Yellowtail Dam Water Supply and Projected Operations January 2019

Forecasted January Operating Range

Forecast Minimum Median Maximum

Monthly Average 1,995 2,120 2,200 Inflow (cfs)

Monthly Average 2,940 2,940 2,940 River Release (cfs)

End of January 3620.4 3621.3 3622.0 Elevation (feet)

January 1 2019 Inflow Forecast April through July Runoff April through July Volume (kaf) 810 Percent of Average 66 Water Year Historic Inflow (kaf) Rank 2018 2,318 3 2017 2,953 1 2016 1,032 32 2015 1,543 17

Bighorn River Basin Map Source DEMIS Mapserver 30 Year Average 1,221

Bighorn Lake Inflow Water Year 2019 600 2019 Inflow Median Forecast 30 Year Average Range between Min and Max Forecast

feet) 500 -

400

300

200

100 Inflow (thousands of acre (thousands Inflow 0 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

CLIMATE Climate Departure from Normal January Climate Outlook December 1 through December 31, 2018 SUMMARY Conditions were generally Precipitation Precipitation drier and cooler than average Departure from Normal (inches) i n the Bighorn River Basin d uring December. There were no weather events in December that significantly changed expected reservoir and river operations.

The one month outlook for January is for a greater chance of below average precipitation and above average temperatures in the Bighorn River Basin. Temperature Temperature Departure from normal (°F)

HPRCC using provisional data NOAA Regional Climate Centers SNOWPACK SUMMARY

The Snow Water Equivalent graphs are a composite of SNOTEL sites within the Bighorn River Basin that is managed by the Department of Natural Resource and Conservation Service (NRCS). The January 1, 2019 SNOTEL data along with streamflow data was used to compute an April through July runoff inflow forecast volume into Bighorn Lake of 810,100 acre-feet, or 66 percent of average.

Wind River

NRCS SNOTEL Sites January 1 Composite Snowpack % of Average

Above Boysen 73

Above Buffalo Bill 91

Bighorn Mountains 94

Above Yellowtail 84

Shoshone River Bighorn River

NRCS Montana Snow Survey Website: https://www.nrcs.usda.gov/wps/portal/nrcs/mt/snow/

Statistical shading breaks at 10th, 30th,50th, 70th, and 90th Percentiles Normal (’81-’10) – Official median calculated from 1981-2010 data Normal (POR) – Unofficial mean calculated from Period of Record data

OPERATIONS REVIEW

Releases were increased to 3,025 cfs on December 11 based on November inflows being greater than forecasted November inflows. Bighorn Lake Operations Water Year 2019 3650 3645 3640 3635 3630 3625 3620 3615 Lake Elevation (feet) Elevation Lake 3610

3605 2019 Top of Joint Use 3600 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep January 1 Storage Conditions Elevation Storage Percent of Percent Feet acre-feet Average Full Bighorn Lake 3627.1 885,574 105 87 Buffalo Bill 5368.0 453,833 107 70 Boysen 4718.0 613,735 112 83

Bighorn Lake Inflow and Release 8,000

7,000 Inflow River Releases Powerplant Generation Capacity

6,000

5,000

4,000

Flow (cfs) Flow 3,000

2,000

1,000

0 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Average December Release Average December Inflow Monthly Avg Percent of Monthly Avg Percent of cfs Average cfs Average Bighorn River 3,005 124 Bighorn Lake 2,160 129 Buffalo Bill River Release 360 128 Buffalo Bill 310 116 Boysen Release 1,000 130 Boysen 690 111

OPERATING OUTLOOK

Releases were increased slightly during December due to inflows during November coming in higher than forecasted. Releases will be decreased in January due to lower forecasted inflows during March. The end of March elevation target is 3617 feet under median inflow conditions.

Median Inflow Conditions (April through July Inflow 810 kaf)

Jan Feb Mar April May June July Boysen Release (cfs) 1,002 1,001 1,002 1,002 1,174 1,200 1,207 Buffalo Bill Release (cfs) 355 357 545 1,309 1,914 2,035 2,109 Tributary Gain (cfs) 761 990 1,179 526 686 1,533 -1,275 Monthly Inflow (cfs) 2,118 2,348 2,726 2,837 3,775 4,768 2,041

Monthly Inflow (kaf) 130.3 130.4 167.6 168.8 232.1 283.7 125.5 Monthly Release (kaf) 176.5 156.3 173.1 162.7 161.3 167.4 175.2

Afterbay Release (cfs) 2,940 2,885 2,885 2,805 2,693 2,884 2,919 River Release (cfs) 2,940 2,885 2,885 2,783 2,500 2,501 2,500

End-of-Month Content (kaf) 839.3 813.4 807.9 814.0 884.8 1,001.1 951.4 End-of-Month Elevation (feet) 3621.3 3617.8 3617.0 3617.9 3627.0 3638.4 3634.1

Minimum Inflow Conditions (April through July Inflow 558 kaf)

Jan Feb Mar April May June July Boysen Release (cfs) 1,000 999 603 701 1,174 1,200 1,200 Buffalo Bill Release (cfs) 355 357 355 686 1,729 1,849 1,924 Tributary Gain (cfs) 641 823 1,039 329 161 -15 -1,711 Monthly Inflow (cfs) 1,996 2,179 1,997 1,716 3,064 3,033 1,413

Monthly Inflow (kaf) 122.7 121.0 122.8 102.1 188.4 180.5 86.9 Monthly Release (kaf) 175.9 134.9 134.0 107.7 120.4 127.7 134.3

Afterbay Release (cfs) 2,940 2,499 2,249 1,881 2,026 2,217 2,252 River Release (cfs) 2,940 2,499 2,249 1,850 1,833 1,833 1,833

End-of-Month Content (kaf) 831.8 817.9 806.7 801.1 869.2 922.0 874.7 End-of-Month Elevation (feet) 3620.3 3618.4 3616.8 3616.0 3625.1 3631.1 3625.8

Maximum Inflow Conditions (April through July Inflow 1,385 kaf)

Jan Feb Mar April May June July Boysen Release (cfs) 1,002 1,001 1,000 1,202 2,088 2,089 2,090 Buffalo Bill Release (cfs) 355 357 844 1,472 2,672 3,143 3,217 Tributary Gain (cfs) 846 1,129 1,355 886 1,758 2,886 -607 Monthly Inflow (cfs) 2,202 2,487 3,199 3,559 6,518 8,117 4,700

Monthly Inflow (kaf) 135.4 138.1 196.7 211.8 400.8 483.0 289.0 Monthly Release (kaf) 176.5 162.7 226.3 224.3 383.0 300.8 246.2

Afterbay Release (cfs) 2,940 3,000 3,750 3,840 6,299 5,126 4,074 River Release (cfs) 2,940 3,000 3,750 3,840 6,201 4,879 3,654

End-of-Month Content (kaf) 844.5 819.9 790.3 777.8 795.6 977.8 1,020.6 End-of-Month Elevation (feet) 3622.0 3618.7 3614.3 3612.3 3615.2 3636.4 3640.0 OPERATING OUTLOOK

Irrigation diversions are expected to be off until April.

Irrigation Demand Outlook

Bighorn Canal (cfs) Jan Feb Mar April May June July Median Forecast 0 0 0 22 194 383 420 Minimum Forecast 0 0 0 30 194 383 420 Maximum Forecast 0 0 0 0 98 247 420

Power Generation Outlook

Current Number of Units Available: 3 Approximate Yellowtail Powerplant Turbine Capacity (cfs): 6,150 cfs Approximate Yellowtail Powerplant Generation Limit (cfs): 4,740 cfs

Yellowtail Powerplant Release (cfs) Jan Feb Mar April May June July Median Forecast 2,870 2,814 2,815 2,734 2,623 2,813 2,849 Minimum Forecast 2,870 2,429 2,179 1,810 1,956 2,146 2,183 Maximum Forecast 2,870 2,930 3,680 3,769 4,681 4,680 4,004

Yellowtail Powerplant Generation (gwh) Jan Feb Mar April May June July Median Forecast 67.2 57.9 65.1 60.7 61.3 65.7 68.6 Minimum Forecast 66.8 48.7 48.1 36.9 43.1 46.9 49.2 Maximum Forecast 67.3 60.8 87.3 86.1 117.5 113.7 101.2

Yellowtail Spill (cfs) Jan Feb Mar April May June July Median Forecast 0 0 0 0 0 0 0 Minimum Forecast 0 0 0 0 0 0 0 Maximum Forecast 0 0 0 0 1,548 375 0

Release Outlook by Outlet

All releases are expected to go through the Yellowtail Powerplant through the end of July based on current median and minimum inflow forecasts. Releases through either the spillway or river outlet works are likely during May and June under maximum inflow conditions.

OPERATING OUTLOOK Projected elevations and the range of river releases are based on the median, minimum, and maximum inflow forecasts. End of month elevations and river releases vary based on the difference between forecasted inflow scenarios between now and March and forecasted April through July runoff. The monthly average flows during April through July range between 1,833 and 6,200 cfs. The current median April-July inflow forecast calls for a drawdown to 3617 feet and under the maximum inflow forecast a drawdown to 3610.6 feet. The rule curves do not apply to the minimum inflow forecast because the volume, 558,000 acre- feet, is under the minimum reservoir fill volume of 727,000 acre-feet.

Bighorn Lake Elevation 3645 Top of Joint Use Pool- 3640' 3640 3635 3630 3625 3620 3615

Elevation (feet) Elevation 3610 Maximum

3605 Median

3600 Minimum 3595 1/1/19 2/1/19 3/1/19 4/1/19 5/1/19 6/1/19 7/1/19 8/1/19

River Release Range 10,000 River Release 9,000 Powerplant Generation Capacity 8,000 7,000 6,000 5,000 4,000

River Flow (cfs) Flow River 3,000 2,000 1,000 0 1/1/19 2/1/19 3/1/19 4/1/19 5/1/19 6/1/19 7/1/19

Contact Us Clayton Jordan Stephanie Micek Chris Gomer [email protected] [email protected] [email protected] 406-247-7334 406-247-7320 406-247-7307

Monthly Operating Plans, Current Conditions, Snowpack and Other Water Management Information https://www.usbr.gov/gp/lakes_reservoirs/wareprts/main_menu.html