1 Application of the Wetted Perimeter Methodology to Identify And

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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 Up to 25 gallons per minute (gpm) would be withdrawn from one or multiple locations from May to November for approximately six years.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:

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.

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.

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-

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) comprising the channel bed, making any correlation between wetted perimeter fluctuation and potential habitat viability highly challenging if not impossible. Only staff gauges were installed at these locations.

Wetted perimeter sites were established in the general vicinity (within approx. 50-100 yards) of each proposed withdrawal site based on cross sectional morphologic suitability. Sites were chosen based on:

- how representative they were of the channel reach; - uniformity of the cross section; - feasibility for accurately measuring discharge; - whether transitions in and out of the cross section were mostly straight (i.e. not in a meander bed); - whether the cross section had tapered channel edges where fluctuation in stage/discharge would yield a discernible change in wetted perimeter that may expose lateral channel bed at low flows; and - areas with limited flow turbulence.

Figure 1. Iron Creek Vicinity Map.

Channel morphology at Iron Water 1 and 4 was not ideal for use of the wetted perimeter methodology. At Iron Water 4, coarse grain sizes and a relatively steep channel gradient with limited cross-sectional uniformity made identification of a suitable cross section difficult. Iron Water 1 was a difficult site because of the expansive wetland complex just upstream. Just downstream of the proposed withdrawal site, the channel becomes steeper and more confined. Nonetheless, monitoring sites where data could be collected with a reasonable level of confidence were able to be established in the vicinity of each of these withdrawal sites.

At these sites, monitoring stations were installed (Figure 2). These consisted of: 1) an OnsetTM HOBO U20-L water level logger, 2) a staff gauge, and 3) a MoultrieTM Wingscapes wildlife camera set up to take a picture of the staff gauge. Water level loggers were installed in a porous approximately 6” long PVC housing and were either zip tied to rebar pounded into the channel bed or attached to the post supporting the staff gauge. At Iron Water 1, a level logger was also attached at the top of the metal post for the purpose of providing atmospheric pressure adjustment. Both the wildlife cameras and water level loggers were set up to log at one hour intervals. The water level loggers measured both temperature and stage.

In addition to the instruments installed at each site, a cross section was monumented using rebar placed above bankfull elevation on either side of the channel. Survey benchmarks at Iron Water 2 and 4 were conspicuous rocks with distinct markings and/or high points that could be consistently revisited. A location was painted along the southern bridge stringer near Iron Water 1 for use as a benchmark.

Monitoring stations were installed on July 26th and 27th, 2017. Following installation, FS personnel visited the sites every 1-3 weeks until the loggers and camera were removed October 2nd, 2017. During these visits, the following measurements were taken:

- Discharge; - Multiple survey elevations: base of each cross section survey pin (rebar on either end of the channel cross section), bankfull elevation at River Right and River Left, water surface elevation at River Right and River Left; - Stage was recorded at each staff gauge site.

Discharge measurements were taken using standard USGS procedures (Buchanan and Somers 1969) at each monitoring station using a Marsh McBirney meter and top setting wading rod. The number of acquired velocity measurements was at times limited by the width of the meter bulb (0.2 ft).

Surveying was completed using an optical level and standard stadia rod. On August 30th and 31st, a full cross section survey was conducted at each of the wetted perimeter monitoring stations where all major topographic breaks were surveyed.

Stage was not monitored at Hootchville Creek during all field visits because of time availability and access difficulty.

Data Post-Processing:

Following data retrieval, water level data were processed using HOBOwareTM Pro software. Stage data for all three sites were pressure corrected using the logger installed above the water surface at Iron

Water 1. Staff gauge heights recorded during field visits along with data from the wildlife camera (only one ended up yielding useful data, explained in further detail below) were used to validate the corrected stage data. Discharge and survey data were processed in Excel.

Despite the short monitoring time frame, stage-discharge and discharge-wetted perimeter relationships were plotted. Wetted perimeter was calculated using discharge data. Upper and lower inflection points were estimated on the curves and compared against cross section data to discern how well the discharge-wetted perimeter curve represented recorded data.

Figure 2. Example Iron Creek flow monitoring station. The monumented cross section is just upstream out of view.

While temperature data were collected during logger deployment, further post-processing of the data has not been completed to date. Observed temperature ranges were not limiting for aquatic life and are not expected to change as a result of the Iron Creek Exploratory Drilling project. Raw temperature data for each logger can be found in Appendix A.

Observed stage data for Hootchville 1 and 2, Iron Water 3, and East Iron Water (staff gauge monitoring only) are on file at the CGNF Billings office but have not been included in this report.

Results:

Water level logger and cross section data:

Five visits were made to the Iron Creek field monitoring sites between their installation date and their removal on October 3rd. High elevation snowpack prevented earlier field reconnaissance and installation and an early snow event required removal of the instruments in early October. Of these five visits, discharge and wetted perimeter survey measurements were completed during four of them. Monitoring frequency ranged from one to three weeks apart. Competing work priorities, weather, and logistical challenges for accessing the site all dictated the frequency of access. Travel to the Iron Creek monitoring sites required either overnight camping or up to two and half hours of travel one way up to approximately 9,000 feet elevation. This is not accounting for the hiking time required to get to the Hootchville sites or from the ATV trail to the flow monitoring sites on Iron Creek.

Wildlife camera installation for monitoring of stage proved only marginally successful. Cameras, when set up, were tested for accuracy by holding a smartphone against the camera lens and taking a picture of the staff gauge. Camera angle and bearing were adjusted as needed to ensure that staff gauges were within the camera frame. This calibration method proved unsuccessful at two of the three monitoring sites. During picture download it was realized that cameras had been mounted too high to see lower stages. While pictures provided site and channel context, including when ice and snow affected stage measurements, they did not provide secondary validation of the accuracy of pressure conversion to stage using atmospheric barometric pressure. The camera at Iron Water 4 was inadvertently not turned on during installation, so no pictures were taken.

Graphs of recorded pressure, conversions to stage, and recorded temperature at each monitoring site can be found in Appendix A. Barometric pressure-adjusted stage at all three sites matched field- observed stage closely, especially at Iron Water 1 and 2 (recall that the logger used for barometric pressure was at Iron Water 1). Maximum divergence between logged (calculated) stage and observed stage was approximately 20% at Iron Water 4 (Table 1). At Iron Water 1 and 2, maximum divergence was approximately 0.9 percent and 5.2 percent, respectively. It stands to reason that divergence between observed and calculated values would be greatest at sites with the greatest distance and elevation difference between them. Further, cross sectional complexity and associated flow turbulence, albeit relatively minor, may have contributed to the observed versus calculated differences at Iron Water 4.

With several notable exceptions, stage incrementally decreased through the course of the field season until September 15th. Limited stage change was observed in the field and in the water level record prior to mid-September, as evidenced by the channel cross section and wetted perimeter surveys (Appendix B). The Placer Basin SNOTEL site (as noted above, elevation 8830 feet) recorded 1.2 inches of precipitation between August 10th and August 16th (Figure 3). A distinct peak was recorded on August 14th at all three monitoring sites at 5:48 AM at Iron Water 4, 8:40 AM at Iron Water 2, and 9:32 AM at Iron Water 1. The size of the peak was most pronounced at Iron Water 2 with two tenths of a foot of stage change. Recorded stage data suggests that another minor storm peak, albeit less pronounced, occurred on August 24th, where approximately 3/10th of an inch of rainfall occurred between August 23rd and 25th. Minimum recorded stage at all three sites occurred on Sept. 12th at Iron Water 1 and 2 and on the morning of Sept. 13th at Iron Water 4.

Between September 15th and 22nd, a substantial storm event hit southcentral Montana, with higher elevations experiencing snow and ice. The Placer Basin SNOTEL site registered approximately 2.9 inches of precipitation between these dates. Between 11 am on 9/21 and 11 am on 9/22, 1.3 inches of

precipitation was measured at the site. Pictures taken using the wildlife cameras during that time frame indicate that pressure measurements (i.e. stages) recorded between approximately September 21st and September 26th in the Iron Creek basin were affected by ice. Stages at Iron Water 4 were recorded as negative on Sept. 14th and for much of the remaining monitoring period, implying significant ice accumulation at the site. A negative value may suggest water was running over the top of an ice shelf formed over the pressure transducer, though no data is available to support that hypothesis.

Stages were high enough to be visible in pictures at Iron Water 1 and 2. When the cameras weren’t obscured by snow, recorded water levels did not appear to be severely affected by ice accumulation; stage measurements appeared to still roughly match those stages recorded by the water level loggers. This observation suggests that wetted perimeter would still be representative of conditions on site at these two locations despite discharge measurements calculated from recorded stages being inevitably suspect when ice is present.

Generally, ice-affected data points should be omitted from further analysis. This was required at Iron Water 4 as a result of having negative recorded stages. Iron Water 1 and 2, however, were approached differently. Given a) the limited data availability at this site due to short monitoring period, b) the observed consistency between recorded stage and observed staff gauge height, and c) the general consistency in response at Iron Water 1 and 2 (suggesting limited influence of ice on recorded response), ice-affected values were left in for analysis purposes. Where applicable, subsequent data analyses have been qualified to denote the inherent uncertainty in further application of these recorded values.

Table 1. Maximum and minimum absolute difference between observed and calculated stage at each monitoring location. Stage (pressure) measurements were logged every hour, so comparison between observed versus calculated stages were based on the nearest hour to the time of observation. For example, a stage observed on 8/17 at 1441 was observed with the stage logged at 1500; a stage logged at 1125 was correlated with the stage logged at 1200. Difference Difference between Percent between observed and difference observed and Percent difference Monitoring location calculated from calculated from observed maximum observed minimum stage absolute stage stage absolute stage (ft) (ft) Iron Water 1 0.020 0.893 0.001 0.044 Iron Water 2 0.025 5.263 0.004 0.896 Iron Water 4 0.064 19.63 0.006 1.345

36 PREC.I-1 (in) 35 PREC.I-2 (in)

Wateryear accumulated precipitation(inches)

8/3/2017 9/2/2017

7/24/2017 8/13/2017 8/23/2017 9/12/2017 9/22/2017 10/2/2017

Date 10/12/2017

Figure 3. Montana (PST) SNOTEL Site Placer Basin (696) (10D24S) Daily series for July 30th- Sept. 30th, 2017. NRCS National Water and Climate Center- Provisional Data- subject to revision as of 2017-November-29. Two precipitation sensors are housed at the same SNOTEL site (PREC.I-1 and PREC.I-2).

Fitting of stage-discharge and discharge-wetted perimeter curves:

Stage-discharge rating curves and discharge-wetted perimeter rating curves were generated for each of the three monitoring sites. With the extremely limited number of stage and discharge measurements taken on-site, an insufficient amount of data was available to know whether the curves are valid other than at the very narrow range of flows that were measured in the field. The minimum number of data points required to achieve statistical validity for a rating curve is not established in the scientific literature. Kennedy (1984) notes that generally rating curves consist of three segments: a low water, medium water, and a high water depth/flow portion. Given the range of flows observed during the monitoring period, stage-discharge rating curves developed here would be likely most representative of low-water and possibly medium-water depths/flows. Despite the uncertainty associated with any curves generated with limited available data, each of these curves has been presented below.

Stage-discharge rating curves are frequently represented by power functions, though portions of the curve may be better represented using a different relationship. At all three monitoring sites, power, logarithmic, and linear functions were each trial fit to the data. To help inform the applicability of each of the response functions, each stage-discharge rating curve was inverted so that discharge could be solved as a function of stage rather than vice versa. Calculated stage from each water level logger was then used to estimate discharge from each stage-measured data point. Though there is potential for substantial error in these calculations due to the limited number of field-observed data points available for validation, this procedure was conducted for two reasons: 1) this provided at least a ballpark idea of range of flows encountered during the 2017 observation window, and 2) these discharges proved to be useful in determining how well a stage-discharge model of a given function type fit the data. The strong agreement between observed and recorded stage supported application of this technique.

At Iron Water 4, a linear trendline was the best model fit with an R2 of better than 0.99 (Figure 4). Use of the inverted model (solved for discharge instead of stage) for calculating stage depths, however, resulted in numerous negative and unreasonably small flow estimates. Application of a power function yielded a more reasonable range of estimated flows through the course of the observation window.

Using the power function, estimated flows through the course of the monitoring period ranged from 0.5 gpm to 1498 gpm (3.3 cfs). Maximum discharge measured in the field was 0.38 cfs (169 gpm) and minimum measured discharge was 0.13 cfs (59.7 gpm).

Maximum discharged measured at Iron Water 2 was 2.2 cfs and minimum measured discharge was 1.1 cfs. Flows at Iron Water 2 were best estimated using a linear function (Figure 5). When a power function was fit to the data and used to estimate flows, the range of estimated flows following the mid- September precipitation event was inordinately high given the range of stages observed by the wildlife camera and lack of field indicators suggesting such high flows. Accordingly, the linear function was retained as the best fit model for the low water data. It is important to qualify the uncertainty associated with application of this method to estimate ranges of flows; application of a low water stage- discharge curve to represent medium to high water flow events and ice-affected pressure measurements both lend themselves to some uncertainty. These data, however, do provide some insight into discharge ranges occurring at this site through the observation period.

Figure 4. Iron Water 4: a) Example linear stage-discharge model fit to field observed data, b) Inverted power function model fit, and c) estimated discharge range through the course of the 2017 monitoring period using the inverted stage-discharge power function model. Note that one cubic foot per second equals 448 gallons per minute. Four data points are included in a and b; two points measured on separate field visits had identical stages and discharges.

Maximum and minimum field-measured discharges at Iron Water 1 were 3.1 cfs and 0.9 cfs, respectively. Similar to Iron Water 2, log and power functions yielded a likely overestimate of flows calculated for high stages at Iron Water 1 (estimated peak flow associated with the September 21st/22nd storm event was on the order of 16 cfs). This suggests that a linear function is the best fit stage- discharge model for low if not also medium water levels at this monitoring site (Figure 6).

Fitted power functions for discharge-wetted perimeter curves at Iron Water 4 and Iron Water 2 yielded R2 values of 0.71 and 0.77, respectively (Figures 7a and 8a). Applying those same functions to the data recorded by the water level logger provided some insight into the range of flows wetted perimeters likely experienced at both sites through the course of the monitoring period (See Figures 7b and 8b).

Comparison between Figure 7a, 7b, and the cross section survey with plotted stages at Iron Water 4 (Figure 1 in Appendix B) suggest that those stages/flows and associated wetted perimeters observed during the monitoring period generally correspond with the appropriate location on the fitted power function. In other words, the discharge-wetted perimeter rating curve appears to be a mostly reasonable representation of conditions encountered on site and reasonable extrapolation of conditions beyond those encountered during field visits.

Figure 5. Iron Water 2: Linear stage-discharge model fit to field observed data (left) and estimated discharge range through the course of the 2017 monitoring period using the inverted stage-discharge power function model (right). Note that only three data points were used here; the fourth data point was removed because of an erroneous discharge-stage relationship, likely due to observation error in the field.

Figure 6. Iron Water 1: Linear stage-discharge model fit to field observed data (left) and estimated discharge range through the course of the 2017 monitoring period using the inverted stage-discharge power function model (right). Note that the monumented cross section at Iron Water 1, downstream of the stage monitoring site, has a thalweg depth roughly half that observed at the Iron Water 1 stage monitoring site. This discrepancy affects the stage correlated with modeled upper and lower wetted perimeter wetted perimeter inflection points and has been compensated for (See Appendix B Figure 3).

Wetted perimeter at extremely low flows (below approximately 0.4 ft of depth) will not change as rapidly as discharge due to a somewhat “V” shaped cross section at Iron Water 4; the extreme lower end of the discharge-wetted perimeter curve may not be an accurate representation of the observed relationship between these two values. Bankfull wetted perimeter calculated from survey data was 9.6 feet, suggesting that range of flows observed through the course of the monitoring period would be best categorized as low to medium (per earlier discussion regarding Kennedy 1984 and representative rating curve flows).

Upper and lower inflection point tangents have been approximated for Iron Water 4 in Figure 7b. The upper inflection point falls at roughly 300 gallons per minute, which is in excess of any flows measured during field visits. The lower inflection point falls at approximately 15 gallons per minute. Numerous recorded stages (flows) during the monitoring period fell below this level (Figure 4c).

Evaluation of Figure 8a and 8b in tandem with the cross section survey and observed stages at Iron Water 2 (Appendix B, Figure 2) suggest a reasonable discharge-wetted perimeter model fit to conditions observed on the ground, more so than with Iron Water 4. Wetted perimeter for bankfull discharge at Iron Water 2 was calculated as 21.1 feet. Also similar to Iron Water 4, this suggests that range of recorded flows never exceeded approximately two thirds of bankfull wetted perimeter and validates the notion that the range of represented flows would best be described as low to medium level.

Figures 7a (left) and 7b (right). Iron Water 4: Stage-wetted perimeter plot for data measured in the field at with a fitted power function (7a) and calculated discharge plotted against wetted perimeter with approximated tangents at upper (dashed line) and lower (dotted line) inflection points (7b). Note that the wetted perimeter and discharge were calculated separately for each recorded data point; use of power-function derived discharges yielded poor representation of wetted perimeter. Wetted perimeter for each recorded point was calculated using a stage-WP power function relationship and discharge was calculated using a separate power function relationship with stage.

Figures 8a (left) and 8b (right). Iron Water 2: Stage-wetted perimeter plot for data measured in the field at with a fitted power function (8a) and calculated discharge plotted against wetted perimeter with approximated tangents at upper (dashed line) and lower (dotted line) inflection points (8b).

Recorded flows fell slightly lower on the wetted perimeter curve than with Iron Water 4. Again, the fitted curve is reasonably representative of those flows and wetted perimeters where inflection points occur in the channel’s cross section (Appendix B Figure 2). A lower proportion of flows fell at or below the lower inflection point at Iron Water 2 than at Iron Water 4. As discussed above, the highest calculated discharges may not be accurate due to the influence of ice.

Approximated inflection points for Iron Water 2 have been plotted in Figure 8b. The upper inflection point fell at approximately 2.6 cfs, which is just above the highest flow measured at Iron Water 2 during field visits. The lower inflection point roughly corresponds with 0.5 cfs. The majority of the recorded stages (flows) fell below the upper inflection point during the monitoring period, but a limited number fell at or below the lower inflection point (Figure 5b).

Discharge-wetted perimeter plots for Iron Water 1 can be found in Figure 9a and 9b. At this site, large rocks along River Left created a backwater pool along the edge of the channel. This site falls at the outlet of a large wetland complex in a channel glide. Accurate wetted perimeter measurements were difficult to attain, as evidenced by the outlier point in the discharge-wetted perimeter plot in Figure 9a and the less accurate model fit. At this site, bankfull wetted perimeter was 34.8 feet, indicating once again that the range of encountered flows was in the low to medium range as it relates to the stage-discharge and discharge-wetted perimeter fluctuation.

Figures 9a (left) and 9b (right). Iron Water 1: Stage-wetted perimeter plot for data measured in the field at with a fitted power function (9a) and calculated discharge plotted against wetted perimeter with estimated tangents at upper and lower inflection points (9b).

Approximated upper inflection point at Iron Water 1 is about 3 cfs and the lower inflection point is about 0.9 cfs. Calculated wetted perimeter upper and lower inflection points again generally correlate with applicable cross sectional morphologic breaks (Appendix B Figure 3). As with Iron Water 2 and 4, the majority of recorded stages (flows) during the monitoring period were below the upper inflection point. A limited number, however, were at or below the lower inflection point.

Discussion/Management Implications:

The flow monitoring methodology implemented during the 2017 field season, for the most part, adequately captured flow variability and stage dynamics for the portion of the field season that the instrumentation was in place. Staff gauges at Hootchville Creek were accessed less frequently than the rest of the gauge network due to the long travel distance required to access. Wildlife cameras would be ideal for monitoring these water withdrawal/ staff gauge sites. Installation of a flume at one or both of the Hootchville proposed withdrawal sites could further validate the limited concern associated with water withdrawal to support drilling operations.

While not properly implemented during the 2017 field season, the site context provided by the wildlife cameras even when stage was not visible proved highly useful and cost effective for understanding what factors were at play in influencing stage (and, by extension, discharge), particularly late in the field season. A tablet-based SD card reader would allow field personnel to check whether cameras have been set at proper elevations to capture stage at the ranges likely to be observed during the monitoring period. Steps will be taken to employ this technology next field season.

There was a limited range of reliable stage/discharge measurements recorded during the monitoring period. Range of observed stages increased with the September storm event, but calculated discharges from those stages may not be valid. In the case of Iron Water 4, negative stage values rendered the data unusable and inaccurate. While a greater range of stages were observed late in the field season resulting from ice-affected stormflows, model estimates of those flows are suspect.

Ranges of calculated flows were similar for Iron Water 2 and Iron Water 1, despite Iron Water 1 having approximately one more square mile of drainage area and another contributing tributary than Iron Water 2. Stormflow response also appeared to be stronger at Iron Water 2 than Iron Water 1. The attenuating effects of wetlands upstream of Iron Water 1 are likely responsible for this difference in response.

Approximated upper and lower wetted perimeter inflection points suggest that the majority of flows during the monitoring period at all three sites fell below the upper wetted perimeter inflection point. Cross sectional channel morphology, however, only is conducive to lateral gravel exposure at Iron Water 2. Wetland contribution to flows at Iron Water 1 and 4 and the coarser grain sizes and steeper gradient at Iron Water 4 likely translate to limited aquatic habitat impacts when flows are between the upper and lower inflection points. At Iron Water 4, data suggest that the lower inflection point corresponds with so little flow that aquatic habitat would be naturally limited. Access to wetland-influenced channel habitat upstream of Iron Water 1 means that likely little impact would be sustained on cutthroat trout during periods of flow below the lower inflection point.

Of all withdrawal sites, water withdrawal at Iron Water 4 would likely have the greatest potential impact on aquatic habitat for the greatest duration of the field season. Discharge is already limited to just less than 0.15 cfs under summer and fall baseflow conditions (Figure 4). This discharge and corresponding stage falls between the approximated upper and lower wetted perimeter inflection point. Under these flow conditions, water withdrawals would at the maximum proposed rate would reduce instream flows by at least 40%, further reducing the already limited aquatic habitat availability during baseflows. These data and analysis generally support modeled findings conducted as a part of the Iron Creek Exploratory Drilling EA (i.e. Efta 2016). Biasing a water withdrawal window toward the beginning of the field season, decreasing withdrawal rates, and/or utilizing storage tanks would ensure that Iron Water 4 discharge and wetted perimeter does not fall below natural baseflow conditions and impair aquatic habitat. More specifically, data suggest that aquatic habitat integrity would be maintained when water withdrawal at the maximum rate (25 gpm) occurs only when discharge at Iron Water 4 is greater than or equal to 0.5 cfs (224 gpm) (0.49 ft stage). For context, the majority of logged stages (and corresponding estimated discharges) fell below this level during the monitoring period. When stream discharge falls to between 100 gpm and 224 gpm, withdrawal rate would need to be reduced to 10 gpm. Prohibiting withdrawal when stream discharge falls below 100 gpm (0.22 cfs) (0.37 ft stage) would minimize the potential for aquatic habitat impairment at this site.

At Iron Water 2, ranges of flows experienced during the latter part of the water year, coupled with the cross sectional morphology that is conducive to macroinvertebrate impairment when lateral gravels are exposed, suggest that there is potential for impairment resulting from water withdrawals. This is particularly of concern late in the water year. Data suggests that flows are generally adequate to support withdrawal of 25 gallons per minute even late in the water year. Of greater concern, however, is potential impact on aquatic habitat when lateral channel gravels (particularly on River Left) would be exposed at a stage of approximately 0.3 ft, which corresponds with the approximated lower inflection point of the wetted perimeter. While a linear stage-discharge function suggests that flows would be negative at this stage, if a power function were used to explain the relationship discharge at this stage would be approximately 0.39 cfs (173 gpm) (Of note- this discrepancy highlights the need to collect more field data next year in order to generate stage-discharge rating curves validated across a wider range of flow levels). Instituting a safety factor of 2, given the data and model accuracy, approximately 0.8 cfs (346 gpm) (0.4 ft stage) may be an appropriate minimum flow target; water withdrawal could only occur when in-channel flows are in excess of this value. The majority of flows through the course of the monitoring period were well in excess of this value, thereby creating a limited concern for constraining withdrawal during project activities.

At Iron Water 1, there is limited concern for habitat impairment as a result of water withdrawals. No restrictions beyond those outlined in the Iron Creek Exploratory Drilling EA are necessary to ensure that aquatic habitat would not be impaired during project implementation.

Limitations of data analysis, presented hypotheses, and conclusions:

Much of this analysis is heavily contingent on the validity of the fitted functions for representing relationships between stage and discharge. These fitted functions are based on relationships between very few data points. While substantial effort has gone into assessing and contextualizing the validity of these modeled relationships, conclusions drawn from these data must be viewed as preliminary until further field validation data can be gathered. At least one more season of field data collection for low

and medium flow levels will be necessary to validate the modeled relationships discussed above for low and medium flows. Next season’s data collection will also try to capture early season flows to begin filling in this critical data gap.

Continuous power or linear functions were used to explain relationships between stage and discharge and discharge and wetted perimeter. Inevitably, these relationships will not be a perfect explanation of what is observed on the ground; channel cross sections are never perfectly smooth and as such continuous curves will always have some inherent error when representing these relationships (e.g. Kean and Smith 2010). The remote nature of this site, however, forces the use of limited field data to infer as much as possible. Identification of inflection points on the curves is also somewhat subjective, which is one limitation of the wetted perimeter methodology (Annear and Conder 1984). Uncertainty or potential error in calculations and estimates presented here have been highlighted. Substantial effort was made to qualify the validity of conclusions or hypotheses made throughout this report and multiple lines of evidence were used to support those conclusions and hypotheses wherever possible. As noted above, next season’s field data collection will provide further validation. General management recommendations presented here will be refined but are not anticipated to substantially change following collection and analysis of additional data.

References:

Annear, T.C., and Conder, A.L. 1984. Relative bias of several fisheries instream flow methods: North American Journal of Fisheries Management, v. 4, p. 531-539.

Buchanan, T.J., and Somers, W.P. 1969. Discharge measurements at gaging stations: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap A8, 65 p. (Also available at https://pubs.usgs.gov/twri/twri3a8/).

California Department of Fish and Wildlife. 2013. Standard Operating Procedure for the Wetted Perimeter Method in California. California Department of Fish and Wildlife Instream Flow Program Standard Operating Procedure DFG-IFP-004, 19 p. Available at: http://www.dfg.ca.gov/water/instream_flow.html.

Daly, C., Halbleib, M., Smith, J.I., Gibson, W.P., Doggett, M.K., Taylor, G.H., Curtis, J., and Pasteris, P.A. 2008. Physiographically-sensitive mapping of temperature and precipitation across the conterminous United States. International Journal of Climatology, 28: 2031-2064.

Efta, J.A. 2016. Application of Wetted Perimeter methodology for evaluating stream flow impacts from drilling water withdrawals, Iron Creek drainage, Beartooth District, Custer Gallatin National Forest. 11 p.

Kean, J. W., and Smith, J.D. 2010. Calculation of stage‐discharge relations for gravel bedded channels. J. Geophys. Res. 115: F03020, doi:10.1029/2009JF001398.

Leathe, Stephen A. and Frederick A. Nelson. 1989. A Literature Evaluation of Montana’s Wetted Perimeter Inflection Point Method for Deriving Instream Flow Recommendations. Montana Department of Fish, Wildlife and Parks. Helena, Montana.

Lohr, S.C. 1993. Wetted stream channel, fish-food organisms and trout relative to the wetted perimeter inflection point instream flow method. PhD Dissertation: Montana State University. 271 p.

Nelson, F.A. 1984. Guidelines for using the wetted perimeter (WETP) computer program of the Montana Department of Fish, Wildlife, and Parks: Bozeman, MT, Montana Department of Fish, Wildlife, and Parks, variously paged.

NetMap. 2016. Virtual watershed and analysis tools. TerrainWorks Inc. www.terrainworks.com

Appendix A. Iron Creek flow monitoring- raw pressure/stage and temperature data.

Figure 1. Iron Water 4 pressure, barometric pressure-corrected stage data derived from pressure, and temperature data for the 2017 monitoring period.

Figure 2. Iron Water 2 pressure, barometric pressure-corrected stage data derived from pressure, and temperature data for the 2017 monitoring period.

Figure 3. Iron Water 1 pressure, barometric pressure-corrected stage data derived from pressure, and temperature data for the 2017 monitoring period.

Appendix B. Channel cross section graphs at each monitoring site.

Figure 1. Iron Water 4 Cross section and water surface elevations for 2017 field visits and approximated upper and lower inflection points calculated from collected data.

Figure 2. Iron Water 2 Cross section and water surface elevations for 2017 field visits and approximated upper and lower inflection points calculated from collected data.

Figure 3. Iron Water 1 Cross section and water surface elevations for 2017 field visits and approximated upper and lower inflection points calculated from collected data. As noted in Figure 6’s caption, stage at the Iron Water 1 monitoring site did not correlate well with stage at the monumented cross section. A calculated ratio between thalweg depth observed during discharge measurements at the monumented cross section and logger-calculated stage was used infer stages corresponding to upper and lower wetted perimeter inflection points.