Snake River Basin
April 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 - 148% (17% above last year), Pacific Creek - 162% (16% above last year), Gros Ventre River - 143% (15% above last year), Hoback River -147% (21% above last year), Greys River - 143% (25% above last year), Salt River - 130% (13% above last year). Snake River Basin above Palisades is estimated to be 46 percent above average (18% 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. Spring Creek Divide was the lowest reporting site (30 percent below average) and Cottonwood Creek was highest (71 percent above average) for March. March precipitation for the basin was 16 percent above average (12 percent ahead of last year). Water-year-to-date precipitation is 37 percent above normal for the Snake River basin. Year-to-date precipitation is 16 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% (13,40 acre feet), Jackson lake 136% (552,300 acre feet compared to 630,800 acre feet last year), and Palisades Reservoir 80% (397,000 acre feet compared to 683,000 acre feet last year). Grassy Lake is about the same as last years storage amount, while Jackson Lake is 12 percent less and Palisades Reservoir is 42 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 29 to 69 percent above normal in the Snake River basin. The 50 percent chance yield near Heise is expected to be 5,970,000 acre feet. Yield from the Snake River above Palisades Reservoir is estimated to be 3,945,000 acre feet; this is 49 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 291,000 acre feet (69 percent more than average). Greys River near Palisades Reservoir is estimated to yield 500,000 acre feet (29 percent more than normal). Salt River near Etna is estimated to have a yield of 530,000 acre feet (33 percent above normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - April 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      1150      1253    |     1300       150    |      1347      1450            869
                                                                    |                       |
SNAKE R abv Palisades Rsvr nr Alpine APR-SEP      3471      3753    |     3945       149    |      4137      4419           2647
                                                                    |                       |
SNAKE nr Heise (2)                   APR-SEP      5343      5717    |     5970       148    |      6223      6597           4048
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       261       279    |      291       169    |       303       321            172
                                                                    |                       |
GREYS R abv Palisades Reservoir      APR-SEP       432       472    |      500       129    |       528       568            388
                                                                    |                       |
SALT abv Reservoir nr Etna           APR-SEP       431       490    |      530       133    |       570       629            400
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 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     13.4     13.4     11.2 |   SNAKE above Jackson Lake   10       117       148
                                                                        |
JACKSON LAKE                           847.0    552.3    630.8    473.2 |   PACIFIC CREEK               3       116       162
                                                                        |
PALISADES                             1400.0    397.0    683.0   1013.5 |   GROS VENTRE RIVER           4       115       143
                                                                        |
                                                                        |   HOBACK RIVER                6       121       147
                                                                        |
                                                                        |   GREYS RIVER                 3       125       143
                                                                        |
                                                                        |   SALT RIVER                  5       113       130
                                                                        |
                                                                        |   SNAKE above Palisades      30       118       146
                                                                        |
==================================================================================================================================
 * 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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