3.4.1.3 Surface Waterbodies and Wetlands

Table 3.4-2 summarizes the areas of aquatic resources identified in the study area by feature class (HDR 2022a).

Table 3.4-2. Acreage Summary of Aquatic Resources Identified within the Aquatic Resources Delineation Study Area

Feature Class Acres

Freshwater Emergent Wetland Ephemeral Channel Intermittent Channel

Perennial Channel

Open Water Habitat

Total

Source: HDR 2022a.

3.4.1.3 Surface Waterbodies and Wetlands

9.05

0.86

0.36

1.72

27.66

39.65

As previously mentioned, the Project will be located in the North Platte River Basin, a subbasin of the Platte River Basin, which is 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 and the existing Seminoe Reservoir (lower reservoir) is located in both the Pathfinder-Seminoe Subbasin and the Medicine Bow Subbasin (Black Canyon 2020b).

Stream types in Carbon County include perennial, ephemeral, and intermittent. Major perennial streams in Carbon County either have headwaters in the Sierra Madre or Medicine Bow Mountains in the southern part of the county or flow into the county from mountainous areas in northern Colorado. Many of the smaller streams in the county that have headwaters in lowland areas are ephemeral, and flow only as a result of local snowmelt and rainfall runoff. Streamflows in intermittent streams vary depending on reach characteristics. Snowmelt runoff, ground-water inflows, and/or springs maintain streamflows throughout most years in some perennial reaches, whereas ephemeral reaches exist where streamflows are less than the losses to seepage, evaporation, and/or diversions (Bartos et al. 2006).

Surface drainage in the Platte River Basin is controlled by topography and is essentially equivalent to physiography. Perennial streams receive a large percentage of their source waters from overland flow associated with snowmelt and rainfall that originate in semihumid and humid mountainous headwater regions and persistent baseflow. Most ephemeral flow occurs in response to springtime snowmelt and to intense, short duration rainfall events characteristic of transient convective thunderstorms. Streamflows are also affected by vegetation, temperature, manmade diversions, and complex interconnections with groundwater (Taucher et al. 2013).

The USGS National Hydrography Dataset (NHD) (USGS 2019) identifies 62 water features within the proposed Footprint of Potential Disturbance. Many of these features were also

identified in the USFWS National Wetlands Inventory (NWI) database (USFWS 2019). The NHD and NWI mapped surface water features within the proposed Footprint of Potential Disturbance were assessed during the June and August 2021 and the August 3, 2022, aquatic resources delineation completed by Black Canyon (HDR 2022a) (included as Appendix B) to determine if their current characteristics and locations matched the datasets. As summarized above, the aquatic resources identified in the survey area were 13 freshwater emergent wetlands, 13 ephemeral streams, 2 intermittent streams, 5 perennial streams, and 1 open-water habitat feature (i.e., Seminoe Reservoir) (HDR 2022a). The extent of these features within the Footprint of Potential Disturbance are shown in the mapbook included in the study report at Appendix B, and each feature class is described in more detail below. The aquatic resources identified in the Footprint of Potential Disturbance are summarized in the study report in Appendix B.

Perennial Channels

The North Platte River is one of the dominant perennial surface-water features in Carbon County as well as in the Project vicinity. The North Platte River flows into the southeast corner of Carbon County from its headwaters in the mountains of northern Colorado. The river flows out the northern edge of the county through Pathfinder Reservoir. It continues through Natrona, Converse, Platte, and Goshen Counties before flowing into Nebraska. The North Platte River is eventually joined by the South Platte River near the City of North Platte, Nebraska. At the confluence, the two rivers combine to form the Platte River, a major tributary to the Missouri River. The river is an important surface-water resource for the region today, providing water for irrigation, industry, municipal, and hydroelectric uses (Bartos et al. 2006).

The largest use of water from the North Platte River is for irrigation. Irrigation along the North Platte probably began with crude ditches used to irrigate irregular patches of land on the flood plain. By the late 1800s, extensive development of the North Platte had taken place, and by 1901, summer flows in the Upper North Platte River and its tributaries had been over-appropriated. To help mitigate this over-appropriation, Reclamation projects were constructed over the next several decades. The two most notable of these Reclamation projects affecting Carbon County are the North Platte and Kendrick Reclamation Projects. Seminoe Dam was constructed in the north-central part of the county between 1936 and 1939 as part of the Kendrick Reclamation Project. Water stored in Seminoe Reservoir is used to help irrigate about 24,000 acres west of Casper, Wyoming. Although development of water resources from the North Platte River and its tributaries has helped to make the region productive economically, it also has changed streamflow characteristics causing a deterioration of riverine habitat (Bartos et al. 2006).

The North Platte River within the proposed Footprint of Potential Disturbance downstream of Seminoe Dam is characterized as a relatively permanent water (RPW), with an average OHWM width of 81 feet. According to the FGDC (2013), this section of the North Platte River is classified as riverine, upper perennial, unconsolidated bottom (R3UB). During the Aquatic Resources Delineation Study (HDR 2022a), observed substrate in this area was dominated by small boulders with areas of cobble/rubble. The stream banks in this area are relatively steep, generally unvegetated, and dominated by boulders and cobble.

Additional perennial streams in the Project vicinity include Austin Creek, Difficulty Creek, Saylor Creek, and Troublesome Creek. Each of these perennial streams have relatively abrupt boundaries between the stream channel and adjacent upland areas due to channel downcutting (i.e., erosion). In some locations, adjacent agricultural activities have degraded riparian vegetation, compromising soil structure along the banks of the streams and increasing bank erosion. In locations along these streams, the historical floodplains have become hydrologically disconnected from the streams. The beds of the perennial channels in the Project vicinity primarily comprise coarse material (e.g., cobble, gravel, and sand), with low proportions of fines (silt and clay). Most perennial stream channels within the Project vicinity are moderately to weakly entrenched and generally lack flanking of riparian scrub-shrub species. The active channels typically include immediate herbaceous hydrophytic vegetation, quickly transitioning to upland vegetation moving up the steep banks. Riparian vegetation included willow species (Salix spp.), Nebraska sedge (Carex nebrascensis), baltic rush (Juncus arcticus), creeping foxtail (Alopecurus arundinaceus), greasewood (Sarcobatus vermiculatus), Wyoming big sagebrush (Artemisia tridentata spp. wyomingensis), rabbitbrush (Chrysothamnus viscidiflorus), Great Basin wildrye (Leymus cinereus), smooth brome (Bromus inermis), prairie junegrass (Koeleria macrantha), Canada thistle (Cirsium arvense), Maximilian sunflower (Helianthus maximiliani), and wild licorice (Glycyrrhiza lepidota) (HDR 2022a).

Intermittent Channels

Streams S-6A, S-6B, S-6C, S-6D, S-6E, and S-14 (Appendix B) are intermittent channels that vary from low to moderate gradients with the average width of the OHWM varying from 2 to 6 feet. During the 2021 field surveys (HDR 2022a), these streams had relatively slow-moving or stagnant water due to the dry season and extreme drought conditions at the time of the delineation. Intermittent streams with no flow contained isolated pools of surface water and/or saturated or inundated emergent wetlands. Documented stream beds mainly consisted of cobble-gravel, mud, and sand, while associated riparian vegetation included upland scrub, dense upland grasses, and/or emergent wetlands.

Ephemeral Channels

All ephemeral streams are associated with the upper reservoir access road or transmission line portion of the Aquatic Resources Delineation Study area. Ephemeral channels (Appendix B) vary from low to moderate gradients with the average width of the OHWM varying from 2 to 12 feet. In general, ephemeral streams within the Project vicinity can be placed into two broad categories: those that include a low-flow channel, active floodplain, and low terrace; and those with a single channel, without differentiation of stream geomorphic features. In general, those that included the stream geomorphic features included more herbaceous vegetation along the stream banks, while those lacking stream geomorphic features mostly only included upland shrub species, such as big sagebrush. Some or all of the following plant species were identified along the ephemeral channel banks: big sagebrush, greasewood, rabbitbrush, tansyleaf tansyaster (Machaeranthera tanacetifolia), western ragweed (Ambrosia psilostachya), dandelion (Taraxacum oficianale), yellow falsify (Tragopogon dubius), Great Basin wildrye (Leymus cinereus), western wheatgrass (Pascopyrum smithii), prairie junegrass (Koeleria macrantha), needle

and thread (Hesperostipa comata), and Indian rice grass (Achnatherum hymenoides). The substrate of ephemeral channels within the Aquatic Resources Delineation Study area primarily consisted of silt/sand, except some that included scattered cobble and even bedrock at higher elevations (HDR 2022a).

Freshwater Emergent Wetlands

Emergent wetlands are areas characterized by erect, rooted, herbaceous hydrophytes, excluding mosses and lichens (FGDC 2013). Emergent wetlands include areas commonly referred to as marshes and wet meadows. Surface water inundation may be relatively brief to permanent, but water depth is sufficiently shallow (usually less than 3 feet) so that rooted plants are emergent in at least part of the wetland.

Thirteen emergent wetlands totaling approximately 9.05 acres were delineated within the study area as part of the Aquatic Resources Delineation Study (Appendix B) (HDR 2022a). Some of these wetlands abutted channels within the study area and some were associated springs and seeps, and others occurred as slope and depressional wetlands. All emergent wetlands identified within the study area were dominated by emergent vegetation, primarily Nebraska sedge and baltic rush. Other dominant species included common spikerush (Eleocharis palustris), dwarf spikerush (Eleocharis parvula), white willow (Salix alba), foxtail barley (Hordeum jubatum), bluejoint (Calamagrostis canadensis), common threesquare (Schoenoplectus pungens), seaside arrowgrass (Triglochin maritima), creeping foxtail, yard knotweed (Polygonum aviculare), and rough cocklebur (Xanthium strumarium) (HDR 2022a).

Seminoe Reservoir

Seminoe Reservoir is currently owned, operated, and maintained by Reclamation and is primarily used for power generation, recreation, municipal use, flood control, and downstream irrigation (Black Canyon 2020b). Seminoe Reservoir is the first and most upstream reservoir on the North Platte River and, therefore, anything upstream from the reservoir is not regulated by a reservoir system for flood mitigation as areas downstream from the reservoir are (Carbon County 2021).

Seminoe Dam and its powerplant are located on the north end of Seminoe Reservoir. At the maximum normal operation pool elevation of 6,357 feet, the storage capacity of Seminoe Reservoir is 1,016,717 acre-feet (ac-ft). The elevation of the gated spillway is 6,307.0 feet (Reclamation 2020a). Water released at the dam is used for irrigation, hydropower, flood control, or stored in downstream federal reservoirs for later use. Seminoe Dam and Seminoe Reservoir along with the Alcova Dam and powerplant, Casper Canal and laterals, and other drainage distributions make up the Kendrick Project (Reclamation 2020b). The Kendrick Project was a cooperative agreement between Reclamation and the State of Wyoming to construct a series of dams and reservoirs to be used to supply irrigation waters to farmers in eastern Wyoming and western Nebraska in the early 1900s. These structures are operated to work together to control river waters. The Casper Alcova Irrigation District (CAID) was formed concurrently and helps to provide

irrigation water by operating and maintaining conveyances serving lands downstream from the headgate, which diverts directly from Alcova Reservoir (State of Wyoming [WY] 2020).

Seminoe Reservoir is 24 miles long from Seminoe Dam to the North Platte River inflow bay and averages 0.8 mile wide. Seminoe Reservoir is bifurcated with the Medicine Bow River forming a second major arm that is 12 miles from the Medicine Bow River inflow to the main body of the reservoir. Seminoe Reservoir has a maximum depth of 206 feet, a mean depth of 50 feet, and an approximate shoreline length of 210 miles. Flows up to approximately 4,000 cfs are released from the reservoir through the Seminoe Powerplant for electrical generation. Flows in excess of 3,000 cfs and up to 48,000 cfs are released through a controlled spillway tunnel (WGFD 2021a).

Water levels in Seminoe Reservoir are highly dependent upon the runoff in a given 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).

Light penetration in the North Platte and Medicine Bow arms of Seminoe Reservoir is greatly reduced by the turbidity of the runoff in early summer. The water clears by midsummer, but a turbid underflow apparently passes through Seminoe Reservoir, causing reduced light availability in Kortes Reservoir and the Miracle Mile in the late summer and fall (Reclamation 1981).

More recent Seminoe Reservoir water surface elevation data obtained as part of the Resident Fisheries Survey Study (HDR 2022b) indicated that Seminoe Reservoir water surface elevations were continuously decreasing during the 2021 sampling events. During 2021, Seminoe Reservoir was being drawn down for a late 2021 test of Seminoe Dam. During the 2021 sampling of Seminoe Reservoir, water surface elevations decreased from 6,323.1 to 6,309.0 feet over the course of sampling with the largest single day decrease of 0.35 foot occurring on June 20, 2021. Although the daily elevation decrease was relatively minor, the seasonal decrease from the beginning to the end of the 2021 Resident Fish Survey Study decreased approximately 14.1 feet (HDR 2022b).

According to the FGDC (2013), large areas of Seminoe Reservoir are classified as L1UBHh (lacustrine, limnetic, unconsolidated bottom, permanently flooded, diked/impounded). Seminoe Reservoir comprises approximately 27.66 acres of the study area on the northeastern side of Seminoe Reservoir (HDR 2022a).

North Platte River Downstream

Downstream of Seminoe Dam and the Project’s Footprint of Potential Disturbance, the North Platte River flows directly into Kortes Reservoir, an 83-acre reservoir that is also operated by Reclamation as part of the Kendrick Project. Kortes Dam is situated in the