Yellowtail Dam & Bighorn Lake 2010 Operation Review Billings, MT January 2011

1. RECLAMATION Managing Water in the West Yellowtail Dam & Bighorn Lake 2010 Operation Review Billings, MT January 2011

2. 2010 Bighorn Lake OperationsUsing the Draft Criteria“A Yo Yo Year”

3. November – March Period Nov-Mar parameters used for calculating release rate April-Oct. gain = 265,000 AF End of Oct. storage = 1,064,000 AF (Elev. 3639.5) Boysen planned release = 900 CFS Buffalo Bill planned release = 360 CFS Based on above Nov-Mar Release set at 2750 cfs

5. Forecasted Nov. – Mar. Inflow acre-feet Forecasted Actual Forecast % of Actual 2010 625,900 646,500 97% • 560,600 580,500 97% • 382,600 382,571 100%

6. February– March Period“Down It Goes” February 1 Apr-Jul Forecast 634,200 AF 57% March 1 Apr-July Forecast 591,000 AF 53% Minimum Forecast for Rule curve 719,000 AF 64% Under minimum rule curve a release of 2,000 cfs is needed

9. Apr-July Rule Curve Analysis“Up She Goes” • Apr 1 April-July Inflow Forecast 625,000 AF 56% • Apr 15 April-July Inflow Forecast 730,000 AF 65% • Based on the April 15 Forecast being greater than 719,000 AF a rule curve was prepared

13. Spring Runoff Forecasts April-July Inflow % of Date Forecast (ac-ft) Average Feb. 1 634,000 57% Mar. 1 591,000 53% Apr. 1 625,000 56% Apr. 15 730,000 65% May 1 900,400 81% May 15 1,121,400 101% June 1 1,263,300 113% Actual 1,504,354 135%

14. August-October Period“and Down Again” • Inflows dropped sharply during the 2nd haft of July and continued to drop and remain low through the end of October. Since most of the snow occurred in the spring it melted rapidly reducing snowpack to supply late summer flows. August –October were unusually dry and warm reducing runoff and increasing irrigation depletions. August –October gains to Bighorn Lake were -53,000 AF compared to an average of 35,800 AF, 88,800 AF less than average.

16. August-October Gains • Actual 2010 Aug-Oct Gains = -53,000 AF • Average Aug-Oct Gains = 35,800 AF • Difference = 88,800 AF • In CFS this difference is 490 CFS • Lowest Gains of Record

18. November – March 2011 Nov-Mar Parameters used for calculating Release Rate 2011 2010 April-Oct. Gain = 201,000 AF 265,000 AF End of Oct. Storage = 938,000 AF 1,064,000 AF Boysen Planned Release 800 CFS 900 CFS Buffalo Bill Planned Release 355 CFS 360 CFS Nov-Mar Release 2,370 CFS 2,750 CFS

22. QUESTIONS & COMMENTS

23. March 31 Target Elevation • Target elevation determined on November 1 prior to accumulation of snowpack in the mountains. • Target elevation is set at a level that will handle both high and low spring runoff • Reservoir level is adjusted by March 31 based on forecasts to deal with expected runoff. Reservoir allowed to gradually fill with average to below average forecasts and to be further drafted with above average forecasts

24. Target Elev. 3620 Compared to 3610 Lake ElevationStorage af 3620 873,600 af 3610803,200 af Diff. 1070,400 af 3645.2 1,140,399 af 3640.01,070,029 af Diff. 5.270,370 af

30. QUESTIONS

31. PEAK BIGHORN RIVER DISCHARGE YearActual PeakEst. With Rule Curve 1995 14,000 CFS 12,000 CFS 1997 11,000 CFS 12,000 CFS 1999 10,000 CFS 12,000 CFS 2009 12,500 CFS 11,000 CFS

32. PEAK BIGHORN LAKE ELEVATION Year Actual Est. With PeakRule CurveDifference 1995 3646.3 3645.6 -0.7 1997 3651.7 3643.9 -7.8 1999 3649.3 3641.5 -7.8 2009 3647.9 3645.5 -2.4

33. PROPOSED RESERVOIR TARGETS Date Existing Proposed Difference _______ Targets Targets (feet) Oct 31 3635 3638-40 +3-5 ft Nov 30 3630 Eliminated +5-7 ft Mar 31 3605-14 3617-21 +7-13 ft July 31 3640 3640 Same

37. Nov.-Mar. Operations • Bighorn River release rate - 2775 cfs • River discharge 275 cfs above 2500 cfs • March 31 Bighorn Lake target level – 3620.6 feet • Current Lake Level – 36??

40. 2010 Forecasted Rule Curve

41. QUESTIONS & COMMENTS