucker 0.18 0.91 0.81 0.59 0.62

Rainbow Trout 0.75 0.23 0.05 0.40 0.35 Sand Shiner 0.71 1.16 0.46 0.21 0.64

Snake River Cutthroat Trout 0.75 0.23 0.05 0.40 0.35

Walleye 1.80 1.96 14.23 2.84 5.21 White Sucker 20.33 10.46 21.89 19.19 17.97

Sum 51.47 57.03 63.22 53.40 55.35

* These rates were calculated based on the maximum pumping capacity based on the current proposed design. However, it is feasible that the Project would be operated below the maximum load, utilizing just one or two turbine units for certain periods of each day. As such, these entrainment rates could be viewed as conservative (i.e., overestimates) of the potential entrainment risk at the proposed Project. 3.5.1.6 Aquatic Environment in the Project Vicinity

As previously mentioned, the Project will be located in the North Platte River Basin, which is a subbasin of the Platte River Basin, composed of the North and South Platte River Basins (Wyoming Water Development Commission 2016). Within the North Platte River Basin, the proposed upper reservoir for the Project will be located in the PathfinderSeminoe Subbasin (HUC 10180003) and existing Seminoe Reservoir (lower reservoir) is

located in both Pathfinder-Seminoe Subbasin and Medicine Bow Subbasin (HUC 10180004) (Black Canyon 2020b).

Seminoe Reservoir

Most of the aquatic habitat existing within the Project vicinity is within Seminoe Reservoir. Seminoe Reservoir is a dimictic lake with a relatively weak and brief summer thermal stratification, isothermal conditions in spring and fall, and ice cover in the winter (Reclamation 1981). However, stratification lasts a relatively short period of time (June to August) due to the continual withdrawal of cooler hypolimnetic waters through the dam’s low-level outlets (Black Canyon Hydro 2020, Reclamation 1981). The lake is further characterized as alkaline, hard, and somewhat saline (Reclamation 1981). Characteristics of Seminoe Reservoir, such as surface area, volume, and maximum and minimum surface elevations are provided in Section 3.4.1.3 above and Exhibit B of this license application.

The North Platte River is the primary water source of Seminoe Reservoir, followed by Medicine Bow River (USEPA 1977). Other smaller tributaries include Cottonwood Creek, Sips Creek, Hurt Creek, Saylor Creek, Austin Creek, and Troublesome Creek (Black Canyon 2020b), as well as Obrien Creek and many draws, ditches, and swales.

The amount of aquatic habitat in Seminoe Reservoir depends on the reservoir surface water elevation and habitat quality characteristics, such as habitat complexity and water quality. A higher surface water elevation provides a greater amount of aquatic habitat, both in total volume and in the littoral zone. The minimum to maximum operating pool elevation can fluctuate up to 67 feet (Stantec 2022).

Based on fish seining investigations primarily conducted to detect changes in relative abundance of common littoral zone fishes in Wyoming reservoirs, Bailey (2005) described Seminoe Reservoir as having very little, if any, aquatic vegetation, with most reservoir substrates dominated by sand and gravel that provide limited habitat complexity. Based on recent observations of exposed aquatic habitat within Seminoe Reservoir during the Resident Fish Survey Study (HDR 2022b), Bailey’s description is still accurate for large areas within Seminoe Reservoir.

As previously described, Black Canyon performed a Resident Fish Survey Study that encompassed approximately the downstream one-third of Seminoe Reservoir. This area was further divided into five sub-areas for ease of sampling and data processing. A detailed description of the aquatic habitat associated with each subsection is provided below. Much of the aquatic habitat consists of gradual to steep drop-offs from the shoreline and the shallow sections of Seminoe Reservoir within the study area are dominated by sandy substrate. General information regarding each subsection surface area, shoreline length and approximate center location is provided in Table 3.5-13 and additional descriptive text of each sample area is provided below. Sampling areas are identified above in Figure 3.5-1.

Table 3.5-13. Resident Fish Survey Study Sample Area Habitat Details

Sample Area Sample Area Nickname Approximate Center Coordinates Approximate Surface Area (acres) Approximate Shoreline Length (miles)

1 The Canyon 42.150438, 106.899217 157 5.3

2 State Campgrounds/ Sunshine Beach 42.134163, 106.883694

3 Sand Draw 42.118771, 106.860005 878 12.4

907 7.1

4 Mini Tetons 42.098204, 106.840693

5 Sand Mountain 42.09483, 106.857945 1,110 9.6

912 8.2

Source: HDR 2022b.

Totals 3,964 42.6

Area 1 – The Canyon

During the 2021 Resident Fish Survey, observed water depths in Area 1 ranged from 0 feet at the shoreline to approximately 125 feet along the thalweg (HDR 2022b), with maximum water depths near Seminoe Dam extending to over 175 feet (Fishermap.Org, 2022). The shoreline of Area 1 is dominated by steep bedrock cliffs with occasional large boulder and bedrock shard complexes which extend deep into the canyon waters making up the dominant visible substrate within the littoral zone. Areas of sand substrate and submerged aquatic vegetation are visually absent in Area 1 except for a few small sand beaches occurring in areas with lower slope. These areas generally consist of a short, gradual slope of sand or fine substrate leading to a significant drop-off a short distance from shore. Shoreline slopes generally ranged from vertical to near vertical and occurred along approximately 90 percent of the shoreline within the Canyon. No tributaries were observed entering this area (HDR 2022b).

Area 2 – “State Campgrounds/Sunshine Beach”

The shoreline of Area 2 is dominated by steep shores with occasional vertical cliffs on the western shore and areas of moderate to gentle slopes on the western shore. The eastern shore is steeper than the eastern shore in most areas with sandstone, red sandstone, and bedrock cliffs and occasional large boulder and bedrock shard complexes; some extending into the littoral zone. Some areas on both the eastern and western shores contain long stepped sand beaches between the boulder and bedrock outcrops. Observed depths in Area 2 ranged from 0 at the shoreline to approximately 100 feet in the thalweg (HDR 2022b). Areas of sand substrate are much more dominant on the eastern shore than the western shore. Submerged aquatic vegetation was visually absent from this area. Three unnamed tributaries were observed entering Seminoe Reservoir from the western shore and Red Spring Draw enters on the eastern shore of Area 2 northwest of Hurt Gulch. These tributaries are assumed to be categorized as ephemeral tributaries.

Area 3- “Sand Draw”

Most of the observed substrate in Area 3 is dominated by sand, with a few areas of boulders and large bedrock shards sporadically located along the shoreline that extend into the littoral zone (HDR 2022b). A small, relatively unvegetated island is located within Seminoe Reservoir in the northwestern portion of Area 3 south of Hurt Gulch. Observed substrate extends into the littoral zone from the island and is dominated by areas of exposed bedrock and scattered areas of boulder and rubble substrate. The western shore is also dominated by sand substrate however, the shoreline slopes are more gradual, with only minimal boulder outcrops extending into the littoral zone (HDR 2022b). This area includes the inlets from Cottonwood Creek and Hurt Gulch on the eastern shore. Observed depths in Area 3 during the Resident Fish Survey ranged from 0 feet at the shoreline to approximately 85 feet, however mapped depths can reach to approximately 160 feet within the thalweg of Area 3 (Fishermap.Org, 2022).

Area 4 – “Mini Tetons”

Most of the shoreline and littoral habitat in this area are gently sloping with sand, gravel, or cobble substrates, the littoral zone surrounding the “Mini Tetons” (two large boulder/bedrock shard outcrops protruding from the reservoir in the mid-eastern portion of this area) and portions of the southern shore (especially the western end) along Horseshoe Ridge consist of bedrock, bedrock shards, and large boulders with a much steeper slope (HDR 2022b). Substrates at the inlet of Saylor Creek consisted of a mix of deep sand and silt. Sips Creek also drains into Area 4 on its eastern shore just north of Saylor Creek (HDR 2022b). Observed depths in Area 4 during the Resident Fish Survey ranged from 0 to approximately 90 feet, however mapped depths can be as great as approximately 140 feet in the thalweg (Fishermap.Org, 2022).

Area 5 – “Sand Mountain”

Much of Area 5 is dominated by sand substrate, ranging from gradual to steep slopes and included the shoreline of “Sand Mountain”15 and the inlet of Hurt Creek on the western shore (HDR 2022b). Sand Mountain is a popular informal recreation area where boaters pull up to the sandy beach to swim and sunbathe. Sand Mountain is steep on the shoreward side and is popularly used for dirt biking and dune climbing. The rest of the western shoreline is also dominated by sand substrate; however, the shoreline slopes are more gradual with only minimal outcrops extending into the littoral zone. Miller Cove on the western shore also contains steep bedrock cliffs extending into the littoral zone. Observed depths in Area 5 during the Resident Fish Survey ranged from 0 feet at the shoreline to approximately 115 feet but can have depths of up to approximately 140 feet (Fishermap.Org, 2022).

15 “Sand Mountain” is located at the eastern end of ID Ridge and the area adjacent to Seminoe Reservoir in this area contains young geologic features dominated by sand dunes.