Natural Resources Conservation Service Idaho Water Supply Outlook Report May 1, 2021

Little Hole in the Wall Creek along the South Fork of the Payette River. Photo by Danny Tappa, May 2, 2021.

April was extremely dry across Idaho, and many sites recorded period-of-record low for the month. Warm and dry conditions have led to faster than normal snowpack melt, and earlier than normal complete melt out appears likely in many basins across the state. With short and long- term weather forecasts not indicating above normal precipitation on the way, Idaho appears in line for continued fast snowmelt, early peak streamflow, and an earlier than normal return to low streamflow (baseflow). IDAHO WATER SUPPLY OUTLOOK REPORT May 1, 2021

Overview When we look back at the water year 2021 (WY21) snowpack, there likely will be a collective shrug acknowledging that it wasn't terrible, but it wasn't good either. Water Supply Outlook Report readers know, however, winter snowpack condition is not the whole water supply story. The dry, depleted soil moisture conditions from last year and this fall, in combination with the dry spring, is leading to earlier and higher than normal irrigation demand. According to the U.S. Drought Monitor, ~47% of Idaho land is under drought conditions. Drought conditions are expected to persist through the summer. Agricultural irrigation water shortages may occur in several basins. The one month outlook from NOAA’s Prediction Center (CPC) forecasts slightly warmer and drier conditions in May for central and southern Idaho. In the Snake River headwaters, warmer than average are expected this month, but there isn’t a clear precipitation signal. Across the rest of the state, there are equal chances of above, below or normal conditions (white color). Above normal precipitation is forecasted in Boundary and Bonner counties in May. The three-month outlook predicts warmer and drier conditions than normal throughout Idaho and the Snake River headwaters.

Snowpack This spring has continued to be exceptionally dry with well below normal precipitation across Idaho. A few storms during April brought minimal snowpack (SWE) increases to the highest elevation sites, but overall, this frozen water resource is transitioning to its liquid form. Throughout Idaho, the snowpack is melting earlier and more quickly compared to the historical average. Peak snowpack occurred up to 26 days earlier than normal at some sites.

The WY21 snowpack was below average, but Idaho didn't set any new low snowpack records. At this point in the snowmelt season, comparing stations rather than basins to each other provides a better apple-to-apple comparison. Water users can investigate how peak snowpack fared in their backyard compared to the historical record with the Interactive Map percentile setting. The percentile feature ranks how good, bad, or average this year’s snowpack is compared to the period of record. The majority of SNOTEL stations south of the Clearwater-Salmon basin divide were below the 50th percentile.

Precipitation April was a very dry month across Idaho with basins only experiencing ~25% to 65% of normal monthly total precipitation (Fig. 1). The majority of SNOTEL sites indicate April precipitation was below the 30th percentile; this pattern was consistent across the entire state with record low precipitation records set. Water year precipitation (Fig. 2) continues to follow the same below normal pattern observed all year: the wettest basins are north of the Salmon-Clearwater divide and the Upper Snake headwaters (81% to 93%), the driest are the Wood and Lost basins (61% to 69%), and the rest of the state are below normal (68% to 80%).

Water supply In general, the dry weather has caused a significant decline in forecasted streamflow volumes compared to last month’s forecasts. There is good agreement in this trend between the NRCS and Northwest River Forecast Center. NRCS generates streamflow forecasts for a range of exceedance volumes in order to provide water users with information specific to their needs. Based on dry soil conditions and lack of spring precipitation, we recommend water users concerned about water shortages use the 70% exceedance probability forecasts (Fig. 5).

The National Water and Climate Center 50% exceedance streamflow forecasts predict streamflow will be near normal in the Clearwater and Panhandle basins (Fig. 4). Basins with 50% to 70% of normal streamflow volume are: Salmon, Weiser, Payette, Boise, Little Lost, Henrys Fork-Teton, Willow-Blackfoot-Portneuf, and Snake Basin above Palisades. Basins with well below normal forecasts for natural streamflow include: Big Wood (16%), Little Wood (31%), Big Lost (34%), Birch-Medicine Lodge-Bear-Camas (9%), Bear River (36%), Goose Creek (45%), Salmon Falls (39%), Bruneau (47%), and Owyhee (24%).

Please see the SWSI table in this report for a prediction of where water supplies could be short. The SWSI index combines the natural streamflow forecasts described above with current reservoir storage and compares it to historical water use in various basins. We expect continued water supply shortages in the Big Wood, Little Wood, Big Lost, Little Lost basins. Diminished agricultural irrigation water supply is also predicted for the Snake River (above Heise), Oakley, Salmon Falls, and the Owyhee basins. Dry conditions across Idaho will continue to impact ranchers who utilize non-irrigated pastures.

For insight into how current weather conditions will influence the timing of peak streamflow, please consult the Northwest River Forecast Center’s website. Streamflow, snowpack, and precipitation data for each basin can be accessed here or on the NRCS interactive map here. For questions about current conditions and water supply impacts, please contact: [email protected], (office) 208-685-6983 or (cell) 208-510-7294. 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

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How forecasts are made

Most of the annual streamflow in the 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 SURFACE WATER SUPPLY INDEX (SWSI) May 1, 2021

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 -0.6 2006 NA Clearwater -2.2 2005 NA Salmon -3.0 2015 NA Weiser -2.4 2013 NA

Payette -1.6 2004 NA

Boise -2.7 2013 - 2.8 Big Wood above Hailey -3.8 1992 - 2.8 Big Wood -3.8 1992 0. 4 Little Wood -4.0 1992 - 1.7

Big Lost -3.9 2004 -0.1

Little Lost -1.9 2015 1. 5

Teton -2.4 2013 - 3.9 Henrys Fork -0.5 2004 - 3.1 Snake (Heise) -1.6 2016 - 1.5 Oakley -1. 7 2001 0.4 Salmon Falls above Jackpot -2.2 2014 NA Salmon Falls -1. 4 2008 - 0.9 Bruneau -2.2 2013 NA Owyhee -2.4 1994 - 1.2 Bear River 1.3 2020 - 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.

Panhandle Basins May 1, 2021

WATER SUPPLY OUTLOOK The month of April was a continuation of abnormally dry conditions during March, with monthly precipitation ~30% to 75% of normal across the Panhandle sub basins (Fig. 1). Record low precipitation from March through April were observed in many cities in and near the region. As of May 1, total water year precipitation within the Panhandle sub basins is ~75% to 95% of normal (Fig. 2). Snowpack ranges from ~35% to 95% of normal (Fig. 3). During April, snowpack has decreased more quickly than normal, with some sites in the Pend Oreille Lake sub basin nearing record decreases for the duration of April. Complete snowmelt has occurred at a SNOTEL station near Mt. Spokane State Park (~4,700 feet elevation); on April 1 this SNOTEL station was 112% of normal and is 0% of normal as of May 1. The snowpack is on track to completely melt out about 10 days earlier than normal in the Kootenai- Pend Oreille Basin. In the Coeur d’Alene-St. Joe basins, snowpack melt timing appears nearly normal at this time with all melt finished around June 29. Storage among the region’s reservoirs as a percent of normal are: Coeur d’Alene at 68% (65% capacity), Pend Oreille at 95% (57% capacity), and Priest Lake at 79% (68% capacity). May through July 50% exceedance streamflow forecasts range from 70% to 106% of normal, and 60% to 99% of normal for the 70% exceedance forecast (Fig. 4, 5). Based on analogous years with similar low soil moisture content, lack of spring precipitation, and comparing past observed to forecasted streamflow conditions, hedging towards the 70% exceedance forecast may be a better predictor of natural streamflow conditions. Equal chances of near, above, and below normal precipitation and temperature are predicted in May according to the NOAA Climate Prediction Center’s 30- day forecast. Panhandle Region Streamflow Forecasts - May 1, 2021 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 MAY-JUL 225 270 305 102% 335 380 300 MAY-SEP 230 280 315 100% 350 400 315 Kootenai R at Leonia 1 & 2 MAY-JUL 4890 5690 6060 106% 6420 7230 5730 MAY-SEP 5790 6660 7060 105% 7450 8320 6730 Boundary Ck nr Porthill MAY-JUL 59 70 77 80% 84 95 96 MAY-SEP 62 73 81 80% 89 100 101 Clark Fork R bl Cabinet Gorge Dam 2 MAY-JUL 6520 7260 7760 90% 8260 9000 8630 MAY-SEP 7090 7950 8540 89% 9130 9990 9580 Pend Oreille Lake Inflow 2 MAY-JUL 7070 7920 8500 88% 9080 9930 9690 MAY-SEP 7710 8710 9380 88% 10100 11100 10700 Priest R nr Priest River 2 MAY-JUL 275 350 405 70% 460 535 580 MAY-SEP 310 395 450 71% 505 590 630 NF Coeur dAlene R at Enaville MAY-JUL 225 285 330 81% 375 440 405 MAY-SEP 255 320 365 82% 410 475 445 St. Joe R at Calder 2 MAY-JUL 580 700 780 102% 865 985 765 MAY-SEP 630 760 845 102% 935 1060 830 Spokane R nr Post Falls 2 MAY-JUL 1030 1260 1420 93% 1570 1800 1530 MAY-SEP 1100 1340 1500 93% 1660 1900 1620 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 April Watershed Snowpack Analysis: May 1, 2021 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2021 2020 Hungry Horse Lake 2686.5 2667.7 2188.0 3451.0 Moyie River 1 80% 97% Flathead Lake 983.4 869.0 971.5 1791.0 Priest River 5 77% 109% Noxon Rapids Reservoir 288.9 291.4 307.4 335.0 Rathdrum Creek 3 37% 89% Lake Pend Oreille 884.2 908.5 931.7 1561.3 Coeur d' Alene River 6 93% 120% Priest Lake 80.8 84.6 101.9 119.3 St. Joe River 4 91% 98% Lake Coeur d' Alene 154.3 218.8 228.0 238.5 Pend Oreille Lake 5 68% 108% Palouse River 2 29% 77% Lower Kootenai 2 83% 112% Pend Oreille-Kootenai 13 74% 109% Coeur d' Alene-St. Joe Total 9 93% 111% Clearwater River Basin May 1, 2021

WATER SUPPLY OUTLOOK Dry conditions continued from March, with April monthly precipitation 35% to 55% of normal in the Clearwater sub basins (Fig. 1). As of May 1, total water year precipitation is ~85% to 105% of normal in the sub basins (Fig. 2). The snowpack in the Clearwater sub basins are ~85% to 105% of normal (Fig. 3). Due to warm temperatures and a lack of snow in April to slow down snowmelt, SWE losses during April were significant in the Clearwater Basin. On April 1, nearly every site within the basin had above normal snowpack and as of May 1, nearly every site within the basin is below normal. SWE losses are occurring nearly ten times faster than normal at Mountain Meadows SNOTEL site bordering the South Fork Clearwater and Selway basins. If no additional snowfall occurs during May, the snowpack in the Clearwater Basin will likely melt out one to two weeks earlier than normal. Dworshak Reservoir is 86% of normal (66% capacity). The Clearwater River at Spalding 50% exceedance forecast for May through July is 88% of normal and is 80% for the 70% exceedance forecast (Fig. 4, 5). Based on similar soil moisture conditions of past years with a dry spring and observed versus forecasted streamflow conditions during dry years, the 70% exceedance forecast may be a better prediction of future streamflow conditions. The NOAA Climate Prediction Center’s 30- day forecast suggests equal chances of near, above, and below normal precipitation and temperature in May in the Clearwater Basin. Clearwater River Basin Streamflow Forecasts - May 1, 2021 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 MAY-JUL 1250 1420 1530 97% 1640 1810 1570 MAY-SEP 1330 1500 1620 97% 1740 1920 1670 Lochsa R nr Lowell MAY-JUL 805 930 1020 90% 1100 1230 1130 MAY-SEP 855 990 1080 89% 1170 1310 1210 Dworshak Reservoir Inflow 2 MAY-JUL 1070 1280 1420 80% 1550 1760 1770 MAY-SEP 1180 1410 1560 81% 1720 1950 1920 Clearwater R at Orofino MAY-JUL 2450 2840 3100 91% 3360 3750 3400 MAY-SEP 2600 3020 3310 91% 3600 4020 3630 Clearwater R at Spalding 2 MAY-JUL 3620 4210 4620 88% 5020 5610 5260 MAY-SEP 3870 4520 4960 88% 5410 6060 5640 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 April Watershed Snowpack Analysis: May 1, 2021 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2021 2020 Dworshak Reservoir 2273.7 2585.0 2646.0 3468.0 NF Clearwater River 8 90% 100% Lochsa River 2 89% 105% Selway River 4 105% 122% SF Clearwater River 1 85% 115% Clearwater Basin Total 15 92% 104% Salmon River Basin May 1, 2021

WATER SUPPLY OUTLOOK A relatively normal winter has transitioned to an extremely dry spring. April precipitation set record lows at several sites and was 36% of normal for the Salmon Basin (Fig. 1). With very low March and April precipitation, the total water year precipitation has fallen to 77% of normal (Fig. 2). The Salmon Basin snowpack is 64% of normal (Fig. 3). Despite a late April storm that brought a small amount of new snow, the snowpack in this basin is melting nearly twice as fast as normal, and is on pace to melt out about two weeks earlier than normal. May to July 50% exceedance streamflow forecasts are ~50 to 60% of normal, and the 70% exceedance forecasts are between 29% and 55% of normal (Fig. 4, 5). Streamflow for the Middle Fork of the Salmon is forecast to be 55% of normal. With the extremely dry spring we recommend users relying on a certain amount of water consider using the 70% exceedance streamflow forecasts. NOAA’s Climate Prediction Center’s 30-day outlook suggests increased chances of above normal temperatures and below normal precipitation, so signs continue to point to early snowpack melt out. Salmon River Streamflow Forecasts - May 1, 2021 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 MAY-JUL 194 290 355 52% 420 515 680 MAY-SEP 250 360 435 54% 510 620 805 Lemhi R nr Lemhi MAY-JUL 1.77 17.5 31 51% 44 63 61 MAY-SEP 3.5 26 42 55% 57 80 76 MF Salmon R at MF Lodge MAY-JUL 210 285 330 55% 380 450 600 MAY-SEP 255 335 390 58% 445 525 675 SF Salmon R nr Krassel Ranger Station MAY-JUL 89 117 136 60% 155 183 225 MAY-SEP 100 130 150 61% 171 200 245 Johnson Ck at Yellow Pine MAY-JUL 64 87 103 60% 118 141 172 MAY-SEP 71 95 112 61% 128 152 184 Salmon R at White Bird MAY-JUL 2010 2510 2860 61% 3200 3700 4660 MAY-SEP 2330 2890 3270 63% 3660 4220 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, 2021 # of % of Median Basin Name Sites 2021 2020 Salmon River ab Salmon 8 60% 78% Lemhi River 4 56% 86% MF Salmon River 3 61% 79% SF Salmon River 3 68% 85% Little Salmon River 4 54% 108% Lower-Middle Salmon 4 59% 81% Salmon Basin Total 21 64% 88% West Central Basins May 1, 2021

WATER SUPPLY OUTLOOK As with much of central Idaho, the fairly normal winter in the West Central basins gave way to a warm and dry spring, and it appears that the snowpack will likely melt out earlier than normal. April precipitation set record lows at several sites and was ~30% to 45% of normal across the West Central basins (Fig. 1). The lack of precipitation in March and April has brought the total water year precipitation down to ~75% to 85% of normal (Fig. 2). The Weiser Basin snowpack is 47% of normal and snowpack melt out is on track to occur about one week earlier than normal. The Payette Basin snowpack is 72% of normal and melt out will likely happen about two weeks earlier than normal. The Boise Basin snowpack is 64% of normal and melt out will likely occur nearly three weeks earlier than normal (Fig. 3). May 1 reservoir storage is near normal in the West Central basins. In the Boise River System, Anderson Ranch, Arrowrock, and Lucky Peak reservoir storage range between 98% and 109% of normal (~70 to 100% of storage capacity). Deadwood and Cascade reservoirs are at 91% and 104% of normal storage, respectively. Mann Creek reservoir is 102% of normal storage. May to July 50% exceedance streamflow forecasts are ~45 to 65% of normal in the Boise Basin, ~55% to 70% in the Payette Basin, and 57% in the Weiser Basin (Fig. 4). With the extremely dry spring some additional consideration should be given to the 70% exceedance streamflow forecasts (see table below). NOAA’s Climate Prediction Center’s 30-day outlook suggests increased chances of above normal temperatures and below normal precipitation, so signs continue to point to early snowpack melt out. West Central Basins Streamflow Forecasts - May 1, 2021 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 MAY-JUL 83 137 173 47% 210 265 370 MAY-SEP 103 159 197 49% 235 290 405 Boise R nr Twin Springs MAY-JUL 193 250 290 63% 335 390 460 MAY-SEP 225 290 330 65% 375 435 510 Mores Ck nr Arrowrock Dam MAY-JUL 14 28 37 54% 46 60 68 MAY-SEP 15.4 30 40 56% 50 64 72 Boise R nr Boise 2 MAY-JUL 305 435 520 55% 610 735 950 MAY-SEP 370 505 600 57% 695 830 1050 Lake Fork Payette R nr McCall MAY-JUL 38 44 49 70% 54 61 70 MAY-SEP 39 46 51 70% 56 63 73 NF Payette R at Cascade 2 MAY-JUL 144 193 225 59% 260 310 380 MAY-SEP 141 195 230 58% 270 320 395 NF Payette R nr Banks 2 MAY-JUL 152 220 265 56% 310 380 475 MAY-SEP 155 225 275 56% 325 395 495 SF Payette R at Lowman MAY-JUL 172 200 220 65% 240 270 340 MAY-SEP 205 235 260 66% 280 315 395 Deadwood Reservoir Inflow 2 MAY-JUL 42 52 59 57% 66 76 104 MAY-SEP 46 58 66 59% 74 85 112 Payette R nr Horseshoe Bend 2 MAY-JUL 445 590 685 59% 780 920 1160 MAY-SEP 505 660 760 58% 865 1020 1310 Weiser R nr Weiser MAY-JUL 79 110 134 57% 160 200 235 MAY-SEP 96 129 155 58% 182 225 265 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 April Watershed Snowpack Analysis: May 1, 2021 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2021 2020 Anderson Ranch Reservoir 311.4 350.0 284.9 450.2 SF Boise River 6 62% 69% Arrowrock Reservoir 185.8 180.8 189.0 272.2 MF & NF Boise Rivers 6 60% 74% Lucky Peak Reservoir 234.5 236.4 219.8 293.2 Mores Creek 5 62% 75% Sub-Basin Total 731.7 767.2 693.7 1015.6 Canyon Creek 1 Deadwood Reservoir 96.1 103.1 105.1 161.9 Boise Basin Total 13 64% 73% Cascade Reservoir 520.0 523.6 501.5 693.2 NF Payette River 8 69% 94% Sub-Basin Total 616.1 626.7 606.6 855.1 SF Payette River 4 69% 84% Lake Lowell 129.3 133.5 125.6 165.2 Payette Basin Total 15 72% 90% Mann Creek Reservoir 10.9 10.7 10.7 11.1 Mann Creek 1 122% 102% Weiser Basin Total 4 47% 99% Wood & Lost River Basins May 1, 2021

WATER SUPPLY OUTLOOK Low April precipitation further amplified drought conditions in the Wood and Lost basins. Despite a late April storm that brought some additional snow to the mountains, early snowpack melt out appears likely in these basins. April precipitation was only ~45 to 55% of normal (Fig. 1). Several sites set low precipitation records in April, with basin-wide water year precipitation levels decreasing to ~60 to 75% of normal across this region (Fig. 2). The Little Lost Basin snowpack is the highest at ~57% of normal, all other basin snowpacks are ~30 to 45% of normal (Fig. 3). A late April storm increased snowpacks a small amount, but it is still melting faster than normal and on pace to melt out two to three weeks earlier than normal across the Wood and Lost basins. Much of the Wood and Lost basins are in severe drought conditions, particularly the Pioneer Mountains which are in extreme drought, and numerous SNOTEL sites have set the lowest two year total precipitation amounts on record. Unless the area receives increased precipitation in late spring and early summer, the areas experiencing extreme drought are likely to expand. Although the snowpack and precipitation have been low throughout the water year, Mackay Reservoir has 99% of its normal May 1 storage, and is at 73% capacity. Little Wood reservoir is at 82% of normal storage, which is 68% of capacity. Magic Reservoir is 33% of normal storage (22% capacity). May to July 50% exceedance streamflow forecasts are well below normal across the basins (Fig. 4). The 50% exceedance forecast for the Big Wood at Hailey is 32% of normal, and only ~6-8% above Magic Reservoir and below Magic Dam. The Little Wood is forecast to be ~32% of normal, the Big Lost ~25 to 40% of normal, and the Little Lost is forecast to be 58% of normal. Surface Water Supply Index (SWSI) values in the Wood and Lost basins indicate significant water shortages are likely, so some additional consideration should be given to the 70% exceedance streamflow forecasts (Fig. 5). Wood and Lost Basins Streamflow Forecasts - May 1, 2021 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 MAY-JUL 0.03 0.95 2.3 9% 4.2 8.2 25 Little Lost R bl Wet Ck nr Howe MAY-JUL 6.9 11.1 14 58% 16.9 21 24 MAY-SEP 7.6 13.5 17.5 58% 21 27 30 Big Lost R at Howell Ranch MAY-JUL 19.2 44 60 41% 77 101 148 MAY-SEP 23 51 70 41% 88 116 169 Big Lost R bl Mackay Reservoir MAY-JUL 2 11.6 28 25% 44 69 111 MAY-SEP 3 27 46 33% 65 94 138 Little Wood R ab High Five Ck MAY-JUL 7.4 12.4 16.6 32% 21 30 52 MAY-SEP 8.8 14.5 19.3 33% 25 34 58 Little Wood R nr Carey 2 MAY-JUL 7.4 12.6 17 31% 22 30 55 MAY-SEP 8.6 14.5 19.4 32% 25 34 61 Big Wood R at Hailey MAY-JUL 15.3 45 65 32% 85 114 205 MAY-SEP 21 55 78 33% 101 135 235 Big Wood R ab Magic Reservoir MAY-JUL 0.17 4.6 11 7% 20 34 147 MAY-SEP 1.62 7.1 13 8% 21 35 159 Camas Ck nr Blaine MAY-JUL 0.5 2.3 5.3 15% 9.6 18.1 35 MAY-SEP 0.6 2.5 5.6 16% 9.9 18.5 36 Big Wood R bl Magic Dam 2 MAY-JUL 0.39 4.8 10.6 6% 18.6 35 176 MAY-SEP 1.57 8 15.3 8% 25 43 191 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 Reservoir Storage (KAF): End of April Watershed Snowpack Analysis: May 1, 2021 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2021 2020 Mackay Reservoir 32.4 42.1 32.8 44.4 Camas-Beaver Creeks 2 11% 41% Little Wood Reservoir 20.5 28.8 25.0 30.0 Birch-Medicine Lodge Creeks 2 71% 97% Magic Reservoir 42.2 157.9 128.0 191.5 Little Lost River 3 57% 74% Big Lost River ab Mackay 4 36% 47% Big Lost Basin Total 5 36% 48% Fish Creek 0 Little Wood ab Resv 4 31% 43% Big Wood River ab Hailey 5 46% 46% Camas Creek 3 0% 0% Birch-Medicine Lodge-Camas-Beaver 4 44% 72% Total Little Wood Basin Total 4 31% 43% Big Wood Basin Total 8 41% 41% Upper Snake River Basins May 1, 2021

WATER SUPPLY OUTLOOK Significantly drier than normal conditions occurred during April in all sub basins. Monthly precipitation ranged from ~40% to 70% (Fig. 1). Including March, only the Portneuf basin received over 60% of normal precipitation over the last two months combined (March & April). As of May 1, water year total precipitation in the Upper Snake is ~80% of normal (Fig. 2). Snowpack conditions in the Henrys Fork- Teton, Snake above Palisades, the Willow-Blackfoot-Portneuf Basin are 77%, 69%, and 91% respectively (Fig. 3). SWE loss has occurred more quickly this April compared to the historical normal rate. The Henrys Fork-Teton and Snake River above Palisades appear to be melting out nearly twice as fast as normal at most SNOTEL sites. NOAA’s official 30-day outlook predicts above normal temperatures and an equal chance of above or below normal precipitation conditions. Reservoir storage in the Upper Snake system is ~120% of normal. As of May 1, the Jackson- Palisades system is at ~140% of normal storage for this time of the year. Streamflow forecasts are below normal for the Upper Snake basins and range from ~30% to 80% for the runoff period (Fig. 4). The Snake River near Heise forecast is 69% of normal. While we typically report the 50% exceedance forecast, users should consider using the 70% exceedance forecast due to the lack of spring precipitation and drier than normal soil conditions (Fig. 5). Upper Snake River Basin Streamflow Forecasts - May 1, 2021 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 MAY-JUL 205 245 275 66% 305 350 415 MAY-SEP 355 405 440 74% 475 530 595 Falls R nr Ashton 2 MAY-JUL 195 220 240 76% 260 285 315 MAY-SEP 250 280 305 79% 330 360 385 Teton R nr Driggs MAY-JUL 60 78 90 67% 103 121 134 MAY-SEP 81 103 119 69% 134 156 173 Teton R nr St Anthony MAY-JUL 165 200 225 70% 250 290 320 MAY-SEP 210 255 280 72% 310 355 390 Henrys Fk nr Rexburg 2 MAY-JUL 610 730 805 69% 885 1000 1170 MAY-SEP 875 1030 1130 72% 1230 1380 1560 Snake R at Flagg Ranch MAY-JUL 205 245 270 64% 295 335 425 MAY-SEP 225 270 300 64% 330 375 470 Snake R nr Moran 2 MAY-JUL 335 400 445 64% 490 555 700 MAY-SEP 370 440 490 63% 540 610 775 Pacific Ck at Moran MAY-JUL 68 94 113 74% 131 158 152 MAY-SEP 74 102 120 75% 139 167 161 Buffalo Fk ab Lava Ck nr Moran MAY-JUL 173 199 215 81% 235 260 265 MAY-SEP 194 225 250 82% 270 300 305 Snake R ab Reservoir nr Alpine 2 MAY-JUL 1150 1300 1400 71% 1500 1660 1960 MAY-SEP 1350 1530 1650 72% 1760 1940 2280 Greys R ab Reservoir nr Alpine MAY-JUL 148 176 195 74% 215 240 265 MAY-SEP 179 210 235 75% 255 290 315 Salt R ab Reservoir nr Etna MAY-JUL 54 99 131 53% 162 205 245 MAY-SEP 91 145 181 58% 220 270 310 Snake R nr Irwin 2 MAY-JUL 1410 1650 1820 68% 1980 2220 2660 MAY-SEP 1710 1990 2190 70% 2380 2660 3150 Snake R nr Heise 2 MAY-JUL 1530 1780 1950 69% 2120 2370 2840 MAY-SEP 1880 2170 2370 70% 2570 2870 3390 Willow Ck nr Ririe 2 MAY-JUL 5.4 11.5 17 40% 23 35 43 Portneuf R at Topaz MAY-JUL 23 29 33 66% 37 44 50 MAY-SEP 31 39 45 67% 51 61 67 Snake R at Neeley 2 MAY-JUL 230 460 660 31% 890 1300 2100 MAY-SEP 240 465 660 29% 895 1300 2260 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 April Watershed Snowpack Analysis: May 1, 2021 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2021 2020 Jackson Lake 667.1 643.1 445.7 847.0 Henrys Fork-Falls River 10 72% 98% Palisades Reservoir 1236.7 964.8 911.7 1400.0 Teton River 9 87% 115% Sub-Basin Total 1903.7 1607.9 1357.4 2247.0 Henrys Fork-Teton 17 77% 105% Henrys Lake 88.5 89.9 83.5 90.4 Snake River ab Jackson Lake 7 55% 104% Island Park Reservoir 125.9 120.4 123.8 135.2 Pacific Creek 2 83% 119% Grassy Lake 12.9 13.6 12.8 15.2 Buffalo Fork 3 83% 115% Sub-Basin Total 227.2 223.9 220.1 240.8 Gros Ventre River 4 86% 121% Ririe Reservoir 62.0 71.6 58.7 80.5 Hoback River 5 68% 125% Blackfoot Reservoir 323.0 211.3 337.0 Greys River 5 84% 136% American Falls Reservoir 1502.4 1667.4 1528.0 1672.6 Salt River 5 78% 148% Basin-Wide Total 3695.2 3893.8 3375.5 4577.9 Snake ab Palisades Resv 25 69% 117% Willow Creek 5 84% 136% Blackfoot River 4 65% 119% Portneuf River 6 110% 80% Willow-Blackfoot-Portneuf 14 91% 112% Southern Snake River Basins May 1, 2021

WATER SUPPLY OUTLOOK Monthly precipitation in the Southern Snake River basins was well below normal and ranges from ~25% to 50% (Fig. 1). Some areas in the Owyhee, Bruneau, and Salmon Falls basins experienced record low precipitation in April. As of May 1, water year total precipitation in the Southern Snake basins ranges from ~65% to 80% of normal (Fig. 2), and the snowpack is ~40% to 60% of normal (Fig. 3). Southern Snake River basins experienced a slightly below normal peak snowpack that occurred near April 1 in every region except the Owyhee Mountains which peaked earlier in March. Although peak conditions on April 1 is near normal timing, the snowpack is melting faster than normal. May 1 basin snowpack conditions compared to the 30-year normal are: Owyhee 43%, Bruneau 51%, Salmon Falls 55%, Goose Creek 48%, and Raft River 61% of normal (Fig. 3). NOAA’s official 30-day outlook predicts above normal temperatures and lower than normal precipitation throughout most of this region. This forecast implies the quicker than normal melt will continue and early complete melt-out is likely.

Current reservoir storage compared to the historical average are: Oakley 84%, Salmon Falls 75%, Wild Horse 111% and Lake Owyhee 76%. Streamflow 50% exceedance forecasts for the May through July period range from ~5% to 60% of normal, and ~0% to 50% of normal for the 70% exceedance forecast (Fig. 4, 5). Based on similar soil moisture conditions of past dry years and observed versus forecasted streamflow conditions during dry years, the 70% exceedance forecast may be a better prediction of future streamflow conditions. Rapid snowmelt coupled with already dry conditions suggest that peak streamflow is likely to be lower and earlier than normal this year. Southside Snake River Basins Streamflow Forecasts - May 1, 2021 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 MAY-JUL 1.99 4.3 6.4 48% 8.8 13.2 13.2 MAY-SEP 2.3 4.9 7.2 50% 9.9 14.7 14.5 Trapper Ck nr Oakley MAY-JUL 1.37 1.83 2.2 59% 2.5 3.1 3.7 MAY-SEP 2.2 2.8 3.2 65% 3.7 4.4 4.9 Oakley Reservoir Inflow MAY-JUL 2.4 4.6 6.5 38% 8.7 12.5 16.9 MAY-SEP 3.3 5.9 8.1 42% 10.7 15 19.4 Salmon Falls Ck nr San Jacinto MAY-JUL 7.6 13.7 18.9 39% 25 35 49 MAY-SEP 9.3 16.1 22 42% 28 39 53 Bruneau R nr Hot Spring MAY-JUL 34 51 66 47% 81 108 140 MAY-SEP 37 56 71 48% 87 115 148 Reynolds Ck at Tollgate MAY-JUL 0.54 1.06 1.52 29% 2.1 3 5.3 MAY-SEP 0.53 1.08 1.55 29% 2.1 3.1 5.4 Owyhee R nr Gold Ck 2 MAY-JUL 0 0 0.34 4% 1.21 3.5 9.6 Owyhee R nr Rome MAY-JUL 2.9 19.6 39 21% 66 118 188 MAY-SEP 6.7 28 50 24% 79 133 205 Owyhee R bl Owyhee Dam 2 MAY-JUL 10.6 35 60 29% 91 150 210 MAY-SEP 24 56 85 35% 119 181 240 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 April Watershed Snowpack Analysis: May 1, 2021 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2021 2020 Oakley Reservoir 28.7 41.2 34.3 75.6 Raft River 1 61% 76% Salmon Falls Reservoir 54.0 103.4 71.6 182.6 Goose-Trapper Creeks 2 48% 75% Wild Horse Reservoir 54.9 73.6 49.4 71.5 Salmon Falls Creek 4 55% 69% Lake Owyhee 406.6 596.8 533.1 715.0 Bruneau River 5 51% 57% Brownlee Reservoir 958.8 998.2 1161.0 1420.0 Reynolds Creek 1 Upper Owyhee 5 52% 26% Owyhee Basin Total 7 43% 22% Bear River Basin May 1, 2021

WATER SUPPLY OUTLOOK Last month was a continuation of the dry water year conditions. Monthly precipitation was 60% of normal (Fig. 1) and total water year precipitation is 73% of normal as of May 1 (Fig. 2). The snowpack is 63% of normal (Fig. 3). The snowpack percent of normal for each sub basin as of May 1 is: Smith-Thomas Forks 81%, Cub River 50%, and Bear Lake 66%. The Bear River Basin experienced slightly below normal peak snowpack conditions near April 1. Peak snowpack occurred approximately a week earlier than normal. NOAA’s official 30-day outlook predicts above normal temperatures and below normal precipitation across this region. This forecast implies quicker than normal melt will continue and early complete melt-out is likely. Storage among the region’s reservoirs as a percent of normal are: Bear Lake at 128% (64% capacity) and Montpelier Reservoir at 124% (84% capacity). Streamflow 50% exceedance forecasts for the May through July period range from 9% to 65% of normal, and 1% to 54% of normal for the 70% exceedance forecast (Fig. 4, 5). Based on similar soil moisture conditions of past dry years and observed versus forecasted streamflow conditions during dry years, the 70% exceedance forecast may be a better prediction of future streamflow conditions (Fig. 5). Rapid melt coupled with already dry conditions suggest that peak streamflow may be lower and earlier than normal this year. Above normal storage in Bear Lake should provide an adequate water supply for irrigators. Bear River Basin Streamflow Forecasts - May 1, 2021 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 44 57 66 59% 75 88 112 APR-SEP 49 64 74 60% 84 99 123 MAY-JUL 41 54 62 60% 70 83 104 Bear R ab Resv nr Woodruff APR-JUL 3.6 14.8 36 30% 58 90 121 APR-SEP 2.6 10.4 34 27% 58 93 128 MAY-JUL 3.2 13 33 31% 53 83 105 Big Ck nr Randolph APR-JUL 0 0.11 0.8 21% 2.1 4.1 3.8 MAY-JUL 0 0.09 0.5 16% 1.66 3.4 3.1 Smiths Fk nr Border APR-JUL 37 47 54 61% 62 72 89 APR-SEP 46 57 65 63% 73 85 104 MAY-JUL 33 43 50 63% 57 67 80 Bear R bl Stewart Dam 2 APR-JUL 0 5.5 20 11% 52 98 183 APR-SEP 0 4.1 25 12% 61 114 205 MAY-JUL 0 1.46 13 9% 43 88 146 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 April Watershed Snowpack Analysis: May 1, 2021 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2021 2020 Bear Lake 833.1 957.4 651.7 1302.0 Smiths-Thomas Forks 5 81% 104% Montpelier Reservoir 3.4 3.4 2.7 4.0 Bear Lake 6 66% 109% Montpelier Creek 2 77% 130% Mink Creek 0 Cub River 1 50% 94% Bear River Total 20 63% 97% Malad River 1 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

Report Created by Issued by Idaho Snow Survey Staff Matthew J. Lohr, Chief Natural Resources Conservation Service Natural Resources Conservation Service Boise, Idaho Washington, DC Email: [email protected]

Released by Corey Loveland, Snow Survey Supervisor Curtis Elke, State Conservationist Danny Tappa, Data Collection Officer (DCO) Natural Resources Conservation Service Mark Robertson, Hydrologist Boise, Idaho Peter Youngblood, Hydrologist Earl Adsley, Hydrologist Cody Brown, Hydrologist John Wilford, Electronics Technician

Erin Whorton, Water Supply Specialist (WSS) [email protected] (o) 208-685-6983 (c) 208-510-7294

Forecasts Provided by Forecast Hydrologist Staff NRCS, National Water and Climate Center Portland, Oregon

Julie Koeberle, Forecast Hydrologist Email: [email protected]

Numerous other agencies and groups provide funding and/or support for the collection, operation and maintenance of the Cooperative Idaho Snow Survey program. Your cooperation is greatly appreciated!

This publication is dedicated to the people, agencies and organizations utilizing this data, information and forecasts for short and long term water management, planning, preparation, recreation and otherwise, for the enhancement of the economy and enrichment of livelihoods.