Snake River Basin (1)
March 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 - 92% (57% of last year at this time), Pacific Creek - 100% (59% of last year at this time), Gros Ventre River - 98% (67% of last year at this time), Hoback River -82% (55% of last year at this time), Greys River - 92% (61% of last year at this time), Salt River - 98% (69% of last year at this time). Snake River Basin above Palisades is estimated to be 93 percent of average (60% 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 below average last month. Base Camp was the lowest reporting site (29 percent of average) and Jackson was the highest (120 percent of average) for February. Monthly precipitation for the basin was 52 percent of average (82 percent of last year). Water-year-to-date precipitation is 81 percent of normal for the Snake River basin. Year-to-date precipitation is 43 percent behind last year.

Reservoir.
Current storage compared to average for the three storage reservoirs in the basin is as follows; Grassy Lake 71% (7,800 acre feet compared to 13,200 last year), Jackson lake 135% (649,600 acre feet compared to 652,300 acre feet last year), and Palisades Reservoir 113% (1,204,500 acre feet compared to 854,700 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 3,680,000 acre feet (91 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,542,000 acre feet (95 percent of normal). The Snake at Moran is expected to yield 785,000 acre feet (90 percent of normal). Pacific Creek at Moran will probably yield 170,000 acre feet (102 percent of average). Greys River above Palisades Reservoir is estimated to yield 345,000 acre feet (89 percent of normal). Salt River near Etna is estimated to have a yield of 383,000 acre feet (96 percent of normal).

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - March 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       606       729    |      785        90    |       841       964            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      2181      2396    |     2542        95    |      2688      2903           2671
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      3064      3431    |     3680        91    |      3929      4296           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       135       156    |      170       102    |       184       205            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       271       315    |      345        89    |       375       419            388
                                                                    |                       |
SALT near Etna                       APR-SEP       276       340    |      383        96    |       426       490            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of February              |        Watershed Snowpack Analysis - March 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      7.8     13.2     11.0 |   SNAKE above Jackson Lake    9        57        92
                                                                        |
JACKSON LAKE                           847.0    649.6    652.3    481.0 |   PACIFIC CREEK               3        59       100
                                                                        |
PALISADES                             1400.0   1204.5    854.7   1063.1 |   GROS VENTRE RIVER           4        67        98
                                                                        |
                                                                        |   HOBACK RIVER                6        55        82
                                                                        |
                                                                        |   GREYS RIVER                 3        61        92
                                                                        |
                                                                        |   SALT RIVER                  5        69        98
                                                                        |
                                                                        |   SNAKE above Palisades      29        60        93
                                                                        |
==================================================================================================================================
 * 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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