Snake River Basin
February 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 - 182% (38% above last year), Pacific Creek - 188% (33% above last year), Gros Ventre River - 169% (31% above last year), Hoback River -174% (28% above last year), Greys River - 172% (36% above last year), Salt River - 167% (22% above last year), Snake above Palisades - 176% (33% 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 much above average last month. East Rim Divide was the lowest reporting site with 93 percent of average for January. January precipitation for the basin was 130 percent of average. Water year to date precipitation is 55 percent above normal for the Snake River basin. Year-to-date precipitation is 25 percent ahead of last year.

Reservoir.
Current storage compared to average for the three storage reservoirs in the basin is as follows; Grassy Lake 121% (13,100 acre feet), Jackson lake 144% (691,600 acre feet), and Palisades Reservoir 114% (1,188,700 acre feet). Grassy Lake is about the same as last years storage amount, while Jackson Lake is 2 percent more and Palisades Reservoir is 13 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 April to September period are forecast to be 46 to 76 percent above normal in the Snake River basin. The 50 percent chance yield from the Snake River above Palisades Reservoir is estimated to be 4,210,000 acre feet; this is 59 percent more than normal. The Snake at Moran is expected to yield 1,290,000 acre feet (48 percent above normal). Pacific Creek at Moran will probably yield 302,000 acre feet (76 percent more than average). Greys River near Palisades Reservoir is estimated to yield 565,000 acre feet (46 percent more than normal). Salt River near Etna is estimated to have a yield of 600,000 acre feet (50 percent above normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - February 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)                 APR-SEP      1091      1228    |     1290       148    |      1352      1489            869
                                                                    |                       |
SNAKE R abv Palisades Rsvr nr Alpine APR-SEP      3712      4009    |     4210       159    |      4411      4708           2647
                                                                    |                       |
SNAKE nr Heise (2)                   APR-SEP      5399      5846    |     6150       152    |      6454      6901           4048
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       262       286    |      302       176    |       318       342            172
                                                                    |                       |
GREYS R abv Palisades Reservoir      APR-SEP       469       526    |      565       146    |       604       661            388
                                                                    |                       |
SALT abv Reservoir nr Etna           APR-SEP       492       556    |      600       150    |       644       708            400
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 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     13.1     13.1     10.8 |   SNAKE above Jackson Lake   10       138       182
                                                                        |
JACKSON LAKE                           847.0    691.6    677.0    479.6 |   PACIFIC CREEK               3       133       188
                                                                        |
PALISADES                             1400.0   1188.7   1365.6   1043.6 |   GROS VENTRE RIVER           4       133       170
                                                                        |
                                                                        |   HOBACK RIVER                6       128       174
                                                                        |
                                                                        |   GREYS RIVER                 3       136       172
                                                                        |
                                                                        |   SALT RIVER                  5       122       167
                                                                        |
                                                                        |   SNAKE above Palisades      30       134       178
                                                                        |
==================================================================================================================================
 * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual flow 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 flow - actual flow may be affected by upstream water management.
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