Idaho Water Supply Outlook Report March 1, 2020

Natural Resources Conservation Service Idaho Water Supply Outlook Report March 1, 2020

Boise Foothills near Bogus Basin Road Snow Course, February 28, 2020 Photo courtesy of Melissa Ghergich

The Bogus Basin Road snow course (5,568 ft), visible above in the sunlit aspect of the image foreground, serves as an important barometer for transition zone snowpack. The March 1 snow measurement yielded 8.6” of snow water equivalent (SWE), which is 130% of normal. Several other snow measurement sites in the Payette and neighboring Boise River basin depict a similar story: above normal snowpack in the ‘lower’ elevation (~5,000 to 6,000 ft) zones. Interestingly, in and near the Boise River basin, all measurement sites below 5,700 ft are reporting above normal SWE, while all stations above 5,700 ft are reporting below normal SWE. Continue reading the full report for snowpack and water outlook details throughout Idaho. Water Supply Outlook Report Federal - State – Private Cooperative Snow Surveys 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: [email protected]

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 ([email protected])*

USDA is an equal opportunity provider and employer. To file a complaint of discrimination, write: USDA, Office of the Assistant Secretary for Civil Rights, Office of Adjudication, 1400 Independence Ave., SW, Washington, DC 20250-9410 or call (866) 632-9992 (Toll-free Customer Service), (800) 877-8339 (Local or Federal relay), (866) 377-8642 (Relay voice users). IDAHO WATER SUPPLY OUTLOOK REPORT

March 1, 2020

SUMMARY

Precipitation

February of 2020, unlike 2019, was not one for the record books in Idaho. Monthly precipitation was below normal for most of Idaho and ranged from ~25 to 125% (Figure 1). The Clearwater River basin received the most precipitation during February (125%). Once again this water-year, the basins that received the least precipitation with respect to normal were the Wood & Lost in south-central Idaho, where new record low monthly precipitation was observed for the month of February. Now five months into the water-year and wet season, and the Wood & Lost basins have yet to see a single month with above normal precipitation (January was near normal). Resulting, water-year precipitation totals are abysmal and are now approaching half of normal for the Oct. 1 – Mar. 1 period (Figure 2). While there’s still time during the remainder of our wet-season to make up ground, it’s looking likely the overall water picture will be below normal in the Wood & Lost basins. Near-term outlooks from NOAA’s Climate Prediction Center suggest an increased likelihood for above normal precipitation in early March, which would certainly be welcomed across south-central Idaho. Monthly and water-year precipitation data for all basins in Idaho can be accessed in tabular form here.

Snowpack

After a favorable weather pattern for mountain snowfall during most of January, high-pressure and drier conditions became much more prevalent during February. That being said, moderate snowpack gains were still observed throughout Idaho during February except in and near the Wood & Lost River basins. Most of Idaho experienced normal to slightly below normal average temperatures, owing in part to sufficient radiative cooling at night resulting from dry (cloudless) weather. As we transition into spring, this is an important process that helps to preserve snowpack because it results in a net energy loss from snow to the atmosphere (the opposite of what’s needed for snowmelt). March 1 snowpack percentages range from 90 to 110% for most of Idaho (Figure 3), except for the Wood & Lost basins (50 to 70%) and northern Idaho (110 to 120%). More snow is needed in the Wood & Lost basins in order to secure an adequate water supply. See Figure 3 for a map of basin specific May 1 snowpack conditions, or access the same information in tabular form here.

Reservoirs & Streamflow

All major reservoir projects in the Middle and Upper Snake basins continue to hold above normal storage, which will help to provide a buffer against anticipated below normal streamflow in and near the Wood & Lost basins. A statewide summary of current reservoir storage can be accessed here. For the majority of Idaho, streamflow forecasts for the primary runoff periods are expected to be ~80 to 120% of normal. As discussed previously related to current snowpack and precipitation totals, the Wood & Lost basins are again the exception with streamflow forecasts ranging from 20 (Camas Creek) to 60% of normal. Full basin specific forecast details can be accessed here and in Figure 4.

Note: The streamflow volumes referenced in this report are the 50% Chance of Exceeding Forecast, unless otherwise noted. IDAHO SURFACE WATER SUPPLY INDEX (SWSI) March 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 where appropriate. 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.

Agricultural Water Most Recent Year Supply Shortage SWSI With Similar SWSI May Occur When BASIN or REGION Value Value SWSI is Less Than

Spokane 0.7 2009 NA Clearwater 1.3 2019 NA Salmon -0.4 2016 NA Weiser -0.4 2005 NA

Payette -1.3 2004 NA

Boise -1.0 2003 - 1.8 Big Wood above Hailey -2.9 2004 - 2.9 Big Wood -1.0 2008 0. 1 Little Wood -1.3 2004 - 1.6

Big Lost -1.3 2013 0.5

Little Lost ------1.2

Teton 0.4 2019 - 4.0 Henrys Fork 0.7 2014 - 2.5 Snake (Heise) 1. 6 2019 - 1.8 Oakley 2.4 2019 - 0.1 Salmon Falls above Jackpot 0.4 2010 NA Salmon Falls 1. 9 1996 - 1.0 Bruneau 0.4 2010 NA Owyhee 1.3 1995 - 2.7 Bear River 2.9 2017 - 3.7

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. 103 Figure 1: Monthly Precipitation 85 58 February 2020 ¯ Monthly Precipitation as a Percentage of the 1981 to 2010 Average PANHANDLE 92 REGION Above >= 150% 92 130 - 149% 110 - 129% 103 Average 90 - 109% 155 Precipitation 70 - 89% 115 50 -69% 152 Below 0 - 49% CLEARWATER BASIN No Data 172 Provisional Data - Subject to Revision

SALMON 81 RIVER

86 BASINS WEST 75 93 167 UPPER SNAKE CENTRAL 75 92 RIVER BASIN BASINS 82 77 149 57 87 82 S 60 IN 37 S 124 75 A B 42 T 164 47 20 S LO 33 & OD 90 120 WO 109 155

72 61 BEAR 107 93 RIVER 119 BASIN 121 91

SOUTHSIDE SNAKE RIVER BASINS 0 25 50 100 150 200 Miles This map is prepared by the USDA-NRCS Idaho Snow Survey Office. http://www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/ Copyright:(c) 2014 Esri Figure 2: Water Year to Date

112 Precipitation March 1, 2020

¯ 95 Basin-wide Water Year Precipitation as a 104 Percentage of the 1981 to 2010 Average

Above >= 150% PANHANDLE 130 - 149% 95 REGION 89 110 - 129% Average 90 - 109% Precipitation 91 70 - 89% 103 50 -69% 93 Below 0 - 49% 98 No Data CLEARWATER Provisional Data - Subject to Revision BASIN 106

SALMON 84 RIVER

76 BASINS WEST 73 82 100 UPPER SNAKE CENTRAL 80 80 RIVER BASIN BASINS 93 83 89 70 76 S 85 76 IN 58 S 98 82 A B 62 T 102 70 52 S LO 68 & OD 82 89 WO 90 110

78 89 BEAR 99 101 RIVER 106 BASIN 100 93

SOUTHSIDE SNAKE RIVER BASINS

0 25 50 100 150 200 Miles This map is prepared by the USDA-NRCS Idaho Snow Survey Office. http://www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/ Copyright:(c) 2014 Esri Figure 3: Percent of Median 115 Snowpack, March 1, 2020 ¯ 124 109 Basin-wide Snow Water Equivilant as a Percentage of the 1981 to 2010 Median

Above >= 150% PANHANDLE 106 REGION 130 - 149% 111 110 - 129% Median 90 - 109% 108 Snowpack 70 - 89% 109 112 50 -69%

110 Below 0 - 49% CLEARWATER No Data BASIN Provisional Data - Subject to Revision 130

SALMON 108 RIVER

85 BASINS WEST 82 103 115 UPPER SNAKE CENTRAL 94 95 RIVER BASIN BASINS 97 95 105 88 89 S 95 87 IN 64 S 106 A 102 B 69 T 111 81 56 S LO 77 & OD 108 109 WO 109 130

90 111 BEAR 106 115 RIVER 125 BASIN 112 106

SOUTHSIDE SNAKE RIVER BASINS

i r s E 4 1 0 2 ) c ( : t h g i r y p o C

/ w o n s / d i / n i a m / s c r n / l a t r o p / s p w / v o g . a d s u . s c r n . w w w / / : p t t h

. e c i f f O y e v r u S w o n S o h a d I S C R N - A D S U e h t y b d e r a p e r p s i p a m s i h T

s e l i M

)

0 0 2 0 5 1 0 0 1 0 5 5 2 0

S N I S A B R E V I R

E K A N S E D I S H T U O S

) ) " *# "

)

*#" *# ) "

N I S A B

)

* #

R E V I R

) "

R A E

B *#

)

*# *# "

* #

W *# * #

) " O

& )

" *# L

O *#

S # *# ) *# T " *

# ) * "

B *## *#

A *

)

" *# S

) "

)

" *# *# I ) "

*# S )

# " * ) " *# *#

*# *# *# *# *#

S N I S A

*# *# B

N I S A B R E V I R

L A R T N E C

E K A N S R E P P

U *#

T S E W

) *# " S N I S A B

R E V I

R *#

N O M L A

S *# n o i s i v e R o t t c e j b u S - a t a D l a n o i s i v o r P

a t a D o N (

) "

e g a r e v A

% 4 2 - 0

w o l e B

* #

N I S A B % 9 4 - 5 2 ) *# "

R E T A W R A E L C

% 9 6 - 0

5 *#

* # * # )

# " % 9 8 - 0

7 *

t s a c e r o F

#) *"

% 9 0 1 - 0 9

e g a r e v A

* # % 9 2 1 - 0 1 1

* * # #

% 9 4 1 - 0 3 1

* #

% 9 7 1 - 0 5 1 * #

e g a r e v A

% 0 8 1 >

N O I G E R

e v o b A

* #

E L D N A H N A P

e g a r e v A 0 1 0 2 o t 1 8 9 1 e h t f o e g a t n e c r e P

* #

a s a w o l F y l u J o t l i r p A d e t s a c e r o F ¯

0 2 0 2 , 1 h c r a M ) "

t s a c e r o F w o l f m a e r t S : 4 e r u g i F

) " ) "

Panhandle Region March 1, 2020

WATER SUPPLY OUTLOOK February precipitation in the Panhandle region hovered around normal with end-of-month totals ranging between ~90 to 105% for individual drainages. Water-year precipitation has maintained near normal levels after the abundant precipitation that was observed during January. February storms brought a fair amount of snow to the mountains in the region, leaving current snowpack totals between ~95 and 125% of normal. Reservoir storage in the Panhandle region ranges from ~50 to 125% of normal, with Lake Pend Oreille at 71%, Hungry Horse Lake at 124%, and Flathead Lake at 96% of normal. Mar. 1 streamflow forecasts are above normal across the Idaho Panhandle and range from 100 to 120% of average. While some of the reservoirs and lakes of the region are holding below normal water currently, it appears plentiful runoff is on the way this spring!

Panhandle Region Streamflow Forecasts - March 1, 2020

Forecast Exceedance Probabilities for Risk Assessment <--Drier------Projected Volume------Wetter--> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) Moyie R at Eastport APR-JUL 310 365 400 107% 435 490 375 APR-SEP 320 375 410 106% 445 500 385 Kootenai R at Leonia 1 & 2 APR-JUL 5090 6380 6970 106% 7560 8850 6600 APR-SEP 6050 7370 7970 105% 8570 9890 7590 Boundary Ck nr Porthill APR-JUL 101 116 126 108% 136 151 117 APR-SEP 105 121 131 107% 141 157 123 Clark Fork R bl Cabinet Gorge Dam 2 APR-JUL 9040 10500 11500 112% 12600 14000 10300 APR-SEP 9900 11500 12600 112% 13700 15400 11300 Pend Oreille Lake Inflow 2 APR-JUL 10300 12000 13100 111% 14300 16000 11800 APR-SEP 11300 13100 14300 112% 15600 17400 12800 Priest R nr Priest River 2 APR-JUL 670 795 880 113% 965 1090 780 APR-SEP 710 840 930 112% 1020 1150 830 NF Coeur dAlene R at Enaville APR-JUL 585 730 825 118% 925 1070 700 APR-SEP 620 765 865 117% 965 1110 740 St. Joe R at Calder 2 APR-JUL 900 1080 1200 114% 1320 1500 1050 APR-SEP 955 1140 1260 113% 1390 1570 1120 Spokane R nr Post Falls 2 APR-JUL 1980 2460 2790 117% 3110 3590 2390 APR-SEP 2050 2540 2870 116% 3200 3690 2480 Spokane R at Long Lake APR-JUL 2270 2760 3090 118% 3420 3910 2620 APR-SEP 2450 2950 3290 115% 3630 4130 2850 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 Reservoir Storage (KAF): End of February Watershed Snowpack Analysis: March 1, 2020 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2020 2019 Hungry Horse Lake 2743.4 2677.6 2209.0 3451.0 Moyie River 6 115% 91% Flathead Lake 776.7 761.1 812.8 1791.0 Priest River 4 124% 105% Noxon Rapids Reservoir 296.4 304.6 313.9 335.0 Rathdrum Creek 4 96% 86% Lake Pend Oreille 561.3 566.6 792.6 1561.3 Coeur d' Alene River 9 111% 93% Priest Lake 50.6 40.9 57.1 119.3 St. Joe River 5 108% 96% Lake Coeur d' Alene 64.8 38.6 132.8 238.5 Spokane River 17 106% 91% Palouse River 2 109% 106% Kootenai ab Bonners Ferry 23 109% 93%

Clearwater River Basin March 1, 2020

WATER SUPPLY OUTLOOK Storms during February left the Clearwater Basin’s end-of-month precipitation totals between 90% and 250% of normal. Water year precipitation is now between 75 and 130% of normal. A particularly strong and wet storm tracked across the Clearwater Basin Feb. 5-8, dropping 2 to 5+ inches of precipitation. Continuous storms passed through the Clearwater Mountains, leaving snowpack totals between ~100 and 140% of normal as of Mar. 1. Sites above 5,000 feet in the region received between 5 to 15 inches of SWE increase during the month. Dworshak Reservoir in the Clearwater Basin is currently at 96% of normal. Streamflow forecasts are between 105 to 120% of normal around the Clearwater Basin. Water supply should be adequate during spring and summer.

Clearwater River Basin Streamflow Forecasts - March 1, 2020

Forecast Exceedance Probabilities for Risk Assessment <--Drier------Projected Volume------Wetter--> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) Selway R nr Lowell APR-JUL 1810 2060 2230 116% 2400 2650 1920 APR-SEP 1890 2150 2330 115% 2510 2770 2020 Lochsa R nr Lowell APR-JUL 1210 1400 1520 108% 1640 1830 1410 APR-SEP 1270 1460 1590 107% 1720 1910 1480 Dworshak Reservoir Inflow 2 APR-JUL 2030 2380 2620 109% 2860 3210 2410 APR-SEP 2170 2530 2780 108% 3030 3390 2570 Clearwater R at Orofino APR-JUL 4020 4620 5030 117% 5440 6040 4310 APR-SEP 4220 4840 5270 116% 5700 6320 4540 Clearwater R at Spalding 2 APR-JUL 6200 7210 7890 115% 8580 9590 6890 APR-SEP 6540 7580 8290 114% 9000 10000 7270 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 Reservoir Storage (KAF): End of February Watershed Snowpack Analysis: March 1, 2020 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2020 2019 Dworshak Reservoir 2273.7 2293.0 2358.0 3468.0 NF Clearwater River 8 112% 100% Lochsa River 2 110% 100% Selway River 4 130% 109% Clearwater Basin Total 15 115% 102%

Salmon River Basin March 1, 2020

WATER SUPPLY OUTLOOK February precipitation was a tale of two sides. The eastern basin within the greater Salmon River watershed received well above normal precipitation while the west received ~50 to 100% of normal. This imbalance during February has not changed the overall picture. All sub-basins range from ~75 to 100% of normal for water-year-to-date precipitation, nearly the same as the end of February. The average for the entire Salmon River basin improved 6% from Feb 1st to 87% of normal. Mar. 1 snowpack totals for the Salmon range from ~70% to 140% of normal, while the basin as a whole is near normal.

There are no major reservoirs to report on in the Salmon River watershed. Streamflow forecasts for the Salmon River range from ~80% to 100% with the Middle Fork Lodge forecasts for April through July and April through September at 84% of normal. Boaters interested in peak streamflow information can start referencing our snow-stream relationship graphs as temperatures begin to rise and ripen and begin to melt the snowpack. Once we get into the melt season, these graphs will be updated biweekly.

Salmon River Streamflow Forecasts - March 1, 2020

Forecast Exceedance Probabilities for Risk Assessment <--Drier------Projected Volume------Wetter--> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) Salmon R at Salmon APR-JUL 350 500 605 78% 705 855 775 APR-SEP 420 590 700 78% 815 985 900 Lemhi R nr Lemhi MF Salmon R at MF Lodge APR-JUL 350 485 580 84% 670 805 690 APR-SEP 400 550 650 84% 745 895 770 SF Salmon R nr Krassel Ranger Station APR-JUL 126 176 210 78% 245 295 270 APR-SEP 138 191 225 78% 260 315 290 Johnson Ck at Yellow Pine APR-JUL 100 140 167 87% 193 235 191 APR-SEP 108 149 177 86% 205 245 205 Salmon R at White Bird APR-JUL 3830 4660 5230 97% 5800 6630 5370 APR-SEP 4250 5150 5770 97% 6380 7290 5940 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: March 1, 2020 # of % of Median Basin Name Sites 2020 2019 Salmon River ab Salmon 9 94% 117% Lemhi River 10 115% 109% MF Salmon River 3 82% 117% SF Salmon River 3 85% 114% Little Salmon River 4 108% 157% Salmon Basin Total 29 101% 119%

West Central Basins

March 1, 2020

WATER SUPPLY OUTLOOK February precipitation for all three West Central Basins was relatively low ranging from ~60 to 80% of normal with the Boise basin receiving the lowest. Water-year-to-date precipitation compared to normal has decreased slightly from Feb. 1, and now ranges from ~75 to 80%. Snowpack is just below normal for the West Central with a range of 90 to 95%. With a few exceptions across the West Central basins, measurements sites below 6,000 feet are reporting more snow water equivalent with respect to normal than are the sites above 6,000 feet. As the snowpack chart above illustrates, peak snowpack doesn’t typically occur until closer to April 1, and we still have quite a bit of new snow needed in order to reach the normal seasonal maximum snowpack.

The reservoir storage for the West Central basin is in good shape with the Boise system at 121% of normal, Payette system at roughly normal (99%), and Lake Lowell and Mann Creek at 108% and 81%, respectively. Streamflow forecasts for the region range from ~60%-95% for the runoff season with the lowest forecast for the South Fork of the Boise and the highest for the upper Payette. Shortages are unlikely in these basins due to a combination of above normal reservoir storage and normal to slightly below normal expected streamflow.

West Central Basins Streamflow Forecasts - March 1, 2020

Forecast Exceedance Probabilities for Risk Assessment <--Drier------Projected Volume------Wetter--> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) SF Boise R at Anderson Ranch Dam 2 APR-JUL 141 225 285 60% 340 425 475 APR-SEP 161 250 310 61% 370 455 510 Boise R nr Twin Springs APR-JUL 300 400 465 79% 535 635 585 APR-SEP 330 435 510 80% 580 685 635 Mores Ck nr Arrowrock Dam APR-JUL 51 82 104 90% 125 157 115 APR-SEP 53 86 108 91% 130 163 119 Boise R nr Boise 2 APR-JUL 585 810 970 77% 1120 1350 1260 APR-SEP 610 845 1010 74% 1170 1410 1360 Lake Fork Payette R nr McCall APR-JUL 53 66 75 94% 84 97 80 APR-SEP 55 68 77 93% 86 100 83 NF Payette R at Cascade 2 APR-JUL 270 365 425 88% 490 580 485 APR-SEP 275 370 435 88% 500 595 495 NF Payette R nr Banks 2 APR-JUL 320 450 540 86% 625 755 625 APR-SEP 330 465 555 87% 645 775 640 SF Payette R at Lowman APR-JUL 210 270 310 78% 355 415 400 APR-SEP 245 310 355 78% 400 465 455 Deadwood Reservoir Inflow 2 APR-JUL 57 78 92 75% 105 126 123 APR-SEP 62 84 99 76% 114 136 131 Payette R nr Horseshoe Bend 2 APR-JUL 745 1040 1230 83% 1430 1720 1480 APR-SEP 815 1120 1330 82% 1530 1840 1630 Weiser R nr Weiser MAR-JUL 250 355 435 82% 530 680 530 APR-JUL 168 240 300 81% 365 475 370 APR-SEP 187 265 325 81% 395 505 400 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 Reservoir Storage (KAF): End of February Watershed Snowpack Analysis: March 1, 2020 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2020 2019 Anderson Ranch Reservoir 313.9 272.0 247.0 450.2 SF Boise River 8 81% 138% Arrowrock Reservoir 243.3 188.2 185.9 272.2 MF & NF Boise Rivers 6 87% 128% Lucky Peak Reservoir 110.8 118.2 120.5 293.2 Mores Creek 4 102% 139% Sub-Basin Total 668.1 578.4 553.4 1015.6 Canyon Creek 4 93% 149% Deadwood Reservoir 90.8 88.6 88.9 161.9 Boise Basin Total 17 91% 138% Cascade Reservoir 449.5 455.8 457.6 693.2 NF Payette River 8 103% 144% Sub-Basin Total 540.4 544.4 546.5 855.1 SF Payette River 5 88% 130% Lake Lowell 105.1 95.6 97.7 165.2 Payette Basin Total 14 95% 135% Mann Creek Reservoir 4.2 2.6 5.2 11.1 Mann Creek 1 85% 134% Weiser Basin Total 8 95% 160%

Wood & Lost River Basin March 1, 2020

WATER SUPPLY OUTLOOK The Wood & Lost basins have the most concerning water outlook in Idaho. Once again in February, monthly precipitation was abysmal and ranged from 20 to 40% of normal. So far this water-year (Oct. 1 to current), January was the only month with near normal precipitation totals. Water-year-to-date precipitation totals decreased and now range from 50 to 60% of normal. As can be expected with below normal precipitation totals, current snowpack in the Wood & Lost basins generally range from 50 to 70% of normal. The Little Lost, however, is an exception and is currently holding a near normal snowpack (~90%). More than anywhere else in Idaho, this region would benefit from a wetter than normal spring. The good news for water users in this area is that the major reservoirs (Mackay, Little Wood, and Magic) are all holding above normal storage at 134, 145, and 186% of average, respectively. On the flip side, the bad news is that the much below normal water input (precipitation) this winter is driving streamflow forecasts to be much below normal for the primary runoff periods (20 to 60%). More abundant precipitation will be needed in order to avoid water shortages.

Wood and Lost Basins Streamflow Forecasts - March 1, 2020

Forecast Exceedance Probabilities for Risk Assessment <--Drier------Projected Volume------Wetter--> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) Camas Ck at Camas APR-JUL 5.4 11.6 17.2 61% 24 35 28 Little Lost R bl Wet Ck nr Howe Big Lost R at Howell Ranch APR-JUL 34 71 96 60% 120 157 159 APR-SEP 39 80 109 61% 137 179 180 Big Lost R bl Mackay Reservoir APR-JUL -1.38 36 61 50% 86 123 123 APR-SEP 9.9 53 82 55% 111 153 150 Little Wood R ab High Five Ck MAR-JUL 12.2 23 32 42% 42 61 77 MAR-SEP 13.4 25 35 43% 46 66 82 Little Wood R nr Carey 2 MAR-JUL 11.6 23 33 38% 45 65 86 MAR-SEP 12.6 25 36 39% 48 70 92 Big Wood R at Hailey APR-JUL 19.2 75 114 49% 152 210 235 APR-SEP 25 88 130 49% 173 235 265 Big Wood R ab Magic Reservoir APR-JUL 3.5 19.6 38 22% 62 109 170 APR-SEP 4.8 23 43 24% 69 119 182 Camas Ck nr Blaine APR-JUL 1.52 8.8 17.2 21% 28 50 82 APR-SEP 1.63 9 17.5 21% 29 50 83 Big Wood R bl Magic Dam 2 APR-JUL 7.4 32 58 23% 92 156 250 APR-SEP 9.8 37 65 25% 101 168 265 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 Reservoir Storage (KAF): End of February Watershed Snowpack Analysis: March 1, 2020 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2020 2019 Mackay Reservoir 39.4 35.4 29.3 44.4 Camas-Beaver Creeks 4 97% 139% Little Wood Reservoir 25.3 17.9 17.4 30.0 Birch-Medicine Lodge Creeks 4 105% 117% Magic Reservoir 135.1 88.6 72.5 191.5 Little Lost River 4 89% 126% Big Lost River ab Mackay 6 61% 147% Big Lost Basin Total 7 64% 145% Fish Creek 3 70% 175% Little Wood River 4 56% 160% Big Wood River ab Hailey 7 69% 129% Camas Creek 5 77% 168% Big Wood Basin Total 12 72% 142%

Upper Snake River Basin March 1, 2020

WATER SUPPLY OUTLOOK February precipitation for the Upper Snake ranged from ~70 to 160% of normal. The larger influxes of precipitation likely came from a combination of cold temperatures and storm tracks. Water-year-to- date precipitation now ranges from ~80 to 110%. Snowpack in the region as of Mar. 1 ranged from ~90 to 130%. With equal chances of above or below precipitation and temperatures for March’s climate outlook only time will tell how winter ends, but as of now conditions are looking fairly normal across the Upper Snake.

Reservoir storage for the Upper Snake continues to shine. As of Mar. 1, the entire region’s combined storage is 89% of capacity which is 133% of normal. The Jackson-Palisades system is ~145% of normal with Palisades nearly full, but that’s unlikely to last due to the need to make room for the upcoming snowmelt runoff. Current forecasts have the Upper Snake runoff season ranging from ~90% to 105% except for the Henry’s Fork which ranges from ~85 to 90%. Notably, the critical Snake River at Heise forecast is 99% of normal for the April-July and April-September forecast periods.

Upper Snake River Basin Streamflow Forecasts - March 1, 2020

Forecast Exceedance Probabilities for Risk Assessment <--Drier------Projected Volume------Wetter--> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) Henrys Fk nr Ashton 2 APR-JUL 325 395 440 83% 485 550 530 APR-SEP 475 550 600 85% 655 730 710 Falls R nr Ashton 2 APR-JUL 270 315 345 95% 375 420 365 APR-SEP 325 385 420 97% 460 515 435 Teton R nr Driggs APR-JUL 112 141 162 105% 182 210 154 APR-SEP 138 176 200 104% 225 265 193 Teton R nr St Anthony APR-JUL 275 340 385 105% 425 490 365 APR-SEP 325 400 450 103% 500 580 435 Henrys Fk nr Rexburg 2 APR-JUL 925 1110 1240 89% 1370 1560 1400 APR-SEP 1190 1420 1580 88% 1750 1980 1790 Snake R at Flagg Ranch APR-JUL 300 370 420 90% 470 540 465 APR-SEP 330 405 460 90% 510 585 510 Snake R nr Moran 2 APR-JUL 465 570 640 84% 715 820 765 APR-SEP 515 630 710 84% 790 905 845 Pacific Ck at Moran APR-JUL 106 135 155 95% 175 205 164 APR-SEP 113 143 164 95% 184 215 173 Buffalo Fk ab Lava Ck nr Moran APR-JUL 220 255 280 100% 305 340 280 APR-SEP 245 290 320 100% 350 390 320 Snake R ab Reservoir nr Alpine 2 APR-JUL 1650 1900 2080 96% 2250 2500 2170 APR-SEP 1880 2180 2380 95% 2580 2880 2500 Greys R ab Reservoir nr Alpine APR-JUL 275 315 340 111% 370 405 305 APR-SEP 315 365 395 110% 425 475 360 Salt R ab Reservoir nr Etna APR-JUL 230 295 345 115% 390 460 300 APR-SEP 280 365 420 114% 475 555 370 Snake R nr Irwin 2 APR-JUL 2370 2740 2990 99% 3250 3620 3010 APR-SEP 2720 3170 3470 99% 3770 4210 3500 Snake R nr Heise 2 APR-JUL 2550 2940 3210 99% 3480 3880 3240 APR-SEP 2960 3430 3750 99% 4070 4540 3780 Willow Ck nr Ririe 2 MAR-JUL 16.7 30 41 61% 54 76 67 Portneuf R at Topaz MAR-JUL 32 42 50 66% 59 73 76 MAR-SEP 40 53 63 68% 73 91 93 Snake R at Neeley 2 APR-JUL 970 1590 2110 80% 2690 3680 2650 APR-SEP 965 1620 2160 77% 2790 3840 2810 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 Reservoir Storage (KAF): End of February Watershed Snowpack Analysis: March 1, 2020 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2020 2019 Jackson Lake 625.3 672.4 434.7 847.0 Henrys Fork-Falls River 9 95% 127% Palisades Reservoir 1339.6 1213.7 925.7 1400.0 Teton River 8 106% 123% Sub-Basin Total 1964.9 1886.0 1360.4 2247.0 Henrys Fork ab Rexburg 17 100% 125% Henrys Lake 87.4 86.1 80.6 90.4 Snake River ab Jackson Lake 8 95% 120% Island Park Reservoir 118.1 118.2 104.7 135.2 Pacific Creek 3 103% 123% Grassy Lake 13.0 13.1 12.1 15.2 Buffalo Fork 1 126% 119% Sub-Basin Total 218.5 217.4 197.4 240.8 Gros Ventre River 4 111% 113% Ririe Reservoir 51.4 50.1 41.2 80.5 Hoback River 5 109% 112% Blackfoot Reservoir 300.1 271.0 181.3 337.0 Greys River 4 125% 108% American Falls Reservoir 1552.6 1558.8 1296.0 1672.6 Salt River 3 130% 108% Basin-Wide Total 4087.5 3983.3 3076.3 4577.9 Snake ab Palisades Resv 22 107% 117% Willow Creek - Ririe 7 108% 142% Blackfoot River 5 109% 132% Portneuf River 7 90% 140% Snake River ab American Falls 43 103% 126%

Southside Snake River Basins March 1, 2020

WATER SUPPLY OUTLOOK Two storms, one on Feb. 5 and the other on Feb. 16, brought most of the precipitation for February. These two storms produced the most snow in the south-central portion of the Southside Snake River Basin. Although February was drier than normal, water year to date precipitation for the Southside Snake is still near normal for all basins. Likewise, snowpack conditions are near, or above normal and range from 105 to 125%. The second half of the month was filled with warm, sunny days and consistently clear, cool nights. These clear nights minimized any warming of the snowpack and there was no significant melt across the Southside Snake sub-basins. Current sub-basin snowpacks with respect to normal are Goose-Trapper Creeks at 125%, Salmon Falls at 112%, Bruneau River at 106%, and Owyhee at 111%. Current reservoir storage expressed as a percent of average is the following for area reservoirs: Oakley 137%, Salmon Falls 177%, Wild Horse 175%, Lake Owyhee 137%. Streamflow forecasts range from 100 to 125% of normal, except for Owyhee R forecasts which are closer to 70%. An above-normal current snowpack coupled with plentiful reservoir storage bodes well for the many users in these basins.

Southside Snake River Basins Streamflow Forecasts - March 1, 2020

Forecast Exceedance Probabilities for Risk Assessment <--Drier------Projected Volume------Wetter--> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) Goose Ck ab Trapper Ck nr Oakley MAR-JUL 17.4 23 28 127% 32 40 22 MAR-SEP 18.2 24 29 121% 34 42 24 Trapper Ck nr Oakley MAR-JUL 4.9 5.8 6.4 108% 7.1 8.1 5.9 MAR-SEP 6 7 7.7 108% 8.4 9.6 7.1 Oakley Reservoir Inflow MAR-JUL 20 27 32 114% 37 46 28 MAR-SEP 22 29 34 110% 40 49 31 Salmon Falls Ck nr San Jacinto MAR-JUL 46 63 76 94% 90 113 81 MAR-SEP 49 66 80 94% 94 117 85 Bruneau R nr Hot Spring MAR-JUL 127 170 200 98% 240 295 205 MAR-SEP 132 177 210 98% 250 305 215 Reynolds Ck at Tollgate MAR-JUL 2.8 4.2 5.3 59% 6.5 8.5 9 MAR-SEP 2.7 4 5.1 56% 6.3 8.3 9.1 Owyhee R nr Gold Ck 2 MAR-JUL 11.8 19 25 89% 32 43 28 APR-JUL 5.3 12 18 82% 25 38 22 Owyhee R nr Rome MAR-JUL 170 270 355 69% 450 605 515 MAR-SEP 182 285 370 70% 465 625 530 APR-JUL 72 155 230 67% 315 475 345 Owyhee R bl Owyhee Dam 2 MAR-JUL 184 300 390 70% 500 680 555 MAR-SEP 210 325 420 72% 525 700 585 APR-JUL 91 180 255 68% 350 510 375 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 Reservoir Storage (KAF): End of February Watershed Snowpack Analysis: March 1, 2020 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2020 2019 Oakley Reservoir 34.7 24.1 25.3 75.6 Raft River 6 115% 111% Salmon Falls Reservoir 83.3 41.3 47.1 182.6 Goose-Trapper Creeks 6 125% 115% Wild Horse Reservoir 60.4 49.6 34.5 71.5 Salmon Falls Creek 8 112% 127% Lake Owyhee 538.7 305.4 392.6 715.0 Bruneau River 8 106% 127% Brownlee Reservoir 930.2 1129.0 1420.0 Reynolds Creek 7 95% 119% Owyhee Basin Total 19 111% 138% Owyhee Basin Snotel Total 8 105% 129%

Bear River Basin March 1, 2020

WATER SUPPLY OUTLOOK The Bear River Basin received near-normal precipitation during February, and water-year precipitation ranges between 74 and 105% of normal. Despite dry conditions in the second half of February, the Bear River Basin snowpack is around normal (~105 - 110%) for Mar. 1. Precipitation in February came from just a couple intermittent storms which were separated by warm and dry periods. Although there wasn’t consistent precipitation, these storms have kept the basin snowpack within the 50th percentile of historic data, indicated by the proximity of the black and red lines in the above snowpack chart. The following sub-basins snowpack with respect to normal are: Malad River 94%, Cub River 100%, Mink Creek 103%, Montpelier Creek 118%. Bear Lake is 70% full and 153% of average. Streamflow forecasts are near normal for the primary runoff period and range from ~85 to 110% of normal. The water outlook is promising for the many water users and recreationalists throughout the Bear River basin.

Bear River Basin Streamflow Forecasts - March 1, 2020

Forecast Exceedance Probabilities for Risk Assessment <--Drier------Projected Volume------Wetter--> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) Bear R nr UT-WY State Line APR-JUL 68 90 104 93% 119 140 112 APR-SEP 75 99 115 93% 132 156 123 Bear R ab Resv nr Woodruff APR-JUL 27 75 107 88% 139 187 121 APR-SEP 25 77 113 88% 148 200 128 Big Ck nr Randolph APR-JUL 0.41 2.5 3.9 103% 5.3 7.4 3.8 Smiths Fk nr Border APR-JUL 69 86 97 109% 108 125 89 APR-SEP 81 100 113 109% 126 145 104 Bear R bl Stewart Dam 2 MAR-JUL 65 134 176 86% 225 295 205 MAR-SEP 64 142 195 85% 250 325 230 APR-JUL 37 105 152 83% 199 265 183 Little Bear at Paradise APR-JUL 15.5 29 38 84% 47 61 45 Logan R nr Logan APR-JUL 80 97 109 98% 121 138 111 Blacksmith Fk nr Hyrum APR-JUL 22 34 43 100% 52 64 43 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 Reservoir Storage (KAF): End of February Watershed Snowpack Analysis: March 1, 2020 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2020 2019 Bear Lake 908.2 833.8 594.1 1302.0 Smiths-Thomas Forks 4 114% 100% Montpelier Reservoir 1.8 1.8 4.0 Bear River ab WY-ID Line 10 110% 112% Montpelier Creek 2 118% 101% Mink Creek 4 103% 119% Cub River 3 100% 118% Bear River ab ID-UT Line 23 106% 115% Malad River 3 94% 140% 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 Boise R nr Boise (2) Moyie R at Eastport – no corrections + Anderson Ranch Res storage change Boundary Ck nr Porthill – no corrections + Arrowrock Res storage change Clark Fork R bl Cabinet Gorge (2) + Lucky Peak Res storage change + Hungry Horse storage change SF Payette R at Lowman - no corrections + Flathead Lake storage change Deadwood Res Inflow (2) + Noxon Res storage change + Deadwood R bl Deadwood Res nr Lowman Whitehorse Rapid gage used create longer term record + Deadwood Res storage change Pend Oreille Lake Inflow (2) Lake Fork Payette R nr McCall – no corrections + Pend Oreille R at Newport, WA NF Payette R at Cascade (2) + Hungry Horse Res storage change + Payette Lake storage change + Flathead Lake storage change + Cascade Res storage change + Noxon Res storage change NF Payette R nr Banks (2) + Lake Pend Oreille storage change + Payette Lake storage change + Priest Lake storage change + Cascade Res storage change Priest R nr Priest R (2) Payette R nr Horseshoe Bend (2) + Priest Lake storage change + Deadwood Res storage change NF Coeur d' Alene R at Enaville - no corrections + Payette Lake storage change St. Joe R at Calder- no corrections + Cascade Res storage change Spokane R nr Post Falls (2) Weiser R nr Weiser - no corrections + Lake Coeur d' Alene storage change Spokane R at Long Lake, WA (2) Wood and Lost Basins + Lake Coeur d' Alene storage change Little Lost R bl Wet Ck nr Howe - no corrections + Long Lake, WA storage change Big Lost R at Howell Ranch - no corrections Big Lost R bl Mackay Res nr Mackay (2) Clearwater River Basin + Mackay Res storage change Selway R nr Lowell - no corrections Little Wood R ab High Five Ck – no corrections Lochsa R nr Lowell - no corrections Little Wood R nr Carey (2) Dworshak Res Inflow (2) + Little Wood Res storage change + Clearwater R nr Peck Big Wood R at Hailey - no corrections - Clearwater R at Orofino Big Wood R ab Magic Res (2) + Dworshak Res storage change + Big Wood R nr Bellevue (1912-1996) Clearwater R at Orofino - no corrections + Big Wood R at Stanton Crossing nr Bellevue (1997 to present) Clearwater R at Spalding (2) + Willow Ck (1997 to present) + Dworshak Res storage change Camas Ck nr Blaine – no corrections Magic Res Inflow (2) Salmon River Basin + Big Wood R bl Magic Dam Salmon R at Salmon - no corrections + Magic Res storage change Lemhi R nr Lemhi – no corrections Upper Snake River Basin MF Salmon R at MF Lodge – no corrections Falls R nr Ashton (2) SF Salmon gage used to create longer term record + Grassy Lake storage change SF Salmon R nr Krassel Ranger Station – no corrections + Diversions from Falls R ab nr Ashton Johnson Creek at Yellow pine – no corrections Henrys Fork nr Ashton (2) Salmon R at White Bird - no corrections + Henrys Lake storage change + Island Park Res storage change West Central Basins Teton R nr Driggs - no corrections Boise R nr Twin Springs - no corrections Teton R nr St. Anthony (2) SF Boise R at Anderson Ranch Dam (2) - Cross Cut Canal into Teton R + Anderson Ranch Res storage change + Sum of Diversions for Teton R ab St. Anthony Mores Ck nr Arrowrock Dam – no corrections + Teton Dam for water year 1976 only + Owyhee R bl Owyhee Dam, OR Henrys Fork nr Rexburg (2) + Lake Owyhee storage change + Henrys Lake storage change + Diversions to North and South Canals + Island Park Res storage change Bear River Basin + Grassy Lake storage change Bear R nr UT-WY Stateline, UT- no corrections + 3 Diversions from Falls R ab Ashton-Chester Bear R abv Res nr Woodruff, UT- no corrections + 6 Diversions from Falls R abv Ashton Big Ck nr Randolph, UT - no corrections + 7 Diversions from Henrys Fk btw Ashton to St. Anthony Smiths Fork nr Border, WY - no corrections + 21 Diversions from Henrys Fk btw St. Anthony to Rexburg Bear R bl Stewart Dam (2) Snake R nr Flagg Ranch, WY – no corrections + Bear R bl Stewart Dam Snake R nr Moran, WY (2) + Rainbow Inlet Canal + Jackson Lake storage change Little Bear R at Paradise, UT - no corrections Pacific Ck at Moran, WY - no corrections Logan R nr Logan, UT - no corrections Buffalo Fork ab Lava nr Moran, WY - no corrections Blacksmith Fk nr Hyrum, UT - no corrections Snake R ab Res nr Alpine, WY (2) + Jackson Lake storage change Reservoir Capacity Definitions (Units in 1,000 Acre-Feet, KAF) Greys R nr Alpine, WY - no corrections Different agencies use various definitions when reporting reservoir capacity and contents. Reservoir storage Salt R nr Etna, WY - no corrections 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 Palisades Res Inflow (2) cases, NRCS reports usable storage which includes active and/or inactive storage. (Revised Feb. 2015) + Snake R nr Irwin Basin- Lake or Dead Inactive Active Surcharge NRCS NRCS Capacity + Jackson Lake storage change Reservoir Storage Storage Storage Storage Capacity Includes + Palisades Res storage change Panhandle Region Snake R nr Heise (2) Hungry Horse 39.73 --- 3451.00 --- 3451.0 Active + Jackson Lake storage change Flathead Lake Unknown --- 1791.00 --- 1791.0 Active + Palisades Res storage change Noxon Unknown --- 335.00 --- 335.0 Active Ririe Res Inflow (2) 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 + Willow Ck nr Ririe Priest Lake 20.00 28.00 71.30 --- 119.3 Dead + Inactive + Active + Ririe Res storage change Clearwater Basin The forecasted natural volume for Willow Creek nr Ririe does not include Dworshak Unknown 1452.00 2016.00 --- 3468.0 Inactive + Active Grays Lake water diverted from Willow Creek drainage through the Clarks West Central Basins Cut diversion and into Blackfoot Reservoir. Anderson Ranch 24.90 37.00 413.10 --- 450.1 Inactive + Active Blackfoot R ab Res nr Henry (2) Arrowrock Unknown --- 272.20 --- 272.2 Active + Blackfoot Res storage change Lucky Peak Unknown 28.80 264.40 13.80 293.2 Inactive + Active The forecasted Blackfoot Reservoir Inflow includes Grays Lake water Lake Lowell 7.90 5.80 159.40 --- 165.2 Inactive + Active Deadwood Unknown --- 161.90 --- 161.9 Active diverted from the Willow Creek drainage through the Clarks Cut diversion Cascade Unknown 46.70 646.50 --- 693.2 Inactive + Active and into Blackfoot Reservoir. Mann Creek 1.61 0.24 11.10 --- 11.1 Active Portneuf R at Topaz - no corrections Wood and Lost Basins American Falls Res Inflow (2) Mackay 0.13 --- 44.37 --- 44.4 Active + Snake R at Neeley Little Wood Unknown --- 30.00 --- 30.0 Active + Jackson Lake storage change Magic Unknown --- 191.50 --- 191.5 Active + Palisades Res storage change Upper Snake Basin + American Falls storage change Jackson Lake Unknown --- 847.00 --- 847.0 Active Palisades 44.10 155.50 1200.00 --- 1400.0 Dead +Inactive +Active + Teton Dam for water year 1976 only Henrys Lake Unknown --- 90.40 --- 90.4 Active Southside Snake River Basins Island Park 0.40 --- 127.30 7.90 135.2 Active + Surcharge Goose Ck nr Oakley - no adjustments Grassy Lake Unknown --- 15.18 --- 15.2 Active Trapper Ck nr Oakley - no adjustments Ririe 4.00 6.00 80.54 10.00 80.5 Active Oakley Res Inflow - flow does not include Birch Creek Blackfoot 0.00 --- 333.50 3.50 333.50 Active (rev. 2/1/2015) + Goose Ck American Falls Unknown --- 1672.60 --- 1672.6 Active + Trapper Ck Southside Snake Basins Salmon Falls Ck nr San Jacinto, NV - no corrections Oakley 0.00 --- 75.60 --- 75.6 Active Salmon Falls 48.00 5.00 182.65 --- 182.6 Active Bruneau R nr Hot Springs - no corrections Wild Horse Unknown --- 71.50 --- 71.5 Active Reynolds Ck at Tollgate - no corrections Lake Owyhee 406.83 --- 715.00 --- 715.0 Active Owyhee R nr Gold Ck, NV (2) Brownlee 0.45 444.70 975.30 --- 1420.0 Inactive + Active + Wildhorse Res storage change Bear River Basin Owyhee R nr Rome, OR – no Corrections Bear Lake 5000.00 119.00 1302.00 --- 1302.0 Active: Owyhee Res Inflow (2) 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 30-Year Average. The 30-year average streamflow for each forecast forecast period. Unless otherwise specified, all streamflow forecasts are period is provided for comparison. The average is based on data from for streamflow volumes that would occur naturally without any upstream 1981-2010. The % AVG. column compares the 50% chance of influences. Water users need to know what the different forecasts exceedance forecast to the 30-year average streamflow; values above represent if they are to use the information correctly when making 100% denote when the 50% chance of exceedance forecast would be operational decisions. The following is an explanation of each of the greater than the 30-year average streamflow. forecasts. AF - Acre-feet, forecasted volume of water are typically in thousands of 90 Percent Chance of Exceedance Forecast. There is a 90 percent acre-feet (KAF). 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 These forecasts are given to users to help make risk-based decisions. less than this forecast value. 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 70 Percent Chance of Exceedance Forecast. There is a 70 percent or less water than planned for. 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 To Decrease the Chance of Having Less Water than Planned for less than this forecast value. 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 Percent Chance of Exceedance Forecast. There is a 50 percent 50% chance that the user will receive less than this amount). To reduce chance that the actual streamflow volume will exceed this forecast value, the risk of having less water than planned for, users can base their and there is a 50 percent chance that the actual streamflow volume will be operational decisions on one of the forecasts with a greater chance of less than this forecast value. Generally, this forecast is the middle of the being exceeded such as the 90 or 70 percent exceedance forecasts. range of possible streamflow volumes that can be produced given current conditions. To Decrease the Chance of Having More Water than Planned for A user might determine that making decisions based on a 50 percent 30 Percent Chance of Exceedance Forecast. There is a 30 percent chance of exceedance forecast is too much risk to take (there is still a chance that the actual streamflow volume will exceed this forecast value, 50% chance that the user will receive more than this amount). To reduce and there is a 70 percent chance that the actual streamflow volume will be the risk of having more water than planned for, users can base their less than this forecast value. operational decisions on one of the forecasts with a lesser chance of being exceeded such as the 30 or 10 percent exceedance forecasts. 10 Percent Chance of Exceedance Forecast. There is a 10 percent chance that the actual streamflow volume will exceed this forecast value, Forecast use example: and there is a 90 percent chance that the actual streamflow volume will be less than this forecast value. Using the 50 Percent Exceedance Forecast. Using the example forecasts shown on the next page, there is a 50% chance that actual *Note: There is still a 20 percent chance that actual streamflow streamflow volume at the Henry’s Fork near Ashton will be less than 280 volumes will fall either below the 90 percent exceedance forecast or KAF between June 1 and Sept. 30. There is also a 50% chance that above the 10 percent exceedance forecast. actual streamflow volume will be greater than 280 KAF.

These forecasts represent the uncertainty inherent in making streamflow Using the 90 and 70 Percent Exceedance Forecasts. If an unexpected predictions. This uncertainty may include sources such as: unknown future shortage of water could cause problems (such as irrigated agriculture), weather conditions, uncertainties associated with the various prediction users might want to plan on receiving 245 KAF during Jun 1 through methodologies, and the spatial coverage of the data network in a given September 30 (from the 70 percent exceedance forecast). There is a 30% basin. chance of receiving less than 245 KAF.

Alternatively, if users determine the risk of using the 70 percent Sept. 30 (from the 30 percent exceedance forecast). There is a 30% exceedance forecast is too great, then they might plan on receiving 198 chance of receiving more than 315 KAF. KAF (from the 90 percent exceedance forecast). There is 10% chance of receiving less than 72 KAF. Alternatively, if users determine the risk of using the 30 percent exceedance forecast is too great, then they might plan on receiving 360 Using the 30 or 10 Percent Exceedance Forecasts. If an unexpected KAF (from the 10 percent exceedance forecast). There is a 10% chance of excess of water could cause problems (such as operating a flood control receiving more than 360 KAF. Users could also choose a volume in reservoir), users might plan on receiving 315 KAF between June 1 and between any of these values to reflect their desired risk level.

Upper Snake River Basin Streamflow Forecasts - June 1, 2015

Forecast Exceedance Probabilities for Risk Assessment <---Drier------Projected Volume------Wetter---> Forecast 90% 70% 50% 30% 10% 30yr Avg Forecast Point Period (KAF) (KAF) (KAF) % Avg (KAF) (KAF) (KAF) Henrys Fk nr Ashton JUN-JUL 72 106 129 56 152 186 230 JUN-SEP 198 245 280 68 315 360 410

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. 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: [email protected]

Forecasts provided by Forecast Hydrologist Staff NRCS, National Water and Climate Center, Portland, Oregon Email: [email protected]

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!