Natural Resources Conservation Service

Idaho Water Supply Outlook Report May 1, 2020

Snow-free landscape near the South Fork Payette River, May 2, 2020 Photo courtesy of Danny Tappa

Across central Idaho, little snow remains below approximately 6,000 feet in elevation after warm and dry weather promoted widespread snowmelt during April. Drier-than-normal soil moisture conditions coupled with poor spring precipitation is not an ideal recipe for efficient snowmelt-runoff. Continue reading the full report for snowpack and water outlook details throughout Idaho.

For more water supply and resource management information:

Contact: Your local county Natural Resources Conservation Service Office Internet Web Address: http://www.id.nrcs.usda.gov/snow/ Natural Resources Conservation Service Snow Surveys 9173 West Barnes Drive, Suite C Boise, Idaho 83709-1574 (208) 378-5700 ext. 5

To join a free email subscription list contact us by email at: IDBOISE-NRCS-SNOW@one.usda.gov

Water Supply Outlook Report Federal - State – Private Cooperative Snow Surveys

How forecasts are made Most of the annual streamflow in the western United States originates as snowfall that has accumulated in the mountains during the winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff that will occur when the snow melts. Measurements of snow water equivalent at selected manual snow courses and automated SNOTEL sites, along with precipitation, antecedent streamflow, and indices of the El Niño / Southern Oscillation are used in computerized statistical and simulation models to produce runoff forecasts. Unless otherwise specified, all forecasts are for flows that would occur naturally without any upstream influences. Forecasts of any kind are not perfect. Streamflow forecast uncertainty arises from three primary sources: (1) uncertain knowledge of future weather conditions, (2) uncertainty in the forecasting procedure, and (3) errors in the data. The forecast, therefore, must be interpreted not as a single value but rather as a range of values with specific probabilities of occurrence. The middle of the range is expressed by the 50% exceedance probability forecast, for which there is a 50% chance that the actual flow will be above, and a 50% chance that the actual flow will be below, this value. To describe the expected range around this 50% value, four other forecasts are provided, two smaller values (90% and 70% exceedance probability) and two larger values (30%, and 10% exceedance probability). For example, there is a 90% chance that the actual flow will be more than the 90% exceedance probability forecast. The others can be interpreted similarly. The wider the spread among these values, the more uncertainty is in the forecast. As the season progresses, forecasts become more accurate, primarily because a greater portion of the future weather conditions become known; this is reflected by a narrowing of the range around the 50% exceedance probability forecast. Users should take this uncertainty into consideration when making operational decisions by selecting forecasts corresponding to the level of risk they are willing to assume about the amount of water to be expected. If users anticipate receiving a lesser supply of water, or if they wish to increase their chances of having an adequate supply of water for their operations, they may want to base their decisions on the 90% or 70% exceedance probability forecasts, or something in between. On the other hand, if users are concerned about receiving too much water (for example, threat of flooding), they may want to base their decisions on the 30% or 10% exceedance probability forecasts, or something in between. Regardless of the forecast value users choose for operations, they should be prepared to deal with either more or less water. (Users should remember that even if the 90% exceedance probability forecast is used, there is still a 10% chance of receiving less than this amount.) By using the exceedance probability information, users can easily determine the chances of receiving more or less water. *Starting in 2020, streamflow forecasts with poor prediction skill (jackknife r2 < 0.34) will no longer be issued. This will primarily affect January and June forecasts, with little change anticipated for February, March, April, and May forecasts. For more information, please contact Danny Tappa (daniel.tappa@usda.gov)*

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IDAHO WATER SUPPLY OUTLOOK REPORT

May 1, 2020

SUMMARY Precipitation With the exception of the Upper Snake River basin in eastern Idaho and western Wyoming, monthly precipitation was below normal for all of Idaho and the surrounding terrain that feed our rivers. The Owyhee River basin was the worst off (~30% normal), while most of Idaho received between ~50 and 80% of normal monthly precipitation. As alluded to previously, the Upper Snake was the exception, ~100 to 120% of normal precipitation was observed in sub-basins above American Falls. Water-year precipitation (Oct. 1 to current) yields a similar story: all of Idaho has received less than normal precipitation since Oct. 1, except the Upper Snake above Palisades (~100% of normal). Persistantly dry conditions continue to plague the Wood & Lost basins, water-year precipitation totals now stand at approximately 65% of normal. Extended outlooks from NOAA’s Climate Prediction Center suggested April would be a dry month for Idaho, which came to fruition. Unfortuantely, current outlooks for May are similarly suggesting an increased chance of above normal temperatures and below normal precipitation. Monthly and water-year precipitation data for all basins in Idaho can be accessed in tabular form here. Snowpack Seasonal peak snowpack is now in the past for nearly all of Idaho’s mountains, as widespread and rapid snowmelt was the dominant snow process during April. Snow loss was observed at nearly all snow measurement stations (SNOTEL) and sites (Snow Courses) below ~9,000 feet during April. As mentioned in the April WSOR, snowpack conditions in the Panhandle, Clearwater, Upper Snake, Bear River, and Southside Snake basins exceeded the normal peak snowpack. That hasn’t changed; however, we did not and will not reach the normal snowpack peak in the Salmon, West Central, and Wood & Lost basins during the 2020 water-year. May 1 snowpack percentages range from ~90 to 120% north of and including the Weiser, Payette, and Salmon; the same is true in eastern Idaho (Upper Snake & Bear River; Figure 1). The Boise, Wood & Lost, and Southside Snake basins have ~40 to 70% of normal snowpack remaining, except for the Owyhee River basin which is now nearly snow-free. See Figure 1 for a map of basin specific April 1 snowpack conditions, or access the same information in tabular form here. Reservoirs & Streamflow All major reservoirs in Idaho are holding near or above normal storage for this time of year. A statewide summary of current reservoir storage can be accessed here. After a less-than-ideal month of precipitation, streamflow forecasts have largely decreased since April 1. Aside from northern Idaho (Panhandle & Clearwater) and far eastern Idaho (Upper Snake & Bear River) where remainder season streamflow is forecast to be near normal, the rest of Idaho should anticipate lower than normal runoff (~40 to 80%). Concern continues for water users in the Wood & Lost basins, where median streamflow forecasts range from ~10 to 55% of normal, and water shortages are now very likely. Full basin specific forecast details can be accessed here and in Figure 2. Note: The streamflow volumes referenced in this report are the 50% Chance of Exceeding Forecast, unless otherwise noted.

IDAHO SURFACE WATER SUPPLY INDEX (SWSI) May 1, 2020 The Surface Water Supply Index (SWSI) is a predictive indicator of surface water availability within a watershed for the spring and summer water use season. The index is calculated by combining pre-runoff reservoir storage (carryover) with forecasts of spring and summer streamflow. SWSI values are scaled from +4.0 (abundant supply) to -4.0 (extremely dry), with a value of zero indicating a median water supply as compared to historical occurrences. The SWSI analysis period is from 1981 to present. SWSI values provide a more comprehensive outlook of water availability by combining streamflow forecasts and reservoir storage. The SWSI index allows comparison of water availability between basins for drought or flood severity analysis. Threshold SWSI values have been determined for some basins to indicate the potential for agricultural irrigation water shortages.

BASIN or REGION

SWSI Value

Most Recent Year With Similar SWSI

Value

Agricultural Water Supply Shortage May Occur When

SWSI is Less Than

Spokane 0.4

2017 NA Clearwater 0.1 2003 NA

Salmon -0.7 2002

NA Weiser -1.0

2014

NA Payette -2.1

2013 NA Boise -2.4 2007 -2.8

Big Wood above Hailey -3.2

2007 NA Big Wood -1.3

2003 0.3 Little Wood -2.1 2013 -1.9

Big Lost -2.1

2007 -0.3 Little Lost -1.3 2015 1.4

Teton 1.0 2018 -3.9 Henrys Fork 1.2

2014 -2.8 Snake (Heise) 1.5

2008 -1.4 Oakley 1.5 2005 0.3

Salmon Falls above Jackpot -0.7 2004 NA Salmon Falls 1.5 1995 -0.8

Bruneau -1.0 2004 NA Owyhee 0.4 2018

-1.1 Bear River 1.8

2012 -3.9

SWSI SCALE, PERCENT CHANCE OF EXCEEDANCE, AND INTERPRETATION

-4 -3 -2 -1 0 1 2 3 4 |------|------|------|------|------|------|------|------|

99% 87% 75% 63% 50% 37% 25% 13% 1% ------------------------------------------------------------------------ |Much | Below | Near Normal | Above | Much | |Below | Normal | Water Supply | Normal | Above | ---------------------------------------------------------------------- NA=Not Available / Not Applicable; Note: The Percent Chance of Exceedance is an indicator of how often a range of SWSI values might be expected to occur. Each SWSI unit represents about 12% of the historical occurrences. As an example of interpreting the above scale, the SWSI can be expected to be greater than -3.0, 87% of the time and less than -3.0, 13% of the time. Half the time, the SWSI will be below and half the time above a value of zero. The interval between -1.5 and +1.5 described as "Near Normal Water Supply," represents three SWSI units and would be expected to occur about one-third (36%) of the time.

SALMON RIVERBASINS

SOUTHSIDE SNAKERIVER BASINS

PANHANDLE REGION

CLEARWATER BASIN

BEAR RIVER BASIN

UPPER SNAKE RIVER BASIN

WEST CENTRAL

BASINS

WOOD & LOST BASINS

22

97

79

57

80

113

63

98

100

97

99

48

76

122

120

80

85

71

108

98

123

97

0

75

74

94

105

56

90

11574

108

149

77

12

77

121

107

117

134

108

41

119

Copyright:(c) 2014 Esri

0 50 100 150 20025Miles

¯

This map is prepared by the USDA-NRCS Idaho Snow Survey Office. http://www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Figure 1: Percent of Median Snowpack, May 1, 2020

Basin-wide Snow Water Equivalent as a Percentage of the 1981 to 2010 Median

MedianSnowpack

Above

Below

Provisional Data - Subject to Revision

>= 150%130 - 149%110 - 129%90 - 109%70 - 89%50 -69%0 - 49%No Data

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SALMON RIVERBASINS

SOUTHSIDE SNAKERIVER BASINS

PANHANDLE REGION

CLEARWATER BASIN

BEAR RIVER BASIN

UPPER SNAKE RIVER BASIN

WEST CENTRAL

BASINS

WOOD & LOST BASINS

Copyright:(c) 2014 Esri

0 50 100 150 20025Miles

¯

This map is prepared by the USDA-NRCS Idaho Snow Survey Office. http://www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Figure 2: Median Water Supply Forecast, May 1, 2020

Forecasted May-July Flow as a Percentage of the 1981 to 2010 Average

Provisional Data - Subject to Revision

#* > 180%

#* 150 - 179%

#* 130 - 149%

#* 110 - 129%

") 90 - 109%

#*

70 - 89%#*

50 - 69%#*

25 - 49%#*

0 - 24%( No Data

AverageForecast

Above Average

BelowAverage

Panhandle Region

May 1, 2020

WATER SUPPLY OUTLOOK

April was drier than normal in the Panhandle region with end-of-the-month precipitation totals ranging

between ~50 and 90% of normal. May 1 water year-to-date totals are near to below normal, with a

range of ~85 to 105%. Widespread snowmelt has begun across the region, and SNOTEL sites below

4,000 feet have completely melted. May 1 snowpack totals are trending near normal (see red line on

SWE plot above) with totals between ~75 and 120%. NOAA's 30-day temperature and precipitation

outlook for May predicts above-normal temperatures and below-normal precipitation. This will likely

increase the already rapid melt rates seen across the region.

Reservoirs in the region are between 83 and 122% of normal. Observed streamflow at stations on the

Saint Joe, Saint Maries, and North Fork Coeur d’Alene rivers were between ~45 and 55% of normal

on April 1. Since then, they have increased to ~89% of normal of observed streamflow at the end of

April. Streamflow forecasts for the runoff period are 100 to 109% of normal.

Priest Lake 84.6 91.2 101.9 119.3

Lake Coeur d' Alene 218.8 209.9 228.0 238.5

Noxon Rapids Reservoir 291.4 299.1 307.4 335.0

Lake Pend Oreille 908.5 947.1 931.7 1561.3

Hungry Horse Lake 2665.1 2798.9 2188.0 3451.0

Flathead Lake 869.0 1143.0 971.5 1791.0

Kootenai ab Bonners Ferry 20 113% 85%

Reservoir Storage (KAF): End of April

Reservoir NameCurrent

(KAF)Last YR

Average

(KAF)

Capacity

(KAF)

Spokane River 12 107% 85%

Palouse River 2 77% 123%

Coeur d' Alene River 6 120% 90%

St. Joe River 4 98% 91%

Priest River 4 108% 95%

Rathdrum Creek 2 89% 35%

Basin Name# of

Sites

% of Median

2020 2019

Moyie River 5 119% 82%

2450 1950

Normals based on 1981-2010 reference period: streamflow, precipitation, & reservoir normals are averages, SWE normals are medians.

1) 90% and 10% exceedance probabilities are actually 95% and 5%

2) Forecasts are for unimpaired flows. Actual flow will be dependent on management of upstream reservoirs and diversions

Watershed Snowpack Analysis: May 1, 2020

1990 2230 1710

MAY-SEP 1610 1860 2030 104% 2200

Spokane R at Long Lake MAY-JUL 1440 1670 1830 107%

1530

MAY-SEP 1300 1540 1700 105% 1860 2100 1620

1070 830

Spokane R nr Post Falls 2 MAY-JUL 1230 1460 1620 106% 1770 2000

875 1000 765

MAY-SEP 645 770 860 104% 945

St. Joe R at Calder 2 MAY-JUL 590 710 795 104%

405

MAY-SEP 365 430 475 107% 520 585 445

750 630

NF Coeur dAlene R at Enaville MAY-JUL 330 395 440 109% 485 545

620 695 580

MAY-SEP 470 555 610 97% 665

Priest R nr Priest River 2 MAY-JUL 435 510 565 97%

9690

MAY-SEP 9540 10500 11200 105% 11900 12900 10700

11600 9580

Pend Oreille Lake Inflow 2 MAY-JUL 8770 9620 10200 105% 10800 11600

9670 10400 8630

MAY-SEP 8650 9520 10100 105% 10700

Clark Fork R bl Cabinet Gorge Dam 2 MAY-JUL 7940 8670 9170 106%

96

MAY-SEP 82 94 101 100% 109 120 101

8120 6730

Boundary Ck nr Porthill MAY-JUL 78 89 96 100% 104 115

6230 7040 5730

MAY-SEP 5590 6460 6850 102% 7250

Kootenai R at Leonia 1 & 2 MAY-JUL 4700 5500 5870 102%

300

MAY-SEP 245 295 330 105% 360 410 315

10%

(KAF)

30yr Avg

(KAF)

Moyie R at Eastport MAY-JUL 235 285 315 105% 350 395

Panhandle Region Streamflow Forecasts - May 1, 2020

Forecast Exceedance Probabilities for Risk Assessment

<--Drier-------------------Projected Volume-------------------Wetter-->

Forecast PointForecast

Period

90%

(KAF)

70%

(KAF)

50%

(KAF) % Avg

30%

(KAF)

Clearwater River Basin

May 1, 2020

WATER SUPPLY OUTLOOK

Few storms passed through the Clearwater Basin during April, leaving precipitation end-of-the-month

totals below normal with a range of 85 to 95%. Water year-to-date totals range from 90 to 100% of

normal. May 1 snowpack totals are trending from above to near normal (see red line on SWE plot

above) with ranges between ~100 and 120%. Significant melting has begun throughout the region

and sites below 3,200 feet have completely melted. However, sites above ~6,200 feet in the North

Fork Clearwater River Basin received net SWE of 1 inch during April. Increased temperatures are

predicted throughout the region, likely increasing melt rates at all elevations.

Storage at Dworshak Reservoir is at 98% of normal. Observed flows at forecasts points in the region

were ~55 to 90% of normal at the beginning of April. They have since increased to ~90 to 120% of

normal at the end of April. Streamflow forecasts for the runoff period are 93 to 111% of normal.

Reservoir NameCurrent

(KAF)Last YR

Average

(KAF)

Capacity

(KAF)

Dworshak Reservoir 2581.6 2921.0 2646.0 3468.0

Selway River 4 122% 112%

Clearwater Basin Total 15 104% 102%

NF Clearwater River 8 100% 99%

Lochsa River 2 105% 110%

Normals based on 1981-2010 reference period: streamflow, precipitation, & reservoir normals are averages, SWE normals are medians.

1) 90% and 10% exceedance probabilities are actually 95% and 5%

2) Forecasts are for unimpaired flows. Actual flow will be dependent on management of upstream reservoirs and diversions

Watershed Snowpack Analysis: May 1, 2020

Basin Name# of

Sites

% of Median

2020 2019

Reservoir Storage (KAF): End of April

5260

MAY-SEP 4550 5200 5650 100% 6090 6740 5640

4470 3630

Clearwater R at Spalding 2 MAY-JUL 4380 4970 5370 102% 5770 6370

3810 4200 3400

MAY-SEP 3060 3480 3760 104% 4050

Clearwater R at Orofino MAY-JUL 2890 3280 3540 104%

1770

MAY-SEP 1390 1620 1780 93% 1940 2170 1920

1380 1210

Dworshak Reservoir Inflow 2 MAY-JUL 1370 1580 1720 97% 1850 2060

1180 1300 1130

MAY-SEP 935 1070 1160 96% 1250

Lochsa R nr Lowell MAY-JUL 885 1010 1090 96%

1570

MAY-SEP 1540 1720 1840 110% 1960 2140 1670

10%

(KAF)

30yr Avg

(KAF)

Selway R nr Lowell MAY-JUL 1460 1620 1740 111% 1850 2010

Clearwater River Basin Streamflow Forecasts - May 1, 2020

Forecast Exceedance Probabilities for Risk Assessment

<--Drier-------------------Projected Volume-------------------Wetter-->

Forecast PointForecast

Period

90%

(KAF)

70%

(KAF)

50%

(KAF) % Avg

30%

(KAF)

Salmon River Basin

May 1, 2020

WATER SUPPLY OUTLOOK

April precipitation for the Salmon amounted to ~70% of normal. This departure from normal will be particularly difficult for the normally wetter, northern region of the Salmon which saw an average of 1-2 inches less precipitation than typical for April. Water-year-to-date precipitation is currently 85%. This is roughly 20% lower than this time last year. Temperatures were cool enough, and the Salmon received enough precipitation as snow to increase SWE slightly at the beginning of April. However, the second half of the month was warm and dry. SWE decreased by an average of 4.3 inches for the 23 sites. This decrease in SWE ranges from roughly zero change at 2 sites above 8,500 feet to 10 inches of loss along the western border of the basin. The Salmon River Basin is roughly 30% melted as of May 1. Streamflow forecasts for the runoff period in the Salmon River range between ~70 and 85%. The Middle Fork Salmon at Middle Fork Lodge is forecasted for ~73% or normal runoff. The Salmon is currently on track to peak in late-May, or early-June. NOAA’s 30-day forecasts for May predict above-normal temperatures and below-normal precipitation for this region. Although only a very small portion of the Southern Salmon Basin is currently abnormally dry, a continued lack in precipitation may raise concern for drought.

Salmon Basin Total 24 91% 112%

SF Salmon River 3 85% 106%

Little Salmon River 4 108% 171%

Lemhi River 7 98% 97%

MF Salmon River 3 80% 106%

Basin Name# of

Sites

% of Median

2020 2019

Salmon River ab Salmon 7 79% 108%

5320 5220

Normals based on 1981-2010 reference period: streamflow, precipitation, & reservoir normals are averages, SWE normals are medians.

1) 90% and 10% exceedance probabilities are actually 95% and 5%

2) Forecasts are for unimpaired flows. Actual flow will be dependent on management of upstream reservoirs and diversions

Watershed Snowpack Analysis: May 1, 2020

4220 4730 4660

MAY-SEP 3430 3990 4380 84% 4760

Salmon R at White Bird MAY-JUL 3030 3540 3880 83%

172

MAY-SEP 109 133 149 81% 165 189 184

225 245

Johnson Ck at Yellow Pine MAY-JUL 100 123 138 80% 154 177

176 205 225

MAY-SEP 123 152 173 71% 193

SF Salmon R nr Krassel Ranger Station MAY-JUL 110 138 157 70%

600

MAY-SEP 375 455 505 75% 560 640 675

101 76

MF Salmon R at MF Lodge MAY-JUL 320 390 440 73% 490 560

62 81 61

MAY-SEP 25 47 63 83% 78

Lemhi R nr Lemhi MAY-JUL 16.6 36 49 80%

680

MAY-SEP 365 475 550 68% 625 735 805

10%

(KAF)

30yr Avg

(KAF)

Salmon R at Salmon MAY-JUL 295 390 455 67% 520 615

Salmon River Streamflow Forecasts - May 1, 2020

Forecast Exceedance Probabilities for Risk Assessment

<--Drier-------------------Projected Volume-------------------Wetter-->

Forecast PointForecast

Period

90%

(KAF)

70%

(KAF)

50%

(KAF) % Avg

30%

(KAF)

West Central Basins

May 1, 2020

WATER SUPPLY OUTLOOK

The West Central basins did not receive much precipitation in April. Monthly precipitation totals compared to normal are 55, 62, and 61% for the Weiser, Payette, and Boise river basins respectively. Water-year-to-date precipitation totals for the West Central Basins now range from ~75 to 80%. Conditions in the area have developed from abnormally dry to moderate drought throughout the month of April. The region’s snowpack melt rates are slightly above normal when compared to our period of record. This region could experience increasingly higher than normal melt rates for the remainder of the season due to current snowpack conditions and the forecasted warm, dry weather. Above 6,000 feet, SNOTEL sites in the West Central Basins range from 10% melted to more than 50% melted as of May 1. All sites below 6,000 feet are melted out except for the two most northern sites—Bear Basin at ~30% melt and West Branch at ~60%. The entire region reservoir storage is currently at 107% of normal and this should help bolster the streamflow after a less than ideal winter. The Boise system is currently 109% of normal with all 3 reservoirs releasing water but continuing to fill as snowpack melts. The Payette system is currently at 101% of normal storage. Streamflow forecasts for the Boise, Payette, and Weiser rivers range from ~50 to 85%. The higher forecasts are for the Payette River and the lower forecasts in the Boise, particularly the South Fork. Flood control releases in the Boise River system are very unlikely this spring, and it’s unclear whether or not the Boise reservoir system will fill. All things considered; water supplies should be adequate across the Boise basin for the 2020 irrigation season.

Mann Creek Reservoir 10.7 10.1 10.7 11.1

Sub-Basin Total 626.7 592.4 606.6 855.1

Lake Lowell 133.5 152.1 125.6 165.2

Deadwood Reservoir 103.1 109.2 105.1 161.9

Cascade Reservoir 523.6 483.2 501.5 693.2

Lucky Peak Reservoir 236.4 234.1 219.8 293.2

Sub-Basin Total 767.2 855.9 693.7 1015.6

Anderson Ranch Reservoir 350.0 407.2 284.9 450.2

Arrowrock Reservoir 180.8 214.6 189.0 272.2

Reservoir Storage (KAF): End of April

Reservoir NameCurrent

(KAF)Last YR

Average

(KAF)

Capacity

(KAF)

Mann Creek 1 102% 147%

Weiser Basin Total 4 99% 179%

SF Payette River 5 90% 131%

Payette Basin Total 13 90% 131%

Boise Basin Total 13 74% 122%

NF Payette River 8 94% 137%

Mores Creek 4 77% 118%

Canyon Creek 1

SF Boise River 6 71% 124%

MF & NF Boise Rivers 6 74% 112%

Watershed Snowpack Analysis: May 1, 2020

Basin Name# of

Sites

% of Median

2020 2019

225 275 265

Normals based on 1981-2010 reference period: streamflow, precipitation, & reservoir normals are averages, SWE normals are medians.

1) 90% and 10% exceedance probabilities are actually 95% and 5%

2) Forecasts are for unimpaired flows. Actual flow will be dependent on management of upstream reservoirs and diversions

MAY-SEP 126 164 193 73%

1310

Weiser R nr Weiser MAY-JUL 107 142 169 72% 198 245 235

1080 1160

MAY-SEP 670 825 925 71% 1030 1180

82 94 112

Payette R nr Horseshoe Bend 2 MAY-JUL 605 745 840 72% 940

MAY-SEP 55 66 74 66%

395

Deadwood Reservoir Inflow 2 MAY-JUL 50 60 67 64% 74 85 104

270 340

MAY-SEP 210 240 265 67% 285 320

430 500 495

SF Payette R at Lowman MAY-JUL 175 205 225 66% 245

MAY-SEP 260 330 380 77%

395

NF Payette R nr Banks 2 MAY-JUL 250 320 365 77% 410 480 475

385 380

MAY-SEP 220 275 310 78% 345 400

67 74 73

NF Payette R at Cascade 2 MAY-JUL 220 270 300 79% 335

MAY-SEP 50 57 62 85%

1050

Lake Fork Payette R nr McCall MAY-JUL 49 55 60 86% 65 72 70

750 950

MAY-SEP 380 520 615 59% 705 845

53 68 72

Boise R nr Boise 2 MAY-JUL 315 445 535 56% 620

MAY-SEP 19.2 34 44 61%

510

Mores Ck nr Arrowrock Dam MAY-JUL 17.6 31 41 60% 50 64 68

395 460

MAY-SEP 230 290 335 66% 375 440

235 290 405

Boise R nr Twin Springs MAY-JUL 196 255 295 64% 335

MAY-SEP 101 157 196 48%

30yr Avg

(KAF)

SF Boise R at Anderson Ranch Dam 2 MAY-JUL 82 135 172 46% 210 260 370

<--Drier-------------------Projected Volume-------------------Wetter-->

Forecast PointForecast

Period

90%

(KAF)

70%

(KAF)

50%

(KAF) % Avg

30%

(KAF)

10%

(KAF)

West Central Basins Streamflow Forecasts - May 1, 2020

Forecast Exceedance Probabilities for Risk Assessment

Wood & Lost River Basin

May 1, 2020

WATER SUPPLY OUTLOOK

The absence of storm tracks in April did not help the Wood and Lost’s current abysmal water year.

April’s total monthly precipitation was less than 75% of normal for the whole region, with the Big Lost

Basin receiving the lowest amount at ~45% of normal and Camas Creek Basin receiving the highest

at ~80% of normal. Water year-to-date precipitation is 60 to 90% of normal. May 1 snowpack totals

are ~10 to 75% of normal. Camas Creek is at 0% due to Couch Summit snow course being melted

while the median for that site is 6” for May 1. SNOTEL sites below 6,500 feet have completely melted,

with a few sites completely melted at ~7,500 feet in the Pioneer and Boulder Mountains. Warmer

weather coupled with below normal precipitation is predicted for May; this will likely increase

snowmelt rates compared to normal.

May 1 reservoir storage totals for the region’s three reservoirs range from 115 to 128%. Observed

streamflow as a percent of normal in the Wood and Lost Basins have decreased during April. This is

likely due to early complete melt in low to mid-elevation areas. Streamflow forecasts range from 10 to

70% of normal.

Magic Reservoir 156.9 182.2 128.0 191.5

Mackay Reservoir 42.1 28.9 32.8 44.4

Little Wood Reservoir 28.8 22.5 25.0 30.0

Big Wood Basin Total 9 52% 125%

Reservoir Storage (KAF): End of April

Reservoir NameCurrent

(KAF)Last YR

Average

(KAF)

Capacity

(KAF)

Big Wood River ab Hailey 6 56% 118%

Camas Creek 3 0% 232%

Fish Creek 0

Little Wood River 3 12% 119%

Big Lost River ab Mackay 4 47% 111%

Big Lost Basin Total 5 48% 108%

Birch-Medicine Lodge Creeks 2 97% 100%

Little Lost River 3 74% 102%

Basin Name# of

Sites

% of Median

2020 2019

Camas-Beaver Creeks 2 41% 54%

62 191

Normals based on 1981-2010 reference period: streamflow, precipitation, & reservoir normals are averages, SWE normals are medians.

1) 90% and 10% exceedance probabilities are actually 95% and 5%

2) Forecasts are for unimpaired flows. Actual flow will be dependent on management of upstream reservoirs and diversions

Watershed Snowpack Analysis: May 1, 2020

33 54 176

MAY-SEP 6.6 17.3 27 14% 40

Big Wood R bl Magic Dam 2 MAY-JUL 4.2 13.1 22 13%

35

MAY-SEP 0.35 2.5 5 14% 8.3 14.9 36

43 159

Camas Ck nr Blaine MAY-JUL 0.26 2.2 4.7 13% 8 14.6

25 45 147

MAY-SEP 3.6 10.9 18 11% 27

Big Wood R ab Magic Reservoir MAY-JUL 0.81 6.9 14.5 10%

205

MAY-SEP 37 71 93 40% 116 150 235

36 61

Big Wood R at Hailey MAY-JUL 29 59 79 39% 99 128

23 32 55

MAY-SEP 9.4 15.5 21 34% 26

Little Wood R nr Carey 2 MAY-JUL 8.2 13.6 18.1 33%

52

MAY-SEP 9.6 15.6 20 34% 26 35 58

115 138

Little Wood R ab High Five Ck MAY-JUL 8.1 13.4 17.7 34% 23 31

64 88 111

MAY-SEP 19.3 48 67 49% 87

Big Lost R bl Mackay Reservoir MAY-JUL 6.8 31 47 42%

148

MAY-SEP 46 74 93 55% 111 139 169

31 30

Big Lost R at Howell Ranch MAY-JUL 40 64 81 55% 97 122

19.8 24 24

MAY-SEP 11.2 17.1 21 70% 25

Little Lost R bl Wet Ck nr Howe MAY-JUL 9.8 14 16.9 70%

30yr Avg

(KAF)

Camas Ck at Camas MAY-JUL 3.2 6.7 9.8 39% 13.5 20 25

Forecast Exceedance Probabilities for Risk Assessment

<--Drier-------------------Projected Volume-------------------Wetter-->

Forecast PointForecast

Period

90%

(KAF)

70%

(KAF)

50%

(KAF) % Avg

30%

(KAF)

10%

(KAF)

Wood and Lost Basins Streamflow Forecasts - May 1, 2020

Upper Snake River Basin

May 1, 2020

WATER SUPPLY OUTLOOK

Unlike the rest of Idaho, circumstances in April allowed the Upper Snake to receive roughly normal to above-normal precipitation. Except for the Henrys Fork-Falls River basin, all sub-basins in the Upper Snake received above-normal precipitation. Water-year-to-date precipitation ranges from ~85 to 110% as of May 1. The Snake River above Palisades Reservoir is 100% of normal. Colder temperatures for this region allowed the snowpack to remain until late April before the snowpack was ripe enough to begin widespread melt near the end of the month. Reservoir storage continues to be exemplary. The entire Upper Snake storage is at 113% of normal,

with the Jackson-Palisades system at ~70% capacity, which represents ~125% of normal. Streamflow

forecasts for the runoff period range from ~85 to 110% with the Snake River near Heise forecast at

~107% of normal. Water managers should have plenty of water for user needs as well as excess

water for aquifer recharge.

Basin-Wide Total 3893.8 3699.3 3375.5 4577.9

Blackfoot Reservoir 323.0 314.5 211.3 337.0

American Falls Reservoir 1667.4 1590.8 1528.0 1672.6

Sub-Basin Total 223.9 217.7 220.1 240.8

Ririe Reservoir 71.6 68.5 58.7 80.5

Island Park Reservoir 120.4 114.3 123.8 135.2

Grassy Lake 13.6 13.7 12.8 15.2

Sub-Basin Total 1607.9 1507.7 1357.4 2247.0

Henrys Lake 89.9 89.7 83.5 90.4

Jackson Lake 643.1 654.2 445.7 847.0

Palisades Reservoir 964.8 853.5 911.7 1400.0

Snake River ab American Falls 41 115% 121%

Reservoir Storage (KAF): End of April

Reservoir NameCurrent

(KAF)Last YR

Average

(KAF)

Capacity

(KAF)

Blackfoot River 3 117% 161%

Portneuf River 5 80% 154%

Snake ab Palisades Resv 24 120% 111%

Willow Creek - Ririe 7 134% 171%

Greys River 4 135% 114%

Salt River 4 149% 131%

Gros Ventre River 4 121% 98%

Hoback River 6 123% 95%

Pacific Creek 2 119% 123%

Buffalo Fork 2 122% 101%

Henrys Fork ab Rexburg 17 107% 117%

Snake River ab Jackson Lake 7 108% 114%

Henrys Fork-Falls River 8 97% 118%

Teton River 9 115% 116%

Normals based on 1981-2010 reference period: streamflow, precipitation, & reservoir normals are averages, SWE normals are medians.

1) 90% and 10% exceedance probabilities are actually 95% and 5%

2) Forecasts are for unimpaired flows. Actual flow will be dependent on management of upstream reservoirs and diversions

Watershed Snowpack Analysis: May 1, 2020

Basin Name# of

Sites

% of Median

2020 2019

2100

MAY-SEP 1420 1900 2230 99% 2560 3050 2260

62 67

Snake R at Neeley 2 MAY-JUL 1370 1810 2110 100% 2410 2850

38 45 50

MAY-SEP 32 40 46 69% 52

Portneuf R at Topaz MAY-JUL 24 29 33 66%

3390

Willow Ck nr Ririe 2 MAY-JUL 14.4 24 31 72% 40 54 43

3480 2840

MAY-SEP 3140 3430 3630 107% 3830 4130

3570 3850 3150

Snake R nr Heise 2 MAY-JUL 2640 2890 3060 108% 3230

MAY-SEP 2900 3180 3380 107%

310

Snake R nr Irwin 2 MAY-JUL 2470 2710 2870 108% 3030 3270 2660

335 245

MAY-SEP 245 300 335 108% 370 425

365 400 315

Salt R ab Reservoir nr Etna MAY-JUL 183 230 260 106% 290

MAY-SEP 290 320 345 110%

2280

Greys R ab Reservoir nr Alpine MAY-JUL 245 270 290 109% 310 335 265

2380 1960

MAY-SEP 2170 2340 2460 108% 2580 2760

340 375 305

Snake R ab Reservoir nr Alpine 2 MAY-JUL 1870 2020 2130 109% 2230

MAY-SEP 265 300 320 105%

161

Buffalo Fk ab Lava Ck nr Moran MAY-JUL 235 265 280 106% 300 325 265

200 152

MAY-SEP 119 147 165 102% 184 210

765 835 775

Pacific Ck at Moran MAY-JUL 111 138 156 103% 174

MAY-SEP 595 665 715 92%

470

Snake R nr Moran 2 MAY-JUL 515 580 625 89% 670 735 700

505 425

MAY-SEP 405 450 480 102% 510 555

1540 1690 1560

Snake R at Flagg Ranch MAY-JUL 370 410 435 102% 465

MAY-SEP 1180 1330 1430 92%

390

Henrys Fk nr Rexburg 2 MAY-JUL 900 1020 1090 93% 1170 1290 1170

425 320

MAY-SEP 365 410 440 113% 470 510

210 235 173

Teton R nr St Anthony MAY-JUL 300 335 360 113% 385

MAY-SEP 159 181 196 113%

385

Teton R nr Driggs MAY-JUL 123 141 153 114% 165 183 134

365 315

MAY-SEP 345 375 400 104% 425 455

540 590 595

Falls R nr Ashton 2 MAY-JUL 270 300 320 102% 335

MAY-SEP 415 470 505 85%

30yr Avg

(KAF)

Henrys Fk nr Ashton 2 MAY-JUL 280 325 355 86% 385 430 415

Forecast Exceedance Probabilities for Risk Assessment

<--Drier-------------------Projected Volume-------------------Wetter-->

Forecast PointForecast

Period

90%

(KAF)

70%

(KAF)

50%

(KAF) % Avg

30%

(KAF)

10%

(KAF)

Upper Snake River Basin Streamflow Forecasts - May 1, 2020

Southside Snake

River Basins

May 1, 2020

WATER SUPPLY OUTLOOK

The Southside Snake River Basin had low amounts of precipitation in April, resulting in ~35 to 80% of normal accumulation across most SNOTEL sites. Water year precipitation continues to be below normal and ranges from ~75 to 100%. Although the month started with near or below normal temperatures, the latter half of the month was consistently warmer than normal with sunny days and scattered rain showers. Also, much of the lower Snake basin snowpack had near-ripe conditions at the beginning of April. A ripe snowpack has all its pore space saturated with liquid water, and a good indicator of this is when snowpack density reaches ~40%. At this phase, any energy entering the snow contributes to melt and runoff. Melt rates across this basin have been above normal, and the remaining snowpack currently ranges from ~20 to 60% of normal. NOAA’s 30-day outlooks predict above-normal temperatures and below-normal precipitation for this region; this bodes for the continuation of rapid snowmelt. Current reservoir storage expressed as a percent of average is the following for area reservoirs: Oakley 120%, Salmon Falls 144%, Wild Horse 136%, and Lake Owyhee 112%. Streamflow forecasts range from ~35 to 80% of normal, except for Owyhee River forecasts which are closer to ~20 - 30%. Above-normal reservoir storage will provide adequate irrigation supplies for the 2020 growing season.

Brownlee Reservoir 998.2 1242.4 1161.0 1420.0

Wild Horse Reservoir 67.4 75.1 49.4 71.5

Lake Owyhee 596.8 646.4 533.1 715.0

Oakley Reservoir 41.2 35.8 34.3 75.6

Salmon Falls Reservoir 103.4 82.2 71.6 182.6

Owyhee Basin Snotel Total 8 22% 63%

Reservoir Storage (KAF): End of April

Reservoir NameCurrent

(KAF)Last YR

Average

(KAF)

Capacity

(KAF)

Reynolds Creek 1

Owyhee Basin Total 8 22% 63%

Salmon Falls Creek 5 63% 116%

Bruneau River 5 57% 114%

Raft River 1 76% 114%

Goose-Trapper Creeks 2 75% 134%

Normals based on 1981-2010 reference period: streamflow, precipitation, & reservoir normals are averages, SWE normals are medians.

1) 90% and 10% exceedance probabilities are actually 95% and 5%

2) Forecasts are for unimpaired flows. Actual flow will be dependent on management of upstream reservoirs and diversions

Watershed Snowpack Analysis: May 1, 2020

Basin Name# of

Sites

% of Median

2020 2019

210

MAY-SEP 28 61 91 38% 126 190 240

142 205

Owyhee R bl Owyhee Dam 2 MAY-JUL 13.2 40 66 31% 99 159

73 127 188

MAY-SEP 9.1 32 56 27% 86

Owyhee R nr Rome MAY-JUL 4.5 24 45 24%

5.4

Owyhee R nr Gold Ck 2 MAY-JUL 0.02 0.81 2 21% 3.7 7.2 9.6

3.4 5.3

MAY-SEP 0.69 1.3 1.82 34% 2.4 3.5

111 142 148

Reynolds Ck at Tollgate MAY-JUL 0.7 1.29 1.79 34% 2.4

MAY-SEP 53 75 92 62%

53

Bruneau R nr Hot Spring MAY-JUL 49 69 86 61% 104 133 140

47 49

MAY-SEP 15.7 24 31 58% 39 51

16.2 21 19.4

Salmon Falls Ck nr San Jacinto MAY-JUL 13.5 21 28 57% 35

MAY-SEP 6.7 10.2 13 67%

4.9

Oakley Reservoir Inflow MAY-JUL 5.3 8.4 10.9 64% 13.7 18.4 16.9

4 3.7

MAY-SEP 2.9 3.6 4 82% 4.6 5.4

13.9 19.5 14.5

Trapper Ck nr Oakley MAY-JUL 1.98 2.5 2.9 78% 3.3

MAY-SEP 4.4 7.8 10.6 73%

30yr Avg

(KAF)

Goose Ck ab Trapper Ck nr Oakley MAY-JUL 3.9 7 9.6 73% 12.5 17.7 13.2

Forecast Exceedance Probabilities for Risk Assessment

<--Drier-------------------Projected Volume-------------------Wetter-->

Forecast PointForecast

Period

90%

(KAF)

70%

(KAF)

50%

(KAF) % Avg

30%

(KAF)

10%

(KAF)

Southside Snake River Basins Streamflow Forecasts - May 1, 2020

Bear River Basin

May 1, 2020

WATER SUPPLY OUTLOOK

Precipitation in the Bear River Basin ranged from ~70 to 100% of normal for the month. Although

April had below-normal precipitation, the total water year precipitation is near normal and ranges

from ~80 to 110%. The Bear River Basin snowpack is near normal (~82 to 100%), but it is

experiencing more rapid melt than normal. The snowpack in this region was already close to ripe

conditions going into April. A ripe snowpack has all its pore space saturated with liquid water; at this

phase, any energy entering the snow contributes to melt and runoff. The latter half of April was filled

with warm, sunny days and scattered rain showers, which have contributed to accelerated

snowmelt.

Bear Lake is 74% full and 147% of average. Streamflow forecasts are near normal for the primary

runoff period and range from ~64 to 100% of normal. The water outlook is promising for the many

water users and recreationalists throughout the Bear River basin.

Bear Lake 957.4 917.2 651.7 1302.0

Montpelier Reservoir 3.4 2.5 2.7 4.0

Malad River 0

Reservoir Storage (KAF): End of April

Reservoir NameCurrent

(KAF)Last YR

Average

(KAF)

Capacity

(KAF)

Cub River 1 94% 101%

Bear River ab ID-UT Line 17 97% 119%

Montpelier Creek 2 130% 105%

Mink Creek 1 82% 111%

Smiths-Thomas Forks 4 104% 98%

Bear River ab WY-ID Line 10 92% 119%

31

Normals based on 1981-2010 reference period: streamflow, precipitation, & reservoir normals are averages, SWE normals are medians.

1) 90% and 10% exceedance probabilities are actually 95% and 5%

2) Forecasts are for unimpaired flows. Actual flow will be dependent on management of upstream reservoirs and diversions

Watershed Snowpack Analysis: May 1, 2020

Basin Name# of

Sites

% of Median

2020 2019

57 43

MAY-JUL 12.7 22 29 94% 36 46

104 116 96

Blacksmith Fk nr Hyrum APR-JUL 21 32 39 91% 46

MAY-JUL 75 87 96 100%

32

Logan R nr Logan APR-JUL 89 101 109 98% 117 129 111

41 45

MAY-JUL 6.7 14 19 59% 24 31

135 180 146

Little Bear at Paradise APR-JUL 16.7 24 29 64% 34

MAY-JUL 30 75 105 72%

183

APR-SEP 59 112 148 72% 184 235 205

96 80

Bear R bl Stewart Dam 2 APR-JUL 57 103 135 74% 167 215

111 123 104

MAY-JUL 62 72 79 99% 86

APR-SEP 83 95 103 99%

3.1

Smiths Fk nr Border APR-JUL 71 81 88 99% 95 105 89

6.9 3.8

MAY-JUL 0.22 1.64 2.8 90% 4 5.7

91 120 105

Big Ck nr Randolph APR-JUL 0.31 2.3 3.6 95% 4.9

MAY-JUL 21 51 71 68%

121

APR-SEP 26 62 86 67% 109 145 128

107 104

Bear R ab Resv nr Woodruff APR-JUL 30 62 84 69% 105 137

109 124 123

MAY-JUL 66 79 87 84% 95

APR-SEP 74 89 99 80%

30yr Avg

(KAF)

Bear R nr UT-WY State Line APR-JUL 71 84 93 83% 102 115 112

Forecast Exceedance Probabilities for Risk Assessment

<--Drier-------------------Projected Volume-------------------Wetter-->

Forecast PointForecast

Period

90%

(KAF)

70%

(KAF)

50%

(KAF) % Avg

30%

(KAF)

10%

(KAF)

Bear River Basin Streamflow Forecasts - May 1, 2020

Streamflow Adjustment List for All Forecasts Published in Idaho Water Supply Outlook Report: Streamflow forecasts are projections of runoff volumes that would occur without influences from upstream reservoirs or diversions. These values are referred to as natural, unregulated or adjusted flows. To make these adjustments, changes in reservoir storage, diversions, and inter-basin transfers are added or subtracted from the observed (actual) streamflow volumes. The following list documents the adjustments made for each forecast point. (Revised Dec. 2018). Panhandle Region Kootenai R at Leonia, MT (2) + Lake Koocanusa storage change Moyie R at Eastport – no corrections Boundary Ck nr Porthill – no corrections Clark Fork R bl Cabinet Gorge (2) + Hungry Horse storage change + Flathead Lake storage change + Noxon Res storage change Whitehorse Rapid gage used create longer term record Pend Oreille Lake Inflow (2) + Pend Oreille R at Newport, WA + Hungry Horse Res storage change + Flathead Lake storage change + Noxon Res storage change + Lake Pend Oreille storage change + Priest Lake storage change Priest R nr Priest R (2) + Priest Lake storage change NF Coeur d' Alene R at Enaville - no corrections St. Joe R at Calder- no corrections Spokane R nr Post Falls (2) + Lake Coeur d' Alene storage change Spokane R at Long Lake, WA (2) + Lake Coeur d' Alene storage change + Long Lake, WA storage change Clearwater River Basin Selway R nr Lowell - no corrections Lochsa R nr Lowell - no corrections Dworshak Res Inflow (2) + Clearwater R nr Peck - Clearwater R at Orofino + Dworshak Res storage change Clearwater R at Orofino - no corrections Clearwater R at Spalding (2) + Dworshak Res storage change Salmon River Basin Salmon R at Salmon - no corrections Lemhi R nr Lemhi – no corrections MF Salmon R at MF Lodge – no corrections SF Salmon gage used to create longer term record SF Salmon R nr Krassel Ranger Station – no corrections Johnson Creek at Yellow pine – no corrections Salmon R at White Bird - no corrections West Central Basins Boise R nr Twin Springs - no corrections SF Boise R at Anderson Ranch Dam (2) + Anderson Ranch Res storage change Mores Ck nr Arrowrock Dam – no corrections

Boise R nr Boise (2) + Anderson Ranch Res storage change + Arrowrock Res storage change + Lucky Peak Res storage change SF Payette R at Lowman - no corrections Deadwood Res Inflow (2) + Deadwood R bl Deadwood Res nr Lowman + Deadwood Res storage change Lake Fork Payette R nr McCall – no corrections NF Payette R at Cascade (2) + Payette Lake storage change + Cascade Res storage change NF Payette R nr Banks (2) + Payette Lake storage change + Cascade Res storage change Payette R nr Horseshoe Bend (2) + Deadwood Res storage change + Payette Lake storage change + Cascade Res storage change Weiser R nr Weiser - no corrections Wood and Lost Basins Little Lost R bl Wet Ck nr Howe - no corrections Big Lost R at Howell Ranch - no corrections Big Lost R bl Mackay Res nr Mackay (2) + Mackay Res storage change Little Wood R ab High Five Ck – no corrections Little Wood R nr Carey (2) + Little Wood Res storage change Big Wood R at Hailey - no corrections Big Wood R ab Magic Res (2) + Big Wood R nr Bellevue (1912-1996) + Big Wood R at Stanton Crossing nr Bellevue (1997 to present) + Willow Ck (1997 to present) Camas Ck nr Blaine – no corrections Magic Res Inflow (2) + Big Wood R bl Magic Dam + Magic Res storage change Upper Snake River Basin Falls R nr Ashton (2) + Grassy Lake storage change + Diversions from Falls R ab nr Ashton Henrys Fork nr Ashton (2) + Henrys Lake storage change + Island Park Res storage change Teton R nr Driggs - no corrections Teton R nr St. Anthony (2) - Cross Cut Canal into Teton R + Sum of Diversions for Teton R ab St. Anthony + Teton Dam for water year 1976 only

Henrys Fork nr Rexburg (2) + Henrys Lake storage change + Island Park Res storage change + Grassy Lake storage change + 3 Diversions from Falls R ab Ashton-Chester + 6 Diversions from Falls R abv Ashton + 7 Diversions from Henrys Fk btw Ashton to St. Anthony + 21 Diversions from Henrys Fk btw St. Anthony to Rexburg Snake R nr Flagg Ranch, WY – no corrections Snake R nr Moran, WY (2) + Jackson Lake storage change Pacific Ck at Moran, WY - no corrections Buffalo Fork ab Lava nr Moran, WY - no corrections Snake R ab Res nr Alpine, WY (2) + Jackson Lake storage change Greys R nr Alpine, WY - no corrections Salt R nr Etna, WY - no corrections Palisades Res Inflow (2) + Snake R nr Irwin + Jackson Lake storage change + Palisades Res storage change Snake R nr Heise (2) + Jackson Lake storage change + Palisades Res storage change Ririe Res Inflow (2) + Willow Ck nr Ririe + Ririe Res storage change The forecasted natural volume for Willow Creek nr Ririe does not include Grays Lake water diverted from Willow Creek drainage through the Clarks Cut diversion and into Blackfoot Reservoir. Blackfoot R ab Res nr Henry (2) + Blackfoot Res storage change The forecasted Blackfoot Reservoir Inflow includes Grays Lake water diverted from the Willow Creek drainage through the Clarks Cut diversion and into Blackfoot Reservoir. Portneuf R at Topaz - no corrections American Falls Res Inflow (2) + Snake R at Neeley + Jackson Lake storage change + Palisades Res storage change + American Falls storage change + Teton Dam for water year 1976 only Southside Snake River Basins Goose Ck nr Oakley - no adjustments Trapper Ck nr Oakley - no adjustments Oakley Res Inflow - flow does not include Birch Creek + Goose Ck + Trapper Ck Salmon Falls Ck nr San Jacinto, NV - no corrections Bruneau R nr Hot Springs - no corrections Reynolds Ck at Tollgate - no corrections Owyhee R nr Gold Ck, NV (2) + Wildhorse Res storage change Owyhee R nr Rome, OR – no Corrections Owyhee Res Inflow (2)

+ Owyhee R bl Owyhee Dam, OR + Lake Owyhee storage change + Diversions to North and South Canals Bear River Basin Bear R nr UT-WY Stateline, UT- no corrections Bear R abv Res nr Woodruff, UT- no corrections Big Ck nr Randolph, UT - no corrections Smiths Fork nr Border, WY - no corrections Bear R bl Stewart Dam (2) + Bear R bl Stewart Dam + Rainbow Inlet Canal Little Bear R at Paradise, UT - no corrections Logan R nr Logan, UT - no corrections Blacksmith Fk nr Hyrum, UT - no corrections Reservoir Capacity Definitions (Units in 1,000 Acre-Feet, KAF) Different agencies use various definitions when reporting reservoir capacity and contents. Reservoir storage terms include dead, inactive, active, and surcharge storage. This table lists the volumes for each reservoir, and defines the storage volumes NRCS uses when reporting capacity and current reservoir storage. In most cases, NRCS reports usable storage which includes active and/or inactive storage. (Revised Feb. 2015) Basin- Lake or Dead Inactive Active Surcharge NRCS NRCS Capacity Reservoir Storage Storage Storage Storage Capacity Includes Panhandle Region Hungry Horse 39.73 --- 3451.00 --- 3451.0 Active Flathead Lake Unknown --- 1791.00 --- 1791.0 Active Noxon Unknown --- 335.00 --- 335.0 Active Lake Pend Oreille 406.20 112.40 1042.70 --- 1561.3 Dead + Inactive + Active Lake Coeur d'Alene Unknown 13.50 225.00 --- 238.5 Inactive + Active Priest Lake 20.00 28.00 71.30 --- 119.3 Dead + Inactive + Active Clearwater Basin Dworshak Unknown 1452.00 2016.00 --- 3468.0 Inactive + Active West Central Basins Anderson Ranch 24.90 37.00 413.10 --- 450.1 Inactive + Active Arrowrock Unknown --- 272.20 --- 272.2 Active Lucky Peak Unknown 28.80 264.40 13.80 293.2 Inactive + Active Lake Lowell 7.90 5.80 159.40 --- 165.2 Inactive + Active Deadwood Unknown --- 161.90 --- 161.9 Active Cascade Unknown 46.70 646.50 --- 693.2 Inactive + Active Mann Creek 1.61 0.24 11.10 --- 11.1 Active Wood and Lost Basins Mackay 0.13 --- 44.37 --- 44.4 Active Little Wood Unknown --- 30.00 --- 30.0 Active Magic Unknown --- 191.50 --- 191.5 Active Upper Snake Basin Jackson Lake Unknown --- 847.00 --- 847.0 Active Palisades 44.10 155.50 1200.00 --- 1400.0 Dead +Inactive +Active Henrys Lake Unknown --- 90.40 --- 90.4 Active Island Park 0.40 --- 127.30 7.90 135.2 Active + Surcharge Grassy Lake Unknown --- 15.18 --- 15.2 Active Ririe 4.00 6.00 80.54 10.00 80.5 Active Blackfoot 0.00 --- 333.50 3.50 333.50 Active (rev. 2/1/2015) American Falls Unknown --- 1672.60 --- 1672.6 Active Southside Snake Basins Oakley 0.00 --- 75.60 --- 75.6 Active Salmon Falls 48.00 5.00 182.65 --- 182.6 Active Wild Horse Unknown --- 71.50 --- 71.5 Active Lake Owyhee 406.83 --- 715.00 --- 715.0 Active Brownlee 0.45 444.70 975.30 --- 1420.0 Inactive + Active Bear River Basin Bear Lake 5000.00 119.00 1302.00 --- 1302.0 Active: Capacity does not include 119 KAF that can be used, historic values below this level are rounded to zero Montpelier 0.21 --- 3.84 --- 4.0 Dead + Active

Interpreting Water Supply Forecasts

Each month, five forecasts are issued for each forecast point and each forecast period. Unless otherwise specified, all streamflow forecasts are for streamflow volumes that would occur naturally without any upstream influences. Water users need to know what the different forecasts represent if they are to use the information correctly when making operational decisions. The following is an explanation of each of the forecasts. 90 Percent Chance of Exceedance Forecast. There is a 90 percent chance that the actual streamflow volume will exceed this forecast value, and there is a 10 percent chance that the actual streamflow volume will be less than this forecast value. 70 Percent Chance of Exceedance Forecast. There is a 70 percent chance that the actual streamflow volume will exceed this forecast value, and there is a 30 percent chance that the actual streamflow volume will be less than this forecast value. 50 Percent Chance of Exceedance Forecast. There is a 50 percent chance that the actual streamflow volume will exceed this forecast value, and there is a 50 percent chance that the actual streamflow volume will be less than this forecast value. Generally, this forecast is the middle of the range of possible streamflow volumes that can be produced given current conditions. 30 Percent Chance of Exceedance Forecast. There is a 30 percent chance that the actual streamflow volume will exceed this forecast value, and there is a 70 percent chance that the actual streamflow volume will be less than this forecast value. 10 Percent Chance of Exceedance Forecast. There is a 10 percent chance that the actual streamflow volume will exceed this forecast value, and there is a 90 percent chance that the actual streamflow volume will be less than this forecast value.

*Note: There is still a 20 percent chance that actual streamflow volumes will fall either below the 90 percent exceedance forecast or above the 10 percent exceedance forecast.

These forecasts represent the uncertainty inherent in making streamflow predictions. This uncertainty may include sources such as: unknown future weather conditions, uncertainties associated with the various prediction methodologies, and the spatial coverage of the data network in a given basin.

30-Year Average. The 30-year average streamflow for each forecast period is provided for comparison. The average is based on data from 1981-2010. The % AVG. column compares the 50% chance of exceedance forecast to the 30-year average streamflow; values above 100% denote when the 50% chance of exceedance forecast would be greater than the 30-year average streamflow. AF - Acre-feet, forecasted volume of water are typically in thousands of acre-feet (KAF). These forecasts are given to users to help make risk-based decisions. Users can select the forecast corresponding to the level of risk they are willing to accept in order to minimize the negative impacts of having more or less water than planned for. To Decrease the Chance of Having Less Water than Planned for A user might determine that making decisions based on a 50 percent chance of exceedance forecast is too much risk to take (there is still a 50% chance that the user will receive less than this amount). To reduce the risk of having less water than planned for, users can base their operational decisions on one of the forecasts with a greater chance of being exceeded such as the 90 or 70 percent exceedance forecasts. To Decrease the Chance of Having More Water than Planned for A user might determine that making decisions based on a 50 percent chance of exceedance forecast is too much risk to take (there is still a 50% chance that the user will receive more than this amount). To reduce the risk of having more water than planned for, users can base their operational decisions on one of the forecasts with a lesser chance of being exceeded such as the 30 or 10 percent exceedance forecasts. Forecast use example: Using the 50 Percent Exceedance Forecast. Using the example forecasts shown on the next page, there is a 50% chance that actual streamflow volume at the Henry’s Fork near Ashton will be less than 280 KAF between June 1 and Sept. 30. There is also a 50% chance that actual streamflow volume will be greater than 280 KAF. Using the 90 and 70 Percent Exceedance Forecasts. If an unexpected shortage of water could cause problems (such as irrigated agriculture), users might want to plan on receiving 245 KAF during Jun 1 through September 30 (from the 70 percent exceedance forecast). There is a 30% chance of receiving less than 245 KAF.

Alternatively, if users determine the risk of using the 70 percent exceedance forecast is too great, then they might plan on receiving 198 KAF (from the 90 percent exceedance forecast). There is 10% chance of receiving less than 72 KAF. Using the 30 or 10 Percent Exceedance Forecasts. If an unexpected excess of water could cause problems (such as operating a flood control reservoir), users might plan on receiving 315 KAF between June 1 and

Sept. 30 (from the 30 percent exceedance forecast). There is a 30% chance of receiving more than 315 KAF. Alternatively, if users determine the risk of using the 30 percent exceedance forecast is too great, then they might plan on receiving 360 KAF (from the 10 percent exceedance forecast). There is a 10% chance of receiving more than 360 KAF. Users could also choose a volume in between any of these values to reflect their desired risk level.

Interpreting Snowpack Plots

Basin snowpack plots represent snow water equivalent indices using the average daily SNOTEL data1 from several sites in or near individual basins. The solid red line (2015), which represents the current water year snowpack water content, can be compared to the normal dashed black line (Median) which is considered “normal”, as well as the SNOTEL observed historical snowpack range for each basin. This allows users to gather important information about the current year’s snowpack as well as the historical variability of snowpack in each basin. The gray shaded area represents the interquartile range (also known as the “middle fifty”), which is the 25th to 75th percentiles of the historical daily snowpack data for each basin. Percentiles depict the value of the average snowpack below which the given percent of historical years fall. For example, the top part of the interquartile range (75th percentile) indicates that the snowpack index has been below this line for 75 percent of the period of record, whereas the reverse is true for the lower part of the interquartile range (25th percentile). This means 50 percent of the time the snowpack index is within the interquartile range (gray area) during the period of record. 1 All data used for these plots come from daily SNOTEL data only and does not include snow course data (collected monthly), whereas the official basin snowpack percent of normal includes both SNOTEL and snow course data, potentially leading to slight discrepancies between plots and official basin percent of normal.

Upper Snake River Basin Streamflow Forecasts - June 1, 2015

Forecast Period

Forecast Exceedance Probabilities for Risk Assessment

30yr Avg(KAF)

<---Drier----------------Projected Volume-------------Wetter--->

Forecast Point 90%(KAF)

70%(KAF)

50%(KAF) % Avg

30%(KAF)

10%(KAF)

Henrys Fk nr Ashton JUN-JUL 72 106 129 56 152 186 230JUN-SEP 198 245 280 68 315 360 410

USDA Natural Resources Conservation Service 9173 West Barnes Drive, Suite C Boise ID 83709-1574

OFFICIAL BUSINESS

Issued by Matthew J. Lohr, Chief Natural Resources Conservation Service Washington, DC Released by Curtis Elke, State Conservationist Natural Resources Conservation Service Boise, Idaho Report Created by Idaho Snow Survey Staff Natural Resources Conservation Service Boise, ID Email: IDBOISE-NRCS-SNOW@one.usda.gov Forecasts provided by Forecast Hydrologist Staff NRCS, National Water and Climate Center, Portland, Oregon Email: julie.koeberle@por.usda.gov Numerous other groups and agencies provide funding and/or cooperative support for the collection, operation and maintenance of the Cooperative Idaho Snow Survey Program. Your cooperation is greatly appreciated!