Snake River Basin
May 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 - 160% (14% above last year), Pacific Creek - 199% (22% above last year), Gros Ventre River - 145% (9% above last year), Hoback River -167% (14% above last year), Greys River - 174% (23% above last year), Salt River - 155% (23% above last year). Snake River Basin above Palisades is estimated to be 61 percent above average (17% above last year). 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. Gros Ventre Summit was the lowest reporting site (23 percent below average) and Cottonwood Creek was highest (125 percent above average) for April. April precipitation for the basin was 46 percent above average (7 percent ahead of last year). Water-year-to-date precipitation is 36 percent above normal for the Snake River basin. Year-to-date precipitation is 14 percent ahead of last year.

Reservoir.
Current storage compared to average for the three storage reservoirs in the basin is as follows; Grassy Lake 117% (13,700 acre feet), Jackson lake 95% (432,200 acre feet compared to 530,700 acre feet last year), and Palisades Reservoir 27% (259,900 acre feet compared to 406,400 acre feet last year). Grassy Lake is about the same as last years storage amount, while Jackson Lake is 19 percent less and Palisades Reservoir is 36 percent less 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 May to September period are forecast to be 33 to 63 percent above normal in the Snake River basin. The 50 percent chance yield near Heise is expected to be 5,540,000 acre feet (51 percent above normal). Yield from the Snake River above Palisades Reservoir is estimated to be 3,879,000 acre feet (60 percent more than normal). The Snake at Moran is expected to yield 1,210,000 acre feet (49 percent above normal). Pacific Creek at Moran will probably yield 266,000 acre feet (63 percent more than average). Greys River near Palisades Reservoir is estimated to yield 460,000 acre feet (33 percent more than normal). Salt River near Etna is estimated to have a yield of 465,000 acre feet (36 percent above normal).

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - May 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)                 MAY-SEP      1088      1172    |     1210       149    |      1248      1332            814
                                                                    |                       |
SNAKE R abv Palisades Rsvr nr Alpine MAY-SEP      3561      3750    |     3879       160    |      4008      4197           2431
                                                                    |                       |
SNAKE nr Heise (2)                   MAY-SEP      5093      5359    |     5540       151    |      5721      5987           3670
                                                                    |                       |
PACIFIC CREEK at Moran               MAY-SEP       237       254    |      266       163    |       278       295            163
                                                                    |                       |
GREYS R abv Palisades Reservoir      MAY-SEP       405       438    |      460       133    |       482       515            345
                                                                    |                       |
SALT abv Reservoir nr Etna           MAY-SEP       378       430    |      465       136    |       500       552            341
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 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     13.7     13.7     11.7 |   SNAKE above Jackson Lake    6       114       160
                                                                        |
JACKSON LAKE                           847.0    432.2    530.7    456.5 |   PACIFIC CREEK               2       122       199
                                                                        |
PALISADES                             1400.0    259.9    406.4    950.0 |   GROS VENTRE RIVER           4       109       145
                                                                        |
                                                                        |   HOBACK RIVER                6       114       167
                                                                        |
                                                                        |   GREYS RIVER                 3       123       174
                                                                        |
                                                                        |   SALT RIVER                  5       123       155
                                                                        |
                                                                        |   SNAKE above Palisades      24       117       161
                                                                        |
==================================================================================================================================
 * 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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