Idaho Water Supply Outlook Report April 1, 2019

Natural Resources Conservation Service Idaho Water Supply Outlook Report April 1, 2019

Owyhee Reservoir in Eastern Oregon near the Idaho Border, March 30, 2019

In 1932, the U.S. Bureau of Reclamation completed the Owyhee Dam and the Owyhee Reservoir started filling for the first time. At the time of completion, the Owyhee Dam was the tallest of its kind in the world (417 feet) and served as a prototype for the Colorado River’s Hoover Dam. In the otherwise arid climate of Eastern Oregon and Southwestern Idaho, the reservoir provides critical irrigation water to approximately 118,000 acres of farmland across both states and four separate irrigation districts. The Owyhee River basin is huge, covering ~7,690 square miles in Oregon, Idaho, and Nevada. To put this in perspective, the entire Boise River basin is ~2,200 square miles! April 1 snowpack in the expansive Owyhee River basin is 130% of normal, and the reservoir is 61% full (88% of average), inflows began to increase in March and will continue in April as we transition into Spring. Please see the full report for details on current snowpack and expected water supplies. 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 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.

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April 1, 2019

SUMMARY

The stage is set. If it were not for February’s abundant precipitation, we would be telling a different story. Snowpacks across Idaho will provide an adequate irrigation supply and then some. The gradual melt of the snowpack in the lowest elevations during March helped to relieve some of the pressure of a rapid melt in these elevations. However, the mid-elevations across the state from the Weiser to the Henrys Fork basin still have a tremendous snowpack that is ripe and ready to melt. This mid-elevation snowpack covers a large area and still has the potential to melt rapidly with the arrival of rain and warmer temperatures.

Spring weather the next few weeks or months will determine how this year’s snowpack melts and fills rivers and reservoirs. Based on the PRISM temperatures maps, March temperatures across the state and most of the West were near to slightly below normal. As we move into April, the days are getting longer which allows for more heating, but night time temperatures are critical, as freezing night time temperatures slows the melt process. Future precipitation intensity and duration is also critical in melting the snow and providing additional runoff. Consecutive days of rain saturates the soils and provides much greater runoff than precipitation falling every other day or every few days. Short-term weather outlooks illustrate that more moisture is on the way and may continue into mid-April.

Reservoir operators are closely watching inflows and streamflow forecasts to assist in their delicate balancing act of making releases while ensuring reservoirs refill after peak inflows occur. More good news is that nearly all the reservoirs and natural lakes will fill this year. This should provide good carryover storage for next year for those starting to think about next year’s water supply. The exceptions are Bear Lake, Salmon Falls and Oakley reservoirs which are not likely to fill even using the 10% chance of exceedance forecasts. Owyhee Reservoir could fill depending upon spring rains falling or not.

SNOWPACK

Snowpacks vary across the state and within watersheds by elevation, so if you like details, be sure to look at the individual sites in your basin. The lowest snowpacks with respect to normal remain in northern Idaho and range from 80 to 100% of median. Interestingly, Bear Mountain SNOTEL at 5,400 feet just north of Clark Fork, Idaho in the Cabinet Mountains, usually one of the higher snow accumulation zones, only has 37.3 inches of snow water (69% of median). Normal April 1 amounts are 54.3 inches. Moving south the snowpack increases to 95% of median in the Clearwater basin and to 109% in the Salmon basin.

This is where it gets interesting – the mid-elevation snowpack in the 6,000 foot elevation band across central Idaho has well above normal snow. Sites in the Weiser are at 150% of median, 300% in the Little Wood basin, and 130% of median in the Henrys Fork where a valley snowpack remains – Island Park SNOTEL site (6,290 feet) is at 125% of median with 47 inches of snow on the ground. Luckily, some of the lower elevation snow has gradually melted filling soil moisture voids from last year’s dry summer. Significant rain or above average temperatures will increase melt rates to levels that the soils cannot absorb and result in rapid increases in tributary streams.

Overall, most basins south of the Salmon Basin range from 110 to 145% of median. The lowest are near 110% of median in the Upper Snake, Mud Lake, Little Lost and Salmon and highest are 150% in the Owyhee and Weiser basins and 160% for the Little Wood. The early April storms are bringing rain in the valleys and snow in the higher elevations. This pattern may continue into mid-April. After more of the mid-elevation snow melts, the higher elevation snowmelt will keep rivers flowing near average or better into the later summer months.

PRECIPITATION

In contrast to February, March precipitation was below average across nearly the whole state. Lowest percentages were a third of average in northern Idaho and increase to 40% in the Clearwater basin. Moving south, precipitation amounts increased more, but only to 50 to 80% of average across central and southern Idaho. Greater precipitation amounts that were closer to near normal fell in Oakley and Bear River basins because the primary March storm track was across the Great Basin. South of the Owyhee, Bruneau, Salmon Falls and Goose headwaters in Nevada and Utah, March precipitation totals reached 150 to 200% of average.

This dry north and wet south pattern is more typical of past El Nino years. When the past two months precipitation for February and March are combined, this pattern really stands out and is interesting to note that basins south of the Salmon River received 100 to 150% of normal February to March totals while basins to the north received only 60 to 100% of normal amounts.

The water year-to-date precipitation pattern also mirrors this El Nino pattern with water year totals at only 75% of average in Idaho’s Panhandle and increasing to 90% in the Clearwater basin. Basins south of the Salmon River have received 100 to 115% of average since October 1, 2018. The exception is the Little Wood basin which is at 125% of its water year-to-date precipitation. It is important to keep in mind that this year’s water year-to-date precipitation totals are only 55 to 80% of their average total annual precipitation while in 2017 Idaho’s central basins had received 20% above their annual water year totals by April 1.

RESERVOIRS

As the snow accumulation season winds down, reservoir operators have a good feel for how much snow is in the mountains to meet and exceed this year’s water supplies. This mountain snowfall information is compiled to produce streamflow forecasts for water managers to monitor inflows and set outflows, while they work to mitigate flooding and ensure reservoirs refill after peak flows occur. This is no easy task since the last piece of the water supply picture comes down to the spring temperatures and precipitation that drives snow melt rates. A warm, dry spring like in 2015 would see high early irrigation demands in mid-April while a cool, wet spring decreases early demands and usually increases water supplies. Reservoir operators will be watching how the remaining mid- elevation snow melts and if the spring weather trends towards dry and warmer weather, or moderate temperatures allow the snow to gradually melt and be easier to manage.

The good news is that enough snow has already fallen in the mountains to ensure adequate irrigation supplies this season. As a result, many of the farmers’ and irrigators’ decisions have already been made thanks to the numerous storms that Mother Nature delivered in February. Releases from reservoirs and surplus volumes will be available for water managers to wisely distribute for Idaho’s numerous uses. Flows over Shoshone Falls and the Snake River near Milner may continue into late June or early July based on current snow levels, reservoir storage, and streamflow forecasts.

STREAMFLOW

After the major jump in streamflow forecasts from February’s dump, forecasts decreased slightly from March 1 because of the below normal March precipitation. Streamflow forecasts mirror the snowpack and increase from northern Idaho to southern Idaho. The lowest forecasts are 75 to 80% of average in the Coeur d’Alene, Priest and Moyie drainages. Dworshak Reservoir Inflow follows this trend with forecasts at 82% of average while the Selway and Lochsa rivers are forecast at about 95%. Streamflow forecasts in the Salmon basin range from 99% of average for the Lemhi River to 109% for the MF Salmon River.

The highest forecasts are mainly across central Idaho with the Weiser River at 136% of average, Boise River at 121%, Camas Creek at 212%, Magic Reservoir Inflow at 162%, Little Wood River at 175%, and Big Lost River at 132%. Willow Creek near Ririe is forecast at 151% of average and Portneuf River is forecast at 125%. The rest of the Upper Snake is forecast at 95 to 115% of average with the important Snake River near Heise forecast at 98%. Watersheds across Idaho’s southern border call for near normal runoff in the Bear River and Owyhee basins, and increase to about 130% of average in the Oakley, Salmon Falls and Bruneau basins.

Shortages are not expected this year. A gradual melt with multiple streamflow peaks would help reduce the impacts of sustained high flows this spring. Any additional rain will also provide a boost in the streamflow. Timing and magnitudes of streamflow peaks will depend on future spring precipitation and temperatures.

Note: The volumes referenced in these narratives are the 50% Chance of Exceeding Forecast, unless otherwise noted. Users may wish to use a different exceedance forecast to reduce their risk of having too much or too little water.

RECREATION

Now is the time to enjoy spring skiing until the snowmelt season really kicks in. The potential remains for some rivers to go big from the abundant mid-elevation snow and potential for rain in early to mid- April, especially where the snow is ripe and melting. If you are an early season river runner, get ready as the Owyhee River near Rome has seen a few peaks, but there is still more snow to melt. Perhaps the Grand Ronde River is an indicator of what may happen in the Owyhee and Bruneau rivers from the remaining snowmelt.

For those that prefer to float on those long summer days, be patient until the snowmelt peak flows have occurred as the higher elevation snow will help sustain flows into the later summer months. Northern Idaho rivers will have a shorter floating season with the snow at only 80 to 95% of median and streamflow forecasts that mirror these percentages, so plan wisely. Here is a link to an article summarizing Idaho’s river running season.

Play it safe and plan your trips wisely as the rivers start to rise. Understand the potential for additional peaks either from rain or the remaining snow. For the early season river runners – know your limits, river skills, and capabilities. Spring precipitation can make or break a forecast so keep an eye on the sky while watching to see if the extreme weather we saw in February makes its way back to Idaho. IDAHO SURFACE WATER SUPPLY INDEX (SWSI) April 1, 2019

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 - 1.5 1998 NA Clearwater - 1.9 2013 NA Salmon - 0 .2 2010 NA Weiser 2.1 1996 NA

Payette 1.1 2008 NA

Boise 1. 3 1999 - 2.2 Big Wood above Hailey 1.3 2011 - 2.9 Big Wood 1.7 1999 0. 1 Little Wood 2. 3 1997 - 1.4

Big Lost 1.9 2018 0.5

Little Lost 0.6 2010 1.2

Teton 0.6 2018 - 4.0 Henrys Fork 2.1 2017 - 2.5 Snake (Heise) 1.5 1998 - 1.8 Oakley 1. 5 2007 - 0.1 Salmon Falls above Jackpot 1.9 2009 NA Salmon Falls 1 .7 1996 - 1.0 Bruneau 1.7 2016 NA Owyhee 0.8 2005 - 2.7 Bear River 1. 3 2012 - 4.0

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.

* # * #

* # Water Supply Forecast

April 1, 2019

* # * ¯ # Forecasted April-July Flow as a

Percentage of the 1981 to 2010 Average * #

Above * # *# > 180% PANHANDLE Average

REGION *# 150 - 179% * # *# 130 - 149% *# 110 - 129%

Average

90 - 109%

"* )#

Forecast * #

CLEARWATER *

# 70 - 89%

BASIN )" )" * # 50 - 69%

* )")" # 25 - 49% Below )" Average 0 - 24% ( No Data Provisional Data - Subject to Revision )" SALMON RIVER *# )" )" BASINS WEST )" UPPER SNAKE CENTRAL *# RIVER BASIN BASINS *# *# # )" * *# S )"*# *# *# IN *# # *# S * )")" A )" )" B )" *#)" T

S *#)"

*# *# LO

*# & * # *# *# *# D )" *# OO )" *# W *#)" )" *# )" *#

*#*# )" *#

*# BEAR RIVER BASIN 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:© 2014 Esri 81 Percent of Median Snowpack 83 ¯ 98 April 1, 2019 Basin-wide Snow Water Equivalent as a Percentage of the 1981 to 2010 Median

88 92 PANHANDLE Above >= 150% REGION 130 - 149%

88 110 - 129% Median 90 - 109% 106 Snowpack 91 70 - 89% CLEARWATER 50 -69% BASIN 92 Below 0 - 49% No Data 103 Provisional Data - Subject to Revision

141 SALMON RIVER

106 BASINS WEST 108 132 101 UPPER SNAKE CENTRAL 146 106 RIVER BASIN BASINS 125 115 106 120 114 S 112 IN 116 S 133 A 116 126 B T 115 S 131 95 161 LO & 175 OD WO 138 104

130 107 139

150

137 121 122 132 120

BEAR RIVER BASIN 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:© 2014 Esri 36 Monthly Precipitation 31 ¯ 35 March 2019 Monthly Precipitation as a Percentage of the 1981 to 2010 Average

33 Above >= 150% 26 PANHANDLE REGION 130 - 149% 110 - 129% 35 Average 90 - 109% 42 Precipitation 70 - 89% 38 CLEARWATER 50 -69% BASIN 43 Below 0 - 49% No Data 49 Provisional Data - Subject to Revision

SALMON 74 RIVER

69 BASINS WEST 55 75 62 UPPER SNAKE CENTRAL 71 55 RIVER BASIN BASINS 31 57 67 60 58 S 57 IN 58 S 57 A 51 65 B T 49 S 62 52 57 LO & 69 D O 48 WO 67

70 86 109

97 81 105

BEAR RIVER BASIN 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:© 2014 Esri 83 Water Year to Date 74 ¯ 77 Precipitation April 1, 2019 Basin-wide Water Year Precipitation as a Percentage of the 1981 to 2010 Average

78 Above >= 150% 71 PANHANDLE REGION 130 - 149% 110 - 129% 79 Average 90 - 109% 86 Precipitation 70 - 89% 85 CLEARWATER 50 -69% BASIN 97 Below 0 - 49% No Data 98 Provisional Data - Subject to Revision

111 SALMON RIVER

100 BASINS WEST 100 105 96 UPPER SNAKE CENTRAL 115 101 RIVER BASIN BASINS 112 111 100 105 105 S 102 IN 109 S 114 A 107 115 B T 104 S 101 105 124 LO & 125 OD WO 108 98

106 101 118

101

115 116 116 108

WATER SUPPLY OUTLOOK A dry winter in the Panhandle ended with the driest month yet – March precipitation totals were only 25 to 45% of average. This left the water year-to-date precipitation ranging from 70% of average in the Coeur d’Alene basin to 90% of average in Rathdrum Creek. April 1 snowpack totals display similar numbers, with most basins between 80% and 90% of normal for this time of year. As daily temperatures increased in mid-March, the snow at most sites below elevations of 5,000 feet started to melt to feed the region’s streams and rivers.

Panhandle natural lakes continue to store below average volumes at 70 to 80% of average capacity for this time of year. Further upstream on the Clark Fork, Montana reservoir systems are storing near or above average volumes. Forecasts for flows in Panhandle rivers are 75 to 80% of normal for the April through July period. Expect significant melt and runoff to begin throughout this month, as the Climate Prediction Center one-month outlook indicates that April will likely be warmer and wetter than average. Panhandle Region Streamflow Forecasts - April 1, 2019 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 225 270 300 80% 330 375 375 APR-SEP 235 280 310 81% 340 385 385 Kootenai R at Leonia 1 & 2 APR-JUL 3700 4580 4980 75% 5380 6260 6600 APR-SEP 4420 5350 5780 76% 6200 7140 7590 Boundary Ck nr Porthill APR-JUL 74 86 94 80% 103 115 117 APR-SEP 77 90 98 80% 107 119 123 Clark Fork R bl Cabinet Gorge Dam 2 APR-JUL 6750 7810 8530 83% 9250 10300 10300 APR-SEP 7300 8500 9310 82% 10100 11300 11300 Pend Oreille Lake Inflow 2 APR-JUL 7670 8860 9670 82% 10500 11700 11800 APR-SEP 8270 9600 10500 82% 11400 12700 12800 Priest R nr Priest River 2 APR-JUL 435 540 610 78% 680 785 780 APR-SEP 470 575 650 78% 725 830 830 NF Coeur dAlene R at Enaville APR-JUL 380 475 535 76% 600 695 700 APR-SEP 410 505 570 77% 635 730 740 St. Joe R at Calder 2 APR-JUL 620 740 820 78% 900 1010 1050 APR-SEP 670 790 875 78% 960 1080 1120 Spokane R nr Post Falls 2 APR-JUL 1220 1570 1810 76% 2040 2390 2390 APR-SEP 1280 1630 1880 76% 2120 2470 2480 Spokane R at Long Lake APR-JUL 1450 1810 2050 78% 2300 2660 2620 APR-SEP 1600 1970 2230 78% 2480 2860 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 March Watershed Snowpack Analysis: April 1, 2019 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2019 2018 Hungry Horse Lake 2573.0 2109.2 2081.0 3451.0 Moyie River 8 81% 123% Flathead Lake 770.7 884.6 762.6 1791.0 Priest River 5 98% 132% Noxon Rapids Reservoir 323.4 310.8 309.9 335.0 Rathdrum Creek 3 84% 100% Lake Pend Oreille 594.5 586.6 773.0 1561.3 Coeur d' Alene River 10 92% 119% Priest Lake 46.1 53.0 67.6 119.3 St. Joe River 6 88% 117% Lake Coeur d' Alene 133.1 133.4 165.5 238.5 Spokane River 17 88% 116% Palouse River 2 106% 108% Kootenai ab Bonners Ferry 26 83% 126% Clearwater River Basin April 1, 2019

WATER SUPPLY OUTLOOK March was this water year’s driest month in the Clearwater Basin region, with monthly precipitation totals at only 41% of normal. This left the water year-to-date precipitation at 90% of average. The little precipitation that came in early March fell as snow, keeping April 1 snowpack totals at 95% of normal. Warmer temperatures in the latter half of the month facilitated melt at elevations below 5,000 feet, resulting in gradually rising streams and rivers across the region.

Dworshak reservoir capacity continues to sit at near normal volumes for this time of year. Streamflow forecasts for the April through July period range from 80% of normal for inflows into Dworshak to near-normal on the Selway River. As the Climate Prediction Center calls for warmer and wetter conditions this month, the snowpack at lower elevations will likely continue to melt out and increase regional streamflows. Clearwater River Basin Streamflow Forecasts - April 1, 2019 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 1530 1730 1860 97% 2000 2190 1920 APR-SEP 1600 1810 1960 97% 2100 2310 2020 Lochsa R nr Lowell APR-JUL 1110 1240 1330 94% 1430 1560 1410 APR-SEP 1160 1300 1400 95% 1500 1640 1480 Dworshak Reservoir Inflow 2 APR-JUL 1510 1790 1970 82% 2150 2430 2410 APR-SEP 1630 1910 2110 82% 2300 2590 2570 Clearwater R at Orofino APR-JUL 3180 3660 3990 93% 4330 4810 4310 APR-SEP 3330 3850 4200 93% 4560 5080 4540 Clearwater R at Spalding 2 APR-JUL 4790 5590 6130 89% 6670 7460 6890 APR-SEP 5060 5900 6470 89% 7050 7890 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 March Watershed Snowpack Analysis: April 1, 2019 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2019 2018 Dworshak Reservoir 2387.7 1553.0 2417.0 3468.0 NF Clearwater River 8 91% 118% Lochsa River 2 92% 126% Selway River 4 103% 134% Clearwater Basin Total 16 95% 122% Salmon River Basin April 1, 2019

WATER SUPPLY OUTLOOK March was relatively calm for the Salmon River basin, at least in terms of weather. Cooler than normal to seasonally normal temperatures left the snowpack relatively undisturbed in March, after the parade of storms brought copious new snow during February. As a whole, the vast Salmon River basin snowpack is above normal at ~110%, with lesser amounts to the east toward Montana and more in the Central Idaho Mountains that make up the Salmon River headwaters. Monthly precipitation was the lowest so far this water-year (63% of average), bringing water-year-to-date precipitation down to a normal level (101%). Weather forecasts for early April suggest a return to more active and wet weather, likely adding to the snowpack in the Salmon River headwaters while causing lower elevation snow to once again join the journey of the water cycle in the liquid phase!

February likely sealed the deal for a normal water supply in the Salmon River basin, and a few wet months leading up to the heat of summer could cause rivers to run bigger than expected, while a few dry months could result in slightly less than normal runoff. At this point, near normal to slightly above normal runoff volumes are anticipated. Water supplies should be plentiful for irrigation, fish, rafts/boats, fishing, and the many other recreational interests in the Salmon River. Salmon River Streamflow Forecasts - April 1, 2019 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 630 745 820 106% 900 1010 775 APR-SEP 730 860 945 105% 1030 1160 900 Lemhi R nr Lemhi APR-JUL 33 57 73 99% 89 113 74 APR-SEP 43 70 89 99% 108 135 90 MF Salmon R at MF Lodge APR-JUL 595 690 750 109% 810 900 690 APR-SEP 665 765 830 108% 900 1000 770 SF Salmon R nr Krassel Ranger Station APR-JUL 225 265 290 107% 315 350 270 APR-SEP 245 280 310 107% 335 370 290 Johnson Ck at Yellow Pine APR-JUL 155 184 205 107% 225 255 191 APR-SEP 166 196 215 105% 240 270 205 Salmon R at White Bird APR-JUL 4270 4860 5250 98% 5650 6240 5370 APR-SEP 4710 5360 5800 98% 6230 6880 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: April 1, 2019 # of % of Median Basin Name Sites 2019 2018 Salmon River ab Salmon 10 106% 111% Lemhi River 10 101% 122% MF Salmon River 3 108% 98% SF Salmon River 3 106% 89% Little Salmon River 4 141% 86% Salmon Basin Total 30 109% 107% West Central Basins April 1, 2019

WATER SUPPLY OUTLOOK Monthly precipitation for March was below normal for the West Central Basins, ranging from 50 to 70% of normal with Bogus Basin SNOTEL the only site receiving near average precipitation in March. Boise, Payette, and Weiser basin's April 1 water year-to-date remain above normal at 105%, 106%, and 115% respectively thanks to a fantastic February. March snowpack accumulation was not significant in the West Central basins, but again due to February’s storms, April snowpack is 130%for both Boise and Payette basins and 146% for Weiser basin. Snowmelt has begun below 6,000 ft in the West Central basins, and sites below 5000 ft are 25% melted or more relative to this year’s peak SWE. Significant melt has not yet occurred above 6,000 ft. Forecasts show temperatures and precipitation will likely be above average for April so we can expect snow melt to increase throughout the month.

Reservoirs were right around where they should be for April 1. The Boise system storage (Anderson Ranch, Arrowrock, and Lucky Peak reservoirs) is 58% full which is 98% of average with Arrowrock the lowest at 70% of average after being drawn down throughout March to make room for runoff as snow melts. The Payette System (Cascade and Deadwood Reservoirs) is 63% full and 97% of average. Median streamflow forecasts are all above average ranging from 110 to 140%. Water supplies will be sufficient for West Central basin water users. West Central Basins Streamflow Forecasts - April 1, 2019 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 510 565 605 127% 640 695 475 APR-SEP 545 600 640 125% 680 735 510 Boise R nr Twin Springs APR-JUL 525 595 640 109% 690 760 585 APR-SEP 570 645 695 109% 745 820 635 Mores Ck nr Arrowrock Dam APR-JUL 117 139 154 134% 169 191 115 APR-SEP 121 144 159 134% 175 198 119 Boise R nr Boise 2 APR-JUL 1300 1440 1530 121% 1630 1760 1260 APR-SEP 1390 1540 1650 121% 1750 1910 1360 Lake Fork Payette R nr McCall APR-JUL 82 90 96 120% 102 111 80 APR-SEP 84 93 99 119% 106 115 83 NF Payette R at Cascade 2 APR-JUL 450 515 560 115% 600 665 485 APR-SEP 460 530 575 116% 620 690 495 NF Payette R nr Banks 2 APR-JUL 575 665 725 116% 790 880 625 APR-SEP 585 680 745 116% 810 905 640 SF Payette R at Lowman APR-JUL 390 430 455 114% 480 520 400 APR-SEP 440 480 510 112% 540 585 455 Deadwood Reservoir Inflow 2 APR-JUL 113 126 135 110% 143 156 123 APR-SEP 120 134 144 110% 154 169 131 Payette R nr Horseshoe Bend 2 APR-JUL 1410 1600 1730 117% 1860 2060 1480 APR-SEP 1510 1710 1850 113% 1990 2190 1630 Weiser R nr Weiser APR-JUL 375 455 515 139% 575 670 370 APR-SEP 405 485 545 136% 610 710 400 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 March Watershed Snowpack Analysis: April 1, 2019 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2019 2018 Anderson Ranch Reservoir 281.5 388.9 245.3 450.2 SF Boise River 8 131% 86% Arrowrock Reservoir 134.2 252.1 190.4 272.2 MF & NF Boise Rivers 6 116% 79% Lucky Peak Reservoir 174.4 236.8 167.5 293.2 Mores Creek 4 126% 70% Sub-Basin Total 590.0 877.8 603.2 1015.6 Canyon Creek 2 191% 77% Deadwood Reservoir 90.0 114.0 90.6 161.9 Boise Basin Total 17 132% 78% Cascade Reservoir 447.1 541.3 462.1 693.2 NF Payette River 8 132% 83% Sub-Basin Total 537.0 655.3 552.7 855.1 SF Payette River 5 120% 88% Lake Lowell 135.5 143.1 113.0 165.2 Payette Basin Total 14 129% 83% Mann Creek Reservoir 5.6 7.7 8.7 11.1 Mann Creek 1 130% 71% Weiser Basin Total 8 146% 73% Wood & Lost River Basin April 1, 2019

WATER SUPPLY OUTLOOK After record snowfall during February, March followed with dry weather and little new snow accumulation. Above approximately 6,000 feet in elevation, minimal snowmelt occurred during March leaving end-of-month snowpack conditions similar to the beginning of March. Much above normal snowpack conditions still exist especially in the 5,500 to 6,500 ft elevation range. As the red line in the above snowpack chart illustrates, snow conditions have exceeded the normal (black dashed line) seasonal peak in the Wood & Lost River basins. Generally, April 1 snowpack conditions range from 120 to 160% of normal, except Fish Creek, which is nearly 200% of normal! March precipitation was generally half of normal across these basins, continuing a water-year trend of less than normal monthly precipitation but with February being the clear outlier (see precipitation graph above). Current weather forecasts suggest a return to more active weather during early April, likely adding to snowpack in the higher elevations and aiding snowmelt and runoff everywhere else.

Mackay Reservoir is 74% of capacity (106% of average), Little Wood is 19% full (28% of average), and Magic is 44% full (94% of average). All three reservoirs are expected to easily fill from this year’s snow-driven runoff. Streamflow forecasts reflect current snowpack conditions, and generally range from 130 to 180% of average for the runoff season. Water supplies will be more than adequate across the Wood & Lost River basins. Wood and Lost Basins Streamflow Forecasts - April 1, 2019 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 30 40 48 171% 56 69 28 Little Lost R bl Wet Ck nr Howe APR-JUL 19.4 25 28 100% 32 37 28 APR-SEP 23 30 35 103% 39 46 34 Big Lost R at Howell Ranch APR-JUL 168 193 210 132% 225 250 159 APR-SEP 188 215 235 131% 255 285 180 Big Lost R bl Mackay Reservoir APR-JUL 125 152 170 138% 189 215 123 APR-SEP 148 180 200 133% 225 255 150 Little Wood R ab High Five Ck APR-JUL 96 112 123 178% 135 154 69 APR-SEP 103 120 132 176% 145 165 75 Little Wood R nr Carey 2 APR-JUL 106 123 135 175% 148 168 77 APR-SEP 113 131 144 173% 158 180 83 Big Wood R at Hailey APR-JUL 210 250 275 117% 300 335 235 APR-SEP 235 275 305 115% 335 375 265 Big Wood R ab Magic Reservoir APR-JUL 122 173 210 124% 255 325 170 APR-SEP 130 183 225 124% 270 340 182 Camas Ck nr Blaine APR-JUL 129 155 174 212% 194 225 82 APR-SEP 129 155 175 211% 195 225 83 Big Wood R bl Magic Dam 2 APR-JUL 270 345 405 162% 465 560 250 APR-SEP 280 360 415 157% 475 575 265 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 March Watershed Snowpack Analysis: April 1, 2019 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2019 2018 Mackay Reservoir 33.0 37.5 31.2 44.4 Camas-Beaver Creeks 4 125% 94% Little Wood Reservoir 5.6 26.7 19.8 30.0 Birch-Medicine Lodge Creeks 4 106% 115% Magic Reservoir 84.0 184.5 89.3 191.5 Little Lost River 4 114% 119% Big Lost River ab Mackay 6 134% 99% Big Lost Basin Total 7 133% 101% Fish Creek 3 193% 73% Little Wood River 4 161% 89% Big Wood River ab Hailey 7 115% 99% Camas Creek 5 175% 67% Big Wood Basin Total 12 132% 90% Upper Snake River Basin April 1, 2019

WATER SUPPLY OUTLOOK The large regional storms the Upper Snake region saw in February came to an end in March. Monthly precipitation totals for all sites above American Falls were only 68% of average, though amounts varied greatly among individual basins. Water year-to-date precipitation remains near- average, and April 1 snowpack totals range from 95% of normal in the Gros Ventre basin to 140% in the Portneuf basin. The lower elevation drainages of southwest Wyoming and southeast Idaho started to see some melt and increasing streamflows mid-month but flows in rivers and streams in the upper elevation drainages remain below average as melting has not yet started higher up.

The region’s reservoir levels range from near normal capacities at Island Park, Henry’s Lake, Grassy Lake, and American Falls, up to 125% of normal at Palisades and 153% of normal at Jackson Lake. Streamflow forecasts for the April through July period range from near normal in headwaters streams and rivers to 150% of normal in Willow Creek near Ririe. Many of the Upper Snake streamflow forecast models do not use the widespread mid-elevation snow, so actual seasonal volumes may fall in the upper half of the forecast range of volumes (10% to 50% exceedance probabilities) because of the boost in runoff from this mid-elevation snow. Upper Snake River Basin Streamflow Forecasts - April 1, 2019 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 470 525 560 106% 600 655 530 APR-SEP 635 695 740 104% 780 845 710 Falls R nr Ashton 2 APR-JUL 335 375 405 111% 430 475 365 APR-SEP 410 460 495 114% 530 580 435 Teton R nr Driggs APR-JUL 127 152 170 110% 187 210 154 APR-SEP 158 190 210 109% 235 265 193 Teton R nr St Anthony APR-JUL 315 365 400 110% 435 485 365 APR-SEP 370 430 470 108% 510 575 435 Henrys Fk nr Rexburg 2 APR-JUL 1220 1370 1480 106% 1580 1740 1400 APR-SEP 1540 1740 1870 104% 2010 2200 1790 Snake R at Flagg Ranch APR-JUL 415 465 500 108% 540 590 465 APR-SEP 450 510 550 108% 590 645 510 Snake R nr Moran 2 APR-JUL 680 755 810 106% 860 940 765 APR-SEP 745 835 895 106% 955 1040 845 Pacific Ck at Moran APR-JUL 111 138 157 96% 175 200 164 APR-SEP 118 146 165 95% 184 215 173 Buffalo Fk ab Lava Ck nr Moran APR-JUL 225 255 275 98% 295 330 280 APR-SEP 250 285 315 98% 340 375 320 Snake R ab Reservoir nr Alpine 2 APR-JUL 1770 1960 2090 96% 2220 2410 2170 APR-SEP 2010 2240 2390 96% 2550 2780 2500 Greys R ab Reservoir nr Alpine APR-JUL 245 280 300 98% 320 355 305 APR-SEP 285 325 350 97% 375 415 360 Salt R ab Reservoir nr Etna APR-JUL 230 290 330 110% 370 430 300 APR-SEP 285 355 405 109% 450 520 370 Snake R nr Irwin 2 APR-JUL 2470 2770 2970 99% 3180 3480 3010 APR-SEP 2840 3200 3450 99% 3690 4050 3500 Snake R nr Heise 2 APR-JUL 2660 2980 3190 98% 3400 3720 3240 APR-SEP 3100 3470 3730 99% 3990 4360 3780 Willow Ck nr Ririe 2 APR-JUL 57 77 92 151% 109 135 61 Portneuf R at Topaz APR-JUL 59 71 80 125% 89 103 64 APR-SEP 74 89 99 122% 111 128 81 Snake R at Neeley 2 APR-JUL 1840 2440 2840 107% 3250 3840 2650 APR-SEP 1900 2550 3000 107% 3440 4090 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 March Watershed Snowpack Analysis: April 1, 2019 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2019 2018 Jackson Lake 660.7 655.7 430.7 847.0 Henrys Fork-Falls River 11 115% 113% Palisades Reservoir 1124.2 1109.4 902.8 1400.0 Teton River 9 116% 103% Sub-Basin Total 1784.9 1765.0 1333.5 2247.0 Henrys Fork ab Rexburg 20 115% 109% Henrys Lake 86.5 82.3 81.3 90.4 Snake River ab Jackson Lake 13 112% 116% Island Park Reservoir 108.8 119.5 111.8 135.2 Pacific Creek 4 116% 121% Grassy Lake 13.3 13.8 12.3 15.2 Buffalo Fork 4 97% 137% Sub-Basin Total 208.7 215.7 205.4 240.8 Gros Ventre River 5 95% 128% Ririe Reservoir 52.2 57.5 44.8 80.5 Hoback River 6 104% 126% Blackfoot Reservoir 276.8 296.8 186.4 337.0 Greys River 4 102% 119% American Falls Reservoir 1521.2 1575.0 1497.0 1672.6 Salt River 5 107% 104% Basin-Wide Total 3843.7 3910.0 3267.1 4577.9 Snake ab Palisades Resv 35 108% 117% Willow Creek - Ririe 7 138% 91% Blackfoot River 5 130% 92% Portneuf River 7 139% 72% Snake River ab American Falls 56 116% 108% Southside Snake River Basins April 1, 2019

WATER SUPPLY OUTLOOK Few March storms in the Southside Snake River region left end-of-month precipitation between 67% and 97% of average. As a result, water-year-to-date precipitation totals are around 110% of average. Strengthened by February’s productive storms, the basins’ April 1 snowpack levels are 120 to 140% of median, which helped to maintain above normal snow levels in this region. Melting has started to occur below 6,700 feet elevation and has reduced SWE levels by 50% or more at SNOTEL sites below 6,000 feet elevation. For example, Mud Flat and Reynolds Creek SNOTEL sites in the Owyhee Mountains have lost 5.0 and 4.2 inches of SWE, respectively, since March 1st.

Reservoir storage levels vary across this region. The five reservoirs’ current percent of capacity are: Brownlee 68%, Oakley 36%, Owyhee 61%, Salmon Falls 26%, and Wildhorse 72%. Streamflow forecasts range from 100 to 140% of average, except for Reynolds Creek forecast at 83% of average. Combining the streamflow forecasts with the reservoir storage will provide adequate irrigation supplies and a good river running season on the Owyhee and Bruneau rivers. Southside Snake River Basins Streamflow Forecasts - April 1, 2019 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 APR-JUL 17.9 23 26 139% 30 36 18.7 APR-SEP 19 24 28 140% 32 39 20 Trapper Ck nr Oakley APR-JUL 4.6 5.3 5.9 118% 6.4 7.3 5 APR-SEP 5.6 6.5 7.1 116% 7.8 8.8 6.1 Oakley Reservoir Inflow APR-JUL 20 26 30 125% 34 41 24 APR-SEP 22 28 33 127% 37 45 26 Salmon Falls Ck nr San Jacinto APR-JUL 65 81 93 133% 106 127 70 APR-SEP 69 85 98 132% 111 132 74 Bruneau R nr Hot Spring APR-JUL 170 210 240 131% 270 325 183 APR-SEP 177 220 250 130% 285 335 192 Reynolds Ck at Tollgate APR-JUL 4.2 5.5 6.4 83% 7.5 9.2 7.7 APR-SEP 4.2 5.5 6.5 82% 7.6 9.4 7.9 Owyhee R nr Gold Ck 2 APR-JUL 13.9 20 25 114% 30 39 22 Owyhee R nr Rome APR-JUL 184 275 350 101% 430 565 345 APR-SEP 200 295 365 100% 450 585 365 Owyhee R bl Owyhee Dam 2 APR-JUL 210 305 380 101% 460 595 375 APR-SEP 235 330 405 100% 490 620 405 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 March Watershed Snowpack Analysis: April 1, 2019 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2019 2018 Oakley Reservoir 27.4 40.3 29.6 75.6 Raft River 5 121% 64% Salmon Falls Reservoir 46.8 98.8 56.0 182.6 Goose-Trapper Creeks 6 122% 65% Wild Horse Reservoir 51.2 65.8 39.2 71.5 Salmon Falls Creek 8 132% 66% Lake Owyhee 434.4 569.8 495.8 715.0 Bruneau River 8 137% 59% Brownlee Reservoir 967.9 937.8 1102.0 1420.0 Reynolds Creek 1 ##### 100% Owyhee Basin Total 20 150% 34% Owyhee Basin Snotel Total 8 132% 47% Bear River Basin April 1, 2019

WATER SUPPLY OUTLOOK March’s storms left end-of-the-month precipitation totals for the Bear River Basin at about 105% of average, and water-year-to-date precipitation totals around 108% of average. March storms have increased snowpack levels for the basin to around 120% of median, an increase of about five percentage points since March 1. With the snowpack now reaching its peak, the snow water equivalent amounts are slightly better than last year on April 1 but much less than the 2017 totals. Reservoir storage in the Bear Lake is currently 140% of average, 66% full while Montpelier Reservoir is 87% of average, 42% full. Streamflow forecasts range from 105 to 115% of average across the basin and are highest in the Bear River headwaters in Utah. Water supplies should be adequate for most if not all the water users. Bear River Basin Streamflow Forecasts - April 1, 2019 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 93 112 125 112% 138 157 112 APR-SEP 104 125 140 114% 155 176 123 Bear R ab Resv nr Woodruff APR-JUL 62 107 137 113% 167 210 121 APR-SEP 66 114 147 115% 180 230 128 Big Ck nr Randolph APR-JUL 2.0 4.2 | 5.7 150% | 7.2 9.4 | 3.8 || Smiths Fk nr Border APR-JUL 72 87 97 109% 107 122 89 APR-SEP 86 103 114 110% 125 142 104 Bear R bl Stewart Dam 2 APR-JUL 87 150 192 105% 235 295 183 APR-SEP 97 167 215 105% 265 335 205 Little Bear at Paradise APR-JUL 30 41 48 107% 55 66 45 Logan R nr Logan APR-JUL 95 111 122 110% 133 149 111 Blacksmith Fk nr Hyrum APR-JUL 29 41 50 116% 59 71 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 March Watershed Snowpack Analysis: April 1, 2019 Current Average Capacity # of % of Median Reservoir Name Last YR Basin Name (KAF) (KAF) (KAF) Sites 2019 2018 Bear Lake 853.7 994.3 611.9 1302.0 Smiths-Thomas Forks 4 100% 102% Montpelier Reservoir 1.7 3.9 1.9 4.0 Bear River ab WY-ID Line 10 118% 88% Montpelier Creek 2 108% 86% Mink Creek 4 124% 74% Cub River 3 119% 76% Bear River ab ID-UT Line 24 120% 84% Malad River 3 165% 65% 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 Shawn Nield, State Soil Scientist Natural Resources Conservation Service Boise, Idaho

Prepared by Idaho Snow Survey Staff Ron Abramovich, Water Supply Specialist Danny Tappa, Hydrologist John Wilford, Electronics Technician Tina Andry, Hydrologist Earl Adsley, Trainee Hydrologist Cody Brown, Trainee Hydrologist

Forecasts and Assistance provided by Gus Goodbody, Forecast Hydrologist Jolyne Lea, Forecast Hydrologist Chris Brown, Forecast Hydrologist NRCS, National Water and Climate Center, Portland, Oregon

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!