Snake River Basin (1)
June 1997

Snow
Snowfall has been well above average in the basin this year. Percent of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson - 155% (23% below last year at this time), Pacific Creek - 176% (13% below last year at this time), Gros Ventre River - 123% (22% less than last year at this time), Hoback River -175% (38% less than last year at this time), Greys River - 159% (6% less than last year at this time), Salt River - 135% (27% less than last year at this time). Snake River Basin above Palisades is estimated to be 45 percent above average (27% less than last year at this time). See the Basin Summary of Snow Courses at the beginning of this report for a detailed listing of snow course information.

Precipitation.
Precipitation across the basin was above average last month. Granite Creek was the lowest reporting site (28 percent below average) and Snake River was highest (162 percent above average) for May. May precipitation for the basin was 7 percent above average (43 percent less than last year). Water-year-to-date precipitation is 33 percent above normal for the Snake River basin. Year-to-date precipitation is 6 percent ahead of last year.

Reservoir.
Current storage compared to average for the three storage reservoirs in the basin is as follows; Grassy Lake 111% (15,100 acre feet), Jackson lake 133% (717,900 acre feet compared to 665,100 acre feet last year), and Palisades Reservoir 87% (921,600 acre feet compared to 695,700 acre feet last year). Grassy Lake is about the same as last years storage amount, while Jackson Lake is 8 percent more and Palisades Reservoir is 32 percent more than last year at this time.

Streamflow.
Streamflow is forecast to be much above average for the basin. The 50 percent chance yields for the June to September period are forecast to be 29 to 82 percent above normal in the Snake River basin. The 50 percent chance yield near Heise is expected to be 4,557,000 acre feet (74 percent above normal). Yield from the Snake River above Palisades Reservoir is estimated to be 3,320,000 acre feet (82 percent more than normal). The Snake at Moran is expected to yield 1,024,000 acre feet (77 percent above normal). Pacific Creek at Moran will probably yield 171,000 acre feet (57 percent more than average). Greys River near Palisades Reservoir is estimated to yield 315,000 acre feet (31 percent more than normal). Salt River near Etna is estimated to have a yield of 296,000 acre feet (29 percent above normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - June 1, 1997
==================================================================================================================================
                                              |  <<====== Drier ======  Future Conditions  =======  Wetter =====>>  |             
                                              |                                                                     |
  Forecast Point                     Forecast |  ==================== Chance Of Exceeding * ======================  |
                                      Period  |    90%       70%    |  50% (Most Probable)  |      30%       10%    |  30-Yr Avg.
                                              | (1000AF)  (1000AF)  |   (1000AF)  (% AVG.)  |    (1000AF)  (1000AF) |    (1000AF)
====================================================================|=======================|=====================================
SNAKE nr Moran (1,2)                 JUN-SEP       868       975    |     1024       177    |      1073      1180            579
                                                                    |                       |
SNAKE R abv Palisades Rsvr nr Alpine JUN-SEP      3095      3229    |     3320       182    |      3411      3545           1823
                                                                    |                       |
SNAKE nr Heise (2)                   JUN-SEP      4118      4379    |     4557       174    |      4735      4996           2621
                                                                    |                       |
PACIFIC CREEK at Moran               JUN-SEP       146       161    |      171       157    |       181       196            109
                                                                    |                       |
GREYS R abv Palisades Reservoir      JUN-SEP       266       295    |      315       131    |       335       364            241
                                                                    |                       |
SALT abv Reservoir nr Etna           JUN-SEP       229       269    |      296       129    |       323       363            229
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of May                   |        Watershed Snowpack Analysis - June 1, 1997
==================================================================================================================================
                                      Usable |  *** Usable Storage ***  |                            Number     This Year as % of
Reservoir                            Capacity|  This      Last          |   Watershed                  of       =================
                                             |  Year      Year     Avg  |                          Data Sites   Last Yr   Average
========================================================================|=========================================================
GRASSY LAKE                             15.2     15.1     14.9     13.6 |   SNAKE above Jackson Lake    5        77       155
                                                                        |
JACKSON LAKE                           847.0    717.9    665.1    540.5 |   PACIFIC CREEK               2        87       176
                                                                        |
PALISADES                             1400.0    921.6    695.7   1055.2 |   GROS VENTRE RIVER           2        68       123
                                                                        |
                                                                        |   HOBACK RIVER                5        62       175
                                                                        |
                                                                        |   GREYS RIVER                 2        94       159
                                                                        |
                                                                        |   SALT RIVER                  3        73       135
                                                                        |
                                                                        |   SNAKE above Palisades      16        73       145
                                                                        |
==================================================================================================================================
 * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual volume will exceed the volumes in the table.

The average is computed for the 1961-1990 base period.

(1) - The values listed under the 10% and 90% Chance of Exceeding are actually 5% and 95% exceedance levels.                  
(2) - The value is natural volume - actual volume may be affected by upstream water management.
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