Snake River Basin
March 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% (21% above last year), Pacific Creek - 169% (18% above last year), Gros Ventre River - 147% (14% above last year), Hoback River -143% (13% above last year), Greys River - 151% (26% above last year), Salt River - 141% (14% above last year). Snake River Basin above Palisades is estimated to be 54 percent above average (20% 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 below average last month. Afton was the lowest reporting site with 38 percent of average for February. February precipitation for the basin was 36 percent below average(36 percent below last year). Water-year-to-date precipitation is 37 percent above normal for the Snake River basin. Year-to-date precipitation is 15 percent ahead of last year.

Reservoir.
Current storage compared to average for the three storage reservoirs in the basin is as follows; Grassy Lake 120% (13,200 acre feet), Jackson lake 136% (652,300 acre feet), and Palisades Reservoir 80% (1,188,400 acre feet). Grassy Lake is about the same as last years storage amount (1 percent less), while Jackson Lake is 3 percent less and Palisades Reservoir is 12 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 35 to 71 percent above normal in the Snake River basin. Yield from the Snake River above Palisades Reservoir is estimated to be 3,985,000 acre feet; this is 51 percent more than normal. The Snake at Moran is expected to yield 1,300,000 acre feet (50 percent above normal). Pacific Creek at Moran will probably yield 294,000 acre feet (71 percent more than average). Greys River near Palisades Reservoir is estimated to yield 525,000 acre feet (35 percent more than normal). Salt River near Etna is estimated to have a yield of 540,000 acre feet (35 percent above normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - March 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      1131      1247    |     1300       150    |      1353      1469            869
                                                                    |                       |
SNAKE R abv Palisades Rsvr nr Alpine APR-SEP      3530      3801    |     3985       151    |      4169      4440           2647
                                                                    |                       |
SNAKE nr Heise (2)                   APR-SEP      5256      5665    |     5943       147    |      6221      6630           4048
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       257       279    |      294       171    |       309       331            172
                                                                    |                       |
GREYS R abv Palisades Reservoir      APR-SEP       444       492    |      525       135    |       558       606            388
                                                                    |                       |
SALT abv Reservoir nr Etna           APR-SEP       429       495    |      540       135    |       585       651            400
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of February              |        Watershed Snowpack Analysis - March 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     13.2     13.3     11.0 |   SNAKE above Jackson Lake   10       121       160
                                                                        |
JACKSON LAKE                           847.0    652.3    674.1    481.0 |   PACIFIC CREEK               3       118       169
                                                                        |
PALISADES                             1400.0    854.7   1188.4   1063.1 |   GROS VENTRE RIVER           4       114       147
                                                                        |
                                                                        |   HOBACK RIVER                6       113       143
                                                                        |
                                                                        |   GREYS RIVER                 3       126       151
                                                                        |
                                                                        |   SALT RIVER                  5       114       141
                                                                        |
                                                                        |   SNAKE above Palisades      30       120       154
                                                                        |
==================================================================================================================================
 * 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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