SUN RIVER PROJECT Operating Guide Greenfields Irrigation District

Operating Guide For Greenfields Irrigation District January 2021

The Sun River Project operational guide is to document the available tools and resources to assist Greenfields Irrigation District (GID) with operational decisions at Gibson Dam and the Sun River Project. This guide is not all-inclusive and is not meant to replace any information in other official Reclamation documents. This guide is a collaborative effort by GID, Reclamation, Sun River Watershed Group, Trout Unlimited, Montana Department of Natural Resource, and the Montana Fish, Wildlife, and Parks to document operational information.

Table of Contents Water Rights...... 3 Storage ...... 3 Natural Flow of the Sun River ...... 3 Broken O Ranch ...... 4 MT FWP Instream Reservation ...... 4 Water Right Agreements ...... 4 Data Used in Operations...... 5 U.S. Geological Survey (USGS)...... 5 National Weather Service (NWS/NOAA) ...... 5 Reclamation – HydroMet ...... 5 Table 1. Sun River Watershed Gage Information...... 6 Natural Resource Conservation Service (NRCS) ...... 7 Figure 1. NRCS SNOTEL Sites ...... 7 Gibson Reservoir Inflow Forecasting Tools ...... 8 Reclamation ...... 8 Figure 2. Example: PyForecast Plots...... 8 Natural Resource and Conservation Service ...... 8 Figure 3. Example: NRCS Forecast ...... 9 Sun River Watershed ...... 9 Figure 4. Example: Estimating Peak Inflow ...... 10 Figure 5. Example: Estimating Peak Inflow Day ...... 10 National Weather Service...... 10 Figure 6. Example: NWS Forecast ...... 11 General Operations ...... 11 Gibson Dam – Spring Operations (March – July)...... 12 Figure 7. Example: Operating Rule Curve ...... 13 Table 2. Gibson Release vs. Gibson Storage...... 13 Figure 8. Example: North & South Fork Flows vs. Temperatures...... 14 Gibson Dam – Summer Operations (July – September) ...... 15 Figure 9. Drought Adjustment to Forecast...... 16 Figure 10. Minimum Flow at Simms Gage ...... 16 Gibson Dam – Winter Operations (October – February) ...... 17 Figure 11. October through March Inflow Forecast Plot ...... 18 Contact Information ...... 19

Water Rights

Water rights for the Sun River Project are based on Montana law. Water rights in Montana are guided by the Prior Appropriation Doctrine, that is, “first in time is first in right.” The Project water rights have been adjudicated in a Temporary Preliminary Decree. These water rights reflect historical use but still need to go through a Preliminary Decree before being finalized in the Montana Water Court. The Project has water rights for storage and for natural flow. The water rights discussed below are the primary water rights that affect project operations.

Storage Storage water rights are held in Reclamation’s name. The Gibson Reservoir water right has a claimed priority date of November 6, 1917 for a volume of 99,058 acre-feet. Gibson Reservoir was authorized as a single purpose, irrigation facility. Water rights were claimed for irrigation, recreation, and fish and wildlife. The water right for filling Gibson Reservoir is based on the capacity of the reservoir as it existed when the water rights were filed in 1981. Gibson Reservoir is on-stream and can legally store water year around. Therefore, water is stored under its priority date whenever it is legally available and there is room to store it up to the volume limit. Gibson Reservoir should only store water when the summation of reservoir inflows and downstream accretions exceed the needs of downstream water right with priorities earlier than November 6, 1917.

Willow Creek Reservoir has water rights from two sources – Willow Creek and the Sun River. The primary use of the water is for irrigation but there are also water right claims for recreation and fish and wildlife. The Willow Creek water rights have priority dates of 1883, and 1906 for the 32,000 acre-feet capacity of Willow Creek Reservoir. Sun River water is delivered to Willow Creek Reservoir through the Willow Creek Feeder Canal with a 1905 priority date. Willow Creek Reservoir is off-stream and water is only diverted to Willow Creek Reservoir under its priority date from April to October.

Pishkun Reservoir has a direct flow right also with a 1905 priority date. The reservoir is also an off-stream reservoir and is filled using this right as well as by transferring storage from Gibson Reservoir. Pishkun Reservoir water is used for irrigation by Greenfields Irrigation District (GID).

Natural Flow of the Sun River Natural flow water rights are held jointly by Reclamation and the Irrigation Districts. There are seven previously decreed water rights with priority dates from 1870-1891 for 31.41 cfs that provide irrigation water for Ft. Shaw Irrigation District (FSID). There is one previously decreed right with an 1880 priority, and one filed right with a 1906 priority date that provides irrigation water to GID. Reclamation filed two additional rights on May 25, 1905, for direct flow from the Sun River. Ft. Shaw and GID share the water available from these rights through an agreement.

Ft. Shaw Irrigation District encompasses 27,273 acres and is allowed to irrigate 13,849 acres in an irrigation season. GID encompasses 133,831 acres and is allowed to irrigate 83,231 acres in an irrigation season.

Broken O Ranch Broken O Ranch has 26 water rights on the Sun River. Thirteen of these rights are for irrigation areas that range from 12 acres to 20,000 acres. The priority dates range from 1871 to 1954. They also have 13 rights for stock use. These priority dates range from 1862 to 1910.

Montana Fish, Wildlife and Parks (MT FWP) Instream Reservation MT FWP has two water rights with 1985 priority dates for instream flow on the Sun River. One right is for 100 CFS and protects a reach on the Sun River from the diversion dam below Gibson Reservoir to Elk Creek (Reach 1). The second instream right is for 130 cfs and protects a reach on the Sun River from Elk Creek to the Missouri River (Reach 2).

These instream reservations are based on the lower inflection point using the wetted perimeter methodology as described in the Upper Missouri River Basin Final Order (1992). This methodology identifies a lower and upper inflection point based on the wetted distance along the bottom and sides of a channel cross-section. Below the lower inflection point, the wetted perimeter changes rapidly with changes in flow. As flows increase above the lower inflection point toward the upper inflection point, changes in wetted perimeter decline. The upper inflection point provides the minimum streamflow that will cover most of a stream’s riffle area resulting in a flow that will protect food-producing capacity (aquatic invertebrate production) and adequate spawning and rearing conditions.

Most water reservations in the Upper Missouri River Basin were based on the upper inflection point; however, the lower inflection point was used for the Sun River based on the severe dewatering and the suppressed fishery at that time. The upper inflection points for Reaches 1 and 2 on the Sun River, are 360 and 220 cfs, respectively, and are the preferred minimum flows. Maintaining flows closer to the upper inflection point would provide a greater benefit to the fishery by allowing for greater food production and habitat. It should be recognized that flows less than 100 and 130 cfs for Reaches 1 and 2, respectively, will result in rapid loss of habitat as these are based on the lower inflection points, and thus 100 and 130 cfs should be considered absolute minimum flows.

Water Right Agreements There is a working agreement between FSID and GID that specifies how storage in Willow Creek Reservoir and how the 1905 direct diversion flow is shared and accounted for between the two Districts. Essentially, Ft. Shaw receives up to 5,544 acre-feet of stored water from Willow Creek Reservoir and the 1905 water rights are divided at the Sun River Diversion Dam, and Ft. Shaw Irrigation District has access to some Project return flows. Please refer to the working agreement between the Districts for specific details.

Senior, non-Project water rights downstream of Sun River Diversion Dam presently require up to about 325 cfs to satisfy diversion needs. The flow of 325 cfs for the senior water rights are 4 partially satisfied by irrigation return flows and natural accretions below the dam and is a collective demand from Gibson to Vaughn during the irrigation season. A minimum flow of 100 cfs is usually passed over the Sun River Diversion Dam. Additional flow needed by the senior water rights during the irrigation season are usually satisfied by releases from Willow Creek Reservoir, which enter the Sun River downstream of the diversion dam but upstream of most senior diversions. Occasionally additional releases from Gibson Reservoir are made to increase the flow over the Diversion Dam for this purpose.

Data Used in Operations

U.S. Geological Survey (USGS) The USGS measures, reviews, approves, displays, and stores streamflow data and maintains gages. Below are the gages the USGS has responsibility for in the Sun River Basin: https://waterdata.usgs.gov/mt/nwis/rt/ . Gages are funded by the USGS and through partnerships with GID, Reclamation, Montana Department of Natural Resources (MT DNRC), MT FWP and the Sun River Watershed Group (SRWG) and can change over time. Please contact the SRWG or USGS for current funding partnerships.

06078500 North Fork Sun River near Augusta MT 06079000 South Fork Sun River near Augusta MT 06080900 Sun River bl Diversion Dam nr Augusta MT 06082200 Sun River bl Willow Cr nr Augusta MT 06085800 Sun River at Simms MT 06088500 Muddy Creek at Vaughn MT 06089000 Sun River near Vaughn MT

National Weather Service (NWS/NOAA) NWS collects, displays and produces many valuable meteorological products. Their data are important during spring runoff, impacting operations prior to and after snowmelt and precipitation events. NWS has more products than can be listed. Contact the local NWS in Great Falls for up to date tools and data. The following are links to some of their products.

Precipitation Forecasts: https://www.wpc.ncep.noaa.gov/qpf/qpf2.shtml Multi-day Outlooks: https://www.cpc.ncep.noaa.gov/ Advance Hydrologic Prediction Service: https://water.weather.gov/ahps2/long_range.php?wfo=tfx Decision Support Page: https://www.weather.gov/mbrfc/idss National Operational Hydrologic Remote Sensing Center: https://www.nohrsc.noaa.gov/interactive/html/map.html

Reclamation – HydroMet Reclamation operates a network of automated hydrologic monitoring stations (HydroMet) throughout the Sun River Watershed. The HydroMet network collects remote field data and 5 transmits it via satellite to provide real-time water management capability. Data can be accessed at: https://www.usbr.gov/gp/hydromet/ . The data are operational and provisional. Reclamation has partnered with GID and the SRWG to transmit and host other gage data in the basin. A summary of streamflow gage sites for the Sun River Basin with ownership and responsibility is shown in Table 1. Gages that are operational, gage ownership, and gage responsibility are subject to change. For current information, please contact the SRWG or Reclamation.

Table 1. Sun River Watershed Gage Information SUN RIVER WATERSHED GAGE INFORMATION BOR USGS Station Name Gage Ownership Primary Data Host Gage Start Primary Data Hydromet DNRC Station Date Users ID ID Maint RESERVOIRS FSDM Ft. Shaw Diversion Dam FSID FSID BOR 05/01/01 FSID/BOR GIBR Gibson Reservoir GID GID BOR 01/01/30 GID/BOR PSHR Pishkun Reservoir GID GID BOR 01/01/36 GID/BOR SRDM Sun River Diversion Dam GID GID BOR 01/01/30 GID/BOR WCR Willow Creek Reservoir GID GID BOR 01/01/36 GID/BOR STREAMS and CANALS BCCM Big Coulee at Beale Diversion SRWG SRWG BOR 10/11/13 SRWG/GID BOCM Broken-O Floweree Canal SRWG SRWG BOR 08/02/00 SRWG/GID MCPM Muddy Creek at Power, MT SRWG SRWG BOR 06/15/13 SRWG/GID SDCM Sun R Valley Ditch Canal SRVDC SRVDC BOR 06/17/02 SRVDC/SRWG SDWM Sun R Valley Ditch Waste SRVDC SRVDC BOR 04/06/13 SRVDC/SRWG SREM Sun River below Elk Creek GID SRWG BOR 07/27/11 SRWG TCPM Tank Coulee near Power, MT SRWG SRWG BOR 10/11/13 SRWG/GID WCFM Willow Creek Feeder Canal GID/FSID GID BOR 04/26/00 GID/BOR USGS 06088500 Muddy Creek at Vaughn, MT USGS/SRWG/CCD USGS USGS 10/01/94 FWP/SRWG NFSM 06078500 North Fork Sun River USGS USGS USGS 05/12/08 BOR/SRWG SFKSM 06079000 South Fork Sun River USGS/BOR BOR USGS 05/09/08 BOR/SRWG SRBD 06080900 Sun River below Diversion Dam USGS/BOR BOR USGS 04/08/16 BOR/SRWG 06082200 Sun River below Willow Creek USGS/SRWG/CCD USGS USGS 10/01/12 SRWG 06085800 Sun River at Simms, MT USGS/SRWG/CCD USGS USGS 10/01/07 FWP/SRWG 06089000 Sun River near Vaughn, MT USGS/NWE USGS USGS 10/01/94 NWE/SRWG DNRC SR 004 Duck Creek Ashuelot/Big Coulee DNRC DNRC DNRC DNRC/SRWG SR 002 Lower Elk Creek DNRC DNRC DNRC DNRC/SRWG SR 003 Lower Mill Coulee DNRC DNRC DNRC DNRC/SRWG SR 001 Upper Elk Creek DNRC DNRC DNRC DNRC/SRWG FSID Adobe Creek at Hwy 200 FSID FSID N/A FSID/BOR Creek at Cemetery Road FSID FSID N/A FSID/BOR FSID A-3 Wasteway FSID FSID N/A FSID/BOR K-ditch Wasteway FSID FSID N/A FSID/BOR Simms Creek at Hwy 21 FSID FSID N/A FSID/BOR

Natural Resource Conservation Service (NRCS) The Montana NRCS Snow Survey Program collects and displays mountain snowpack and precipitation information via the SNOwpack TELemetry (SNOTEL). Common applications of snow survey products include streamflow forecasts, water supply management, flood control, climate modeling, recreation, and conservation planning. SNOTEL sites report snow water equivalents (SWE), snow depth, precipitation, and air temperatures. The NRCS creates comparisons of SWE, such as percent of normal, snowpack trends and projection graphs, precipitation to date, and other graphics. Monthly water supply reports are issued January through June. The NRCS office in Bozeman, MT is responsible for QA/QC of the data and can assist with any questions. NRCS daily SWE is also available in Reclamation’s Hydromet database. More information on SNOTEL sites and water supply can be found at: https://www.nrcs.usda.gov/wps/portal/nrcs/mt/snow/ .SNOTEL sites that are near the Gibson Reservoir watershed are shown in Figure 1 below:

Figure 1. NRCS SNOTEL Sites

In addition, three snow courses are maintained in partnership between the United States Forest Service (USFS) and Montana NRCS that provide useful information for estimating the snowpack in the Sun River basin. These snow courses are Freight Creek, Wrong Ridge, and Wrong Creek. These snow courses sites have a period of record dating back to 1948 and 1949, respectively, 7 among the longest periods of record within the state. Wrong Creek and Wrong Ridge are the only snow measurement sites located in the Sun River basin measuring the headwaters of the North Fork of the Sun River. Snow courses are not automated like SNOTEL sites, and physical snow measurements are collected by hand at the site. As a result, the USFS collects measurements from Snow Courses near the first of March, April, and May.

Gibson Reservoir Inflow Forecasting Tools

During most years, close to 90 percent of the Sun River flow originates upstream of Gibson Reservoir. Gibson Reservoir receives most of the yearly water supply during the spring months of April through July. The water supply is driven by mountain snowpack and spring rainfall. Several tools are available to assist in determining and/or estimating the springtime runoff inflow volume. Forecasting procedures and tools continue to evolve and be updated; therefore, specific runoff equations or methods are not described herein. Forecasts and procedures should be updated yearly with recent data.

Reclamation Reclamation prepares April-July runoff estimates near the first of each month from January through July. The runoff forecasts are computed using Reclamation’s PyForecast program. Generated forecasts statistically correlate mountain snowpack, watershed precipitation, streamflow and climate indices to reservoir inflow. The program and user guide is available and can be download from github at : https://github.com/usbr/PyForecast. An excerpt of the output is shown in Figure 2.

Figure 2. Example: PyForecast Plots

Natural Resource and Conservation Service Runoff forecasts above Gibson Dam are also generated by the NRCS. NRCS includes their forecasts in a water supply report: https://www.nrcs.usda.gov/wps/portal/nrcs/mt/snow/ . In addition, NRCS estimates snowmelt peak dates which are predictors for peak streamflow,

8 typically issued after May 1. An example of the NRCS water supply forecast for Gibson Reservoir is shown in Figure 3.

Figure 3. Example: NRCS Forecast

USDA NRCS National Water & Climate Center * - DATA CURRENT AS OF: March 05, 2020 11:54:36 AM - Based on March 01, 2020 forecast values / PRELIMINARY MISSOURI RIVER BASIN FORECASTS 50% % of max 30% 70% min 30-yr Forecast Point period (KAF) avg (KAF) (KAF) (KAF) (KAF) avg Gibson Reservoir Inflow APR-JUL 415 105 515 455 375 315 395 APR-SEP 455 103 565 500 410 345 440

Sun River Watershed Group The SRWG, contracting with Mr. Phil Farnes of Snowcap Hydrology, has developed an excel spreadsheet forecasting tool which estimates April through July inflow volume. The spreadsheet utilizes regression equations based exclusively on SWE data and historical record from the five SNOTEL sites and three Snow Courses identified in the data section, above. The May 1 inflow forecasting equation also includes April precipitation data. See forecasting spreadsheet for specifics.

Peak inflow into Gibson Reservoir occurs during snowmelt runoff and is important for managing storage levels during runoff. The spreadsheet estimates the peak day inflow through correlation to historic daily April through July inflow volume. The example below in Figure 4 shows if total forecasted April-July inflow to Gibson Reservoir is 500,000 acre-feet (500 kaf), the corresponding peak inflow will be about 7,000 cfs. There is also a May through July inflow volume forecast vs. peak inflow correlation.

Figure 4. Example: Estimating Peak Inflow

The spreadsheet estimates the date of peak runoff based upon SNOTEL site half melts or zero values. There are graphs for 5 SNOTEL sites with Badger Pass and Mount Lockhart using 1/2 melt and Dupuyer Creek, Waldron and Wood Creek using the day snow melts to zero. For example, the graph in Figure 5 shows the date on which the Badger Pass SNOTEL site will likely reach half of its maximum SWE for the year. This date can then be correlated to the date on which the peak inflow to Gibson Reservoir will arrive.

Figure 5. Example: Estimating Peak Inflow Day Day SWE Half-Melt Badger Pass vs Gibson Peak Inflow Day 1979-2020 6/29 6/19 6/9 5/30 5/20 5/10 4/30

4/20 Day PeakDay inflow 5/3 5/13 5/23 6/2 6/12 6/22 7/2 7/12 Day SWE Half-Melt

National Weather Service During the early spring runoff season, the NWS Missouri Basin River Forecast Center prepares short-term forecasts for inflows into Gibson Reservoir. The forecasts are on a 6-hour time step 10 with an outlook of up to 10-days. The forecasts factor in forecasted temperatures (for snowmelt rate) and precipitation by using the 5-day forecasted and 6-10 day average temperatures and precipitation. Forecasts are provided via email from the NWS Missouri Basin River Forecast Center. An example of a Gibson Reservoir inflow forecast is shown in Figure 6.

Figure 6. Example: NWS Forecast :GIBSON RES MT - SUN RIVER HSA - TFX :LATEST ESTIMATED FLOW 2.22 KCFS AT 1200Z ON 0626

.ER AGSM8 20200626 Z DC202006261325/DH18/QIIFF/DIH6 :Flow Fcst / 00Z / 06Z / 12Z / 18Z .E1 :0626: / 2.294 .E2 :0627:/ 2.341 / 2.369 / 2.392 / 2.412 .E3 :0628:/ 2.428 / 2.442 / 2.455 / 2.406 .E4 :0629:/ 2.380 / 2.505 / 2.935 / 3.767 .E5 :0630:/ 5.33 / 6.83 / 7.80 / 8.30 .E6 :0701:/ 8.51 / 8.57 / 8.50 / 8.30 .E7 :0702:/ 7.99 / 7.62 / 7.21 / 6.80 .E8 :0703:/ 6.37 / 5.94 / 5.54 / 5.18 .E9 :0704:/ 4.842 / 4.540 / 4.269 / 4.026 .E10 :0705:/ 3.806 / 3.608 / 3.434 / 3.277 .E11 :0706:/ 3.133 / 3.002 / 2.885

General Operations

Operations of Gibson Dam and Reservoir are performed by GID. Reclamation’s Montana Area Office (MTAO), Facility Operations and Maintenance Division are responsible for incidental flood control, protection of the structure, and operational guidance to GID. The authorized purpose of operations is to provide water supply for irrigation on the Sun River Project. Normal procedures fill the reservoir each year in May or June, while regulating excess runoff that provide incidental flood control and prevent the reservoir water surface from rising above elevation 4724.0 feet.

For downstream flood control purposes, releases above 10,000 cfs at the Sun River Diversion Dam should be avoided.

The USGS gage below the Sun River Diversion Dam (06080900) is the operational gage used in determining flows to the Sun River, with the remainder of Gibson releases going to the Pishkun and Willow Creek supply canals.

Whenever an adequate water supply is available, releases from Gibson Reservoir should be maintained at rates to sustain flows in the Sun River at 100 cfs or higher, below the Sun River Diversion Dam and 130 cfs, or higher, below Elk Creek as measured at the USGS Simms gage. This is normally required to achieve downstream river flows for fisheries and recreation activities. During extraordinary droughts, releases may need to be decreased below these flow targets. During these drought years, coordination and consultation with MTAO and MT FWP should occur to determine appropriate flows over Diversion Dam. Maintaining flows in the Sun River at or near a preferred minimum flow of 220 cfs, based on the upper inflection point of the wetted perimeter studies, would result in increasing benefits to the fishery.

Willow Creek Reservoir receives significant recreational use and MT FWP recommends maintaining minimum water elevation of 4128 ft in Willow Creek Reservoir for the benefit of the fishery and recreational use.

Gibson Dam – Spring Operations (March – July)

Reclamation prepares a monthly operational model (ROMS) of the Sun River Basin with a range of possible inflow volumes based upon the Pyforecast program. Other forecasts can also be modeled as requested. The model output assists in determining if excess water (water above conservation releases and storage requirements) is available. If there is excess water, then excess water shall be released before the spring runoff starts, which is normally initiated in April. A daily excel spreadsheet can also be utilized as a modeling in conjunction with the NWS daily inflow forecasts.

However, care must be taken that if flows are increased during the spring rainbow trout spawning season, that flows are not subsequently reduced below the upper inflection point levels of 360 and 220 cfs in Reaches 1 and 2, respectively until after runoff. Maintaining flows at these levels will ensure that deposited redds remain wetted.

The SRWG, contracting with Mr. Phil Farnes of Snowcap Hydrology, developed an operating rule curve guide to compute outflow based on April through July forecasted inflow volume. The spring outflow can be set based on the April through July forecasted inflow and amount of carry- over storage. An example of the curve guide is shown in Figure 7. The “kaf to Store” is the amount of storage space in Gibson before it is full. For example, “20 kaf to Store” (the blue line below) means there is 20,000 acre-feet (20 kaf) until Gibson is full. If the April through July forecasted inflow is 320 kaf, then Gibson outflow should be set at 3,000 cfs. In contrast, if there is “80 kaf to Store” or 80,000 acre-feet until Gibson is full (red line below), and the April-July forecasted inflow is 320 kaf, then Gibson outflow should be set at 1,400 cfs. When forecasts indicate outflows of less than 2,000 cfs, efforts should be made to increase storage before runoff.

Figure 7. Example: Operating Rule Curve Gibson Reservoir Operating Curves 1200 80 1000 40 800 kaf to Store 80 600 60 0

40 Jul Inflow Forecast, kaf Forecast, Inflow Jul

- 400 20

200 0 Bias Apr Bias

0 0 1000 2000 3000 4000 5000 6000 7000 8000 Outflow, cfs

For more detailed information and up to date data, see the Sun River Watershed Forecasting spreadsheet.

It is recognized that outflow of the Gibson Dam river outlet works is limited by reservoir elevation and the pressure head it creates. Table 2 shows outlet works maximum release at a given elevation and content.

Table 2. Gibson Release vs. Gibson Storage Gibson Reservoir Gibson Reservoir Approximate Maximum Storage (kaf) Elevation (ft) Outlet Works Flow (cfs) 14 4630 2010 19.4 4640 2150 32.1 4660 2400 48.4 4680 2630 69.0 4700 2840 83.2 4712 2970 88.3 4716 3000

When making release changes over the Diversion Dam, it is recommended, when possible, to not exceed daily increments of 10% of the total flow, for both increases and decreases.

The spillway crest is gated above the concrete crest which is at an elevation of 4712.0 feet (83,248 acre-feet). For flood and dam safety considerations, the spillway gates remain open until after the peak inflow has occurred, refer to Sun River Watershed forecasting peak inflow tool or 13 the NRCS peak inflow forecast. Gibson Reservoir should not be full before the day of peak inflow. The volume of the peak inflow and releases helps estimate how many acre-feet of storage will be added during peak run-off. For example, 5,000 cfs inflow and 2,000 cfs outflow for one day is approximately 5,940 acre-feet of storage ((5,000 cfs – 2,000 cfs) * 1.98 = 5,940 af/day). The top 12 feet of gated storage, above the concrete crest, approximately 15,500 acre- feet (elevation of 4724 acre-feet), shall be filled as inflows recede after the runoff peak. The peak normally occurs during late May or June; refer to the estimated peak day inflow spreadsheet. When inflow forecasts are much below normal, the spillway gates may be closed earlier to ensure filling the active conservation pool in consultation with Reclamation. Once Gibson Reservoir has filled or reached its maximum level during spring runoff, releases are set to maintain the reservoir at or below elevation 4724.0.

The dam operating personnel should be aware of impending storms and the amount of rain that has occurred or is expected to occur. When any weather or runoff conditions exist, or are forecasted, which could require a change in reservoir operations or warnings of high flow, the dam operator shall immediately report such conditions to Reclamation. The NWS in Great Falls, Montana, has been requested to notify Reclamation of heavy rains in the Missouri River Basin above Augusta, Montana. When notified of heavy rain, Reclamation will alert the dam operator.

Snowmelt timing and volumes in the basin are driven by temperatures, precipitation and antecedent conditions. The air temperature at the North Fork station can be an indicator for estimating increases and decreases of inflows. Temperatures are also recorded at SNOTEL sites. When daily stream flow is diurnal then there is snowmelt occurring and when the diurnal variation is nearly gone the contribution from snowmelt is very small. Figure 8 demonstrates how the North Fork and South Fork streamflow gages react to temperature fluctuations.

Figure 8. Example: North & South Fork Flows vs. Temperatures

Gibson Dam – Summer Operations (July – September)

After the spring runoff is over, releases during the remainder of the irrigation season from July through September are adjusted as necessary to meet irrigation demands of the Sun River Project and Basin water needs. A minimum storage content of about 5,000 acre-feet at elevation 4608.5, shall be maintained in Gibson Reservoir to prevent the flushing of silt from the bottom of the reservoir into the Sun River.

SRWG developed a procedure for estimating the minimum flow that can be maintained at the Sun River at Simms streamflow gage while ensuring adequate water deliveries for irrigation. After the inflows have been determined by using the May - September forecast and useable storage and forecast adjustment, the target value for minimum flows at the Simms gage can be obtained. This is the amount of water that should pass by the Fort Shaw diversion. When total water supply is about average, minimum flows at Simms gage should be maintained at or above the absolute minimum flow of 130 cfs, based on MT FWP water reservation for the lower Sun River. Maintaining a flow at the preferred minimum flow of 220 cfs (upper inflection point) during average to above average water years, will result in an increased benefit to the fishery by providing increased food production, more suitable habitat, and lower water temperatures.

The annual minimum flow at the Simms gage incorporates a drought adjustment. The minimum flow is conservatively lower in below-average water years. In average or above-average water years, there is adequate runoff to make adjustments if spring precipitation is less than expected and therefore the minimum flow does not need adjustment. However, during low water years, it is difficult to make up the difference between expected and actual spring rainfall because runoff usually drops off earlier than average. To compensate for the drop off in flow, the first step is to make a drought adjustment to the May through September forecast. The adjusted forecast is called the “operations forecast.” The table below shows the drought adjustment, resulting in the operations forecast:

• When the runoff forecast is well above average, then the forecast is not adjusted. • When the forecast is near average, then the operations forecast is about 5% less than the actual forecast. • When the forecasted runoff is about 50% of average, then the operations forecast is about 20% less than the actual forecast.

The procedure adjusts operations for risk of drought in low years but does not add to the flows in high years, as would using a constant value for adjustment in all years. Figure 9 shows the drought adjustment (percentage reduction in the May-September forecast) to be made to the May-September total inflow forecast to create the operations forecast.

Figure 9. Drought Adjustment to Forecast Inflow Gibson Res May-Sep 15 10 5 0 -5 0 200 400 600 800 1000 1200 -10 -15 -20

-25 Correction to Forecast to Correction -30 -35 -40 May-Sep Forecast

The second step is to determine the minimum flow at the Simms gage from the resulting operations forecast. For example, when total water supply is near average, 500 kaf for Gibson inflows between May-September, then the minimum flow at the Simms gage should be set at 130 cfs, Figure 10. The inflow for operations includes the adjusted forecast and current reservoir storage.

Figure 10. Minimum Flow at Simms Gage Min Flow@ Simms 180 160 140 120 100 80

60 Min @ Simms @ Min 40 20 0 0 200 400 600 800 May-September Sun Inflows for Operations

For more detailed calculations refer to the Estimating Proposed Minimum Flows for Sun River at Simms within the SRWG forecasting spreadsheet.

Gibson Dam – Winter Operations (October – February)

Reclamation prepares a winter forecast in October and November. The winter release rate is based on a Reclamation study, Proposed Non-Irrigation Season Release Criteria for Gibson Dam (Reclamation, 2007). Winter release criteria are inflows into Gibson Reservoir and storage content of Gibson, Pishkun, and Willow Creek Reservoirs. In addition, the ROMS model can be used with the forecasted winter inflows to determine a level of acceptable water supply risk to GID. Water supply risk can be quantified by using historic spring runoff volumes and by varying the end of March content target.

The SRWG has developed a guide to assist in determining the Gibson Reservoir winter outflow based upon a correlation of October through March total inflows and inflows to Gibson Reservoir at the start of snowpack accumulation at different SNOTEL sites. See the example in Figure 13 correlating to the Mt. Lockhart SNOTEL. Figure 11 shows that measuring the average inflow into Gibson Reservoir over five days after snow starts collecting at the Mount Lockhart SNOTEL site correlates well with total October to March inflow. For example, if average inflows to Gibson are 400 cfs after first snow accumulation at Mount Lockhart SNOTEL, then total winter inflows to Gibson are likely to be approximately 112 kaf (112,000 af). The Waldron SNOTEL and October inflows are also used to estimate total winter inflow, to increase the accuracy of the estimate.

The information is used to calculate an estimated winter release rate. A 100 cfs release over Diversion Dam is 198 af/day (100 cfs * 1.98 = 198 af/day). Over 181 days, (Oct-March), this is 35,838 acre-feet. Figure 13 shows that most years have at least twice this amount of winter inflow (not including accumulated snowpack). In nearly all years, the winter outflow set in October can be 100 cfs or greater with a minimized risk of losing any reservoir storage to winter releases. In those rare years, outflows can be adjusted downward from 100 cfs if warranted after the February 1 forecast. For more detailed decision-making information, see the SRWG Forecasting spreadsheet.

Figure 11. October through March Inflow Forecast Plot Oct-Mar Inflow Gibson Reservoir 1969-2020

225 200 y = 0.2051x + 26.557 175 R² = 0.758 150 125

100 Marinflow, kaf - 75

Oct 50 25 0 0 100 200 300 400 500 600 700 800 900 Average Inflow for 5 days after snow starts at Mt Lockhart, cfs

The MT FWP prefers a flow target during winter at Simms between the absolute minimum 130 cfs and the preferred minimum flow of 220 cfs based on the lower and upper inflection points of the wetted perimeter studies. It is preferred that the winter flows at Simms are not lower than the flow rate set in October. October is when brown trout spawning occurs and if the flows are reduced below October rates, areas where trout have spawned earlier can be dewatered and the redds will be lost. Currently, water is diverted down the Floweree Canal by the Broken O during winter for stockwater purposes. These withdrawals must also be considered when determining instream minimum flows. Reclamation, GID, and MT FWP will meet in late September to discuss fall and winter flow release plans.

Adjustments to the winter release rate may be required as the water supply conditions change throughout the fall and winter. However, changes in the river release rate from December through February should be minimized to prevent ice jam issues. Release reductions can be made in February if necessary, to conserve water, but these changes should be made gradually allowing the river ice cover to settle slowly, not contributing to further ice jams. Increasing the releases during February should be avoided if river ice cover exists.

Contact Information

Reclamation Stephanie Micek Water Operations 2900 4th Avenue North Billings, MT 59101 [email protected], (406) 247-7320

GID Erling Juel Greenfields Irrigation District Manager 105 West Central Avenue P.O. Box 157 Fairfield, MT 59436 [email protected], (406) 467-2533

Sun River Watershed Group Tracy Wendt Watershed Coordinator PO Box 7312 Great Falls, MT 59406 [email protected], (406) 214 2868

MT FWP Jason Rhoten Region 4 Fishery Manager 4600 Giant Springs Rd Great Falls, MT 59405 [email protected], (406) 454-5853

Trout Unlimited Laura Ziemer Senior Counsel and Water Policy Advisor [email protected], (406) 599-2606

NRCS Lucas Zukiewicz Water Supply Specialist 10 East Babcock Street Rm 443 Bozeman, MT 59715 [email protected], (406) 587-6843