Application of the Wetted Perimeter Methodology to Identify and Mitigate Potential Impacts from Proposed Exploratory Drilling- Iron Creek, Beartooth R.D., Custer Gallatin National Forest J.A. Efta (East Side Hydrologist, Custer Gallatin National Forest) and C. Sestrich (A-B Zone Fish Biologist, Custer Gallatin National Forest) 2/12/2018 Introduction: Sibanye Stillwater Mining Company (SSMC) has proposed to withdraw water from up to seven stream locations within the Iron Creek drainage as a part of the Iron Creek/West Fork Stillwater Exploratory Drilling Project Plan of Operations. Up to 25 gallons per minute (gpm) would be withdrawn from one or multiple locations from May to November for approximately six years. Beyond regulatory requirements for maintenance of in-stream flows, Yellowstone cutthroat trout persist in the drainage, a USFS Region 1 sensitive species. As such, there is a need to evaluate and mitigate the effects on aquatic ecosystems resulting from water withdrawals. Hydrologic analysis associated with the Iron Creek Drilling Environmental Assessment used the Wetted Perimeter Method for Determining Streamflow Requirements for Habitat Protection (or, more generally, the Wetted Perimeter Method) to assess potential impacts from water withdrawals (sensu Nelson 1984; also Annear and Conder 1984; Lohr 1993; California Dept. of Fish and Wildlife 2013). The Wetted Perimeter Method is the analysis methodology adopted by the Montana Department of Natural Resources and Conservation for determining minimum flow requirements for protection of fish. The method is based on the general fact that, as flow increases in a channel at or near baseflow, wetted perimeter increases more quickly than when flow nears the channel’s maximum width. This relationship manifests itself as a distinctly asymptotic exponential relationship, which becomes more pronounced in cross sections more rectangular in shape (ex. Figure 1). There commonly tend to be two inflection points- or, on the ground, two topographic breaks in a channel cross section- at which wetted perimeter rate of increase changes with flow increases. The uppermost inflection point (also referred to as the incipient asymptote, hereafter referred to as the upper inflection point) has been deemed most appropriate for maintenance of aquatic habitat integrity, particularly for sensitive species such as Yellowstone cutthroat trout, because food production is optimized when flows equal or exceed this level. In general, this inflection point correlates with the maximum wetted width of channel (ex. Figure 1). The lower inflection point (also referred to as a breakpoint, but hereafter referred to as the lower inflection point), in turn, correlates with a point lower in the cross sectional profile where macroinvertebrate habitat tends to steeply decline with reductions in flow (Leathe and Nelson 1989). The above mentioned hydrologic analysis, synthesized in Efta 2016 and discussed within the Iron Creek Exploratory Drilling EA Water Resources Report, suggested that there may be risk to aquatic habitat from water withdrawals in Iron Creek during portions of the water year. Accordingly, there was a need to gather field data to better understand whether model implications may come to fruition during operations. In 2017, a flow monitoring network was designed and installed in the Iron Creek drainage. Monitoring objectives were as follows: 1 Figure 1. Photos from roughly the same location on Picket Pin Creek (one drainage north of Iron Creek) looking downstream correlated with plotted wetted perimeter-flow points on the curve. Photo a) was taken June 27, 2016, Photo b) was taken August 1st, 2016, See continuation on Page 3. White arrows identify the same rock in each picture. Point b is just below the upper inflection point of the wetted perimeter-flow curve. Note the differences in habitat availability at each flow stage, especially on the right side of each picture. 2 Figure 1 (continued). Photos from roughly the same location on Picket Pin Creek (one drainage north of Iron Creek) looking downstream correlated with plotted wetted perimeter-flow points on the curve. Photo c) was taken September 19th, 2016. White arrows identify the same rock in each picture. Point b is just below the upper inflection point of the wetted perimeter-flow curve. Note the differences in habitat availability at each flow stage, especially on the right side of each picture. 3 1. Set up and calibrate stage monitoring equipment in anticipation of flow monitoring during project implementation 2. Monitor seasonal flow fluctuation in Iron Creek and its correlation with the channel’s wetted perimeter 3. Determine minimum discharge at water withdrawal sites required to maintain aquatic habitat protection 4. Determine efficacy of use of the Wetted Perimeter Methodology in steep, high elevation environments. 5. Establish stage-discharge and discharge-wetted perimeter relationships in the vicinity of proposed withdrawal sites along Iron Creek. Hydrologic/Geomorphic Context (partially excerpted from the water resources report in the Iron Creek Drilling EA): Iron Creek falls within the Lower West Fork Stillwater River subwatershed (6th Hydrologic Unit Code (HUC) 100700050203). Elevations in the Iron Creek drainage range from nearly 9300 feet along the watershed divide to approximately 6200 feet near the forest boundary. Across these elevations, watershed hydrology is strongly dependent on timing and magnitude of seasonal snowmelt (generally occurring in May and June). Average annual precipitation in Nye is approximately 18 inches (Weather Underground station Cathedral Mountain Ranch KMTNYE#2, elevation 5259 feet). Roughly 1.75 miles from the project area, average annual precipitation at the Placer Basin SNOTEL site (elevation 8830) is approximately 37 inches. This is further corroborated by PRISM data, which estimates average annual precipitation near the subwatershed divide as approximately 38 inches (Daly et al. 2008). Iron Creek is approximately seven square miles in area (Figure 1). The majority of the drainage immediately adjacent to Iron Creek is mapped as glacial deposits (Geraghty 2013). Bedrock outcrops can be found on both sides of the Iron Creek drainage and lie beneath the surficial deposits that occupy the valley. Bedrock consists of multiple horizons of the Stillwater complex and Cambrian sedimentary rock. There are several mapped faults in the Iron Creek drainage (Geraghty 2013). Surface deposits of colluvium and alluvium serve as local controls on near-surface hydrology. These surface deposits were observed in low gradient locations in the upper half of the drainage, in some cases covered with a relatively thin mantle of poorly developed soils. Variation in sorting of these deposits has resulted in substantial local variability in near-surface hydrology. Upstream of the old bridge crossing Iron Creek, glacial deposits are impounding flow, elevating the water table and creating a wetland complex approximately 20 acres in size. Multiple smaller wetlands can be observed throughout the drainage, likely formed through similar conditions. Conversely, along the old Hootchville Road to the west of the old bridge, some areas display facultative wetland species in close proximity to upland high elevation species, likely due to near surface lateral flow continuity associated with large pore spaces in the colluvium. Downstream of the bridge site, the Iron Creek valley bottom narrows and steepens. The confluence with Hootchville Creek falls within this portion of the channel. Field reconnaissance of Hootchville Creek suggests that the stream is primarily spring fed. While upper portions of the channel fall within low-to- 4 moderate gradient ranges (less than 5%), NetMap data suggest that gradients near Hootchville Creek’s confluence with Iron Creek and Iron Creek itself likely approach, if not exceed, 10% (NetMap 2016). Flow measurements and modeling comparing Picket Pin Creek, the next drainage to the north, to Iron Creek suggest that the wetlands found in the upper part of the Iron Creek drainage are providing substantial buffering capacity to flow within Iron Creek towards the end of the water year (August and September); flows appear to remain generally higher later in the year in Iron Creek than in Picket Pin Creek (Efta 2016, unpublished data). Monitoring Network Design and Measurement Schedule: The Iron Creek flow monitoring network was collaboratively established between SSMC, Hydrometrics (hydrology consultant for SSMC), and the Forest Service. During a field meeting in late July 2017, representatives from each organization evaluated each proposed water withdrawal site in the Iron Creek drainage. Three of the four withdrawal sites in Iron Creek upstream of the bridge were deemed to be appropriate locations for application of the wetted perimeter methodology. The fourth site, Iron Water 3, was similar enough morphologically and close enough in distance to Iron Water 2 that only a staff gauge was installed. The wetted perimeter methodology was deemed not applicable at other sites (Hootchville Creek 1 and 2 and East Iron Water) because flows were largely influenced by springs and wetlands; water storage in these areas will likely offset water withdrawals associated with drilling, posing minimal risk to aquatic habitat. Further, as a result of having less variable annual hydrograph fluctuation and generally lower gradients, these sites generally had more bank stabilizing vegetation and finer grains (sand and silt)