Snake River Basin (1)
February 1998

Snow
Snowfall has been close to average in the basin this year. Percent of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake - 102% (56% of last year at this time), Pacific Creek - 113% (60% of last year at this time), Gros Ventre River - 96% (57% of last year at this time), Hoback River -89% (49% of last year at this time), Greys River - 96% (56% of last year at this time), Salt River - 102% (61% of last year at this time). Snake River Basin above Palisades is estimated to be 101 percent of average (56% of 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. East Rim Divide was the lowest reporting site (122 percent of average) and Bedford was highest (224 percent of average) for January. Monthly precipitation for the basin was 144 percent of average (105 percent of last year). Water-year-to-date precipitation is 88 percent of normal for the Snake River basin. Year-to-date precipitation is 45 percent behind last year.

Reservoir.
Current storage compared to average for the three storage reservoirs in the basin is as follows; Grassy Lake 75% (8,100 acre feet compared to 13,100 last year), Jackson lake 137% (655,900 acre feet compared to 691,600 acre feet last year), and Palisades Reservoir 123% (1,284,200 acre feet compared to 1,188,700 acre feet last year).

Streamflow.
Streamflow is forecast to be near to slightly below average for the basin. The 50 percent chance yield near Heise is expected to be 3,730,000 acre feet (92 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,464,000 acre feet (92 percent of normal). The Snake at Moran is expected to yield 810,000 acre feet (93 percent of normal). Pacific Creek at Moran will probably yield 177,000 acre feet (107 percent of average). Greys River above Palisades Reservoir is estimated to yield 355,000 acre feet (92 percent of normal). Salt River near Etna is estimated to have a yield of 380,000 acre feet (95 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - February 1, 1998
==================================================================================================================================
                                              |  <<====== 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 near Moran (1,2)               APR-SEP       619       750    |      810        93    |       870      1001            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      2029      2288    |     2464        92    |      2640      2899           2671
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      2979      3426    |     3730        92    |      4034      4481           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       143       163    |      177       107    |       191       211            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       269       320    |      355        92    |       390       441            388
                                                                    |                       |
SALT near Etna                       APR-SEP       266       334    |      380        95    |       426       494            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 1, 1998
==================================================================================================================================
                                      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      8.1     13.1     10.8 |   SNAKE above Jackson Lake    9        56       102
                                                                        |
JACKSON LAKE                           847.0    655.9    691.6    479.6 |   PACIFIC CREEK               3        60       113
                                                                        |
PALISADES                             1400.0   1284.2   1188.7   1043.6 |   GROS VENTRE RIVER           4        57        96
                                                                        |
                                                                        |   HOBACK RIVER                6        49        89
                                                                        |
                                                                        |   GREYS RIVER                 3        56        96
                                                                        |
                                                                        |   SALT RIVER                  5        61       102
                                                                        |
                                                                        |   SNAKE above Palisades      29        56       101
                                                                        |
==================================================================================================================================
 * 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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