Snake River Basin (1) January 1999

Snow
Snowfall has been close to average in the basin so far this year. Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 109 percent (151% of last year at this time), Pacific Creek -- 113 percent (149% of last year at this time), Gros Ventre River -- 107 percent (143% of last year at this time), Hoback River - - 94 percent (167% of last year at this time), Greys River -- 93 percent (173% of last year at this time), Salt River -- 97 percent (160% of last year at this time). Snake River Basin above Palisades is 105 percent of average (155% 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. Monthly precipitation, for the basin, was 85 percent of average (106 percent of last December). December percentages range from 18 to 100 percent. Water-year- to-date precipitation is 94 percent of normal for the Snake River basin. This is 147 percent of last years total as of January 1. Year to date percentages range from 45 to 112 percent.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake 120 percent (12,600 acre feet compared to 8200 last year), Jackson lake 125 percent (590,000 acre feet compared to 644,300 acre feet last year), and Palisades Reservoir 117 percent (1,207,300 acre feet compared to 1308,400 acre feet last year).

Streamflow
The most probable April through September runoff yield forecast is near average for the basin. The Snake near Moran is expected to yield 890,000 acre feet (102 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,858,000 acre feet (107 percent of normal). The 50 percent chance yield near Heise is expected to be 4,100,000 acre feet (101 percent of normal). Pacific Creek at Moran is expected to yield about 173,000 acre feet (104 percent of average). Greys River above Palisades Reservoir is estimated to yield 375,000 acre feet (97 percent of normal). Salt River near Etna is estimated to have a yield of 380,000 acre feet (95 percent of normal).

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                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - June 1, 1998
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                                              |  <<====== 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)               JUN-SEP       379       486    |      535        92    |       584       691            579
                                                                    |                       |
SNAKE above Palisades (2)            JUN-SEP      1655      1797    |     1894       104    |      1991      2133           1823
                                                                    |                       |
SNAKE near Heise (2)                 JUN-SEP      2303      2528    |     2680       102    |      2832      3057           2622
                                                                    |                       |
PACIFIC CREEK at Moran               JUN-SEP        72        86    |       96        92    |       106       120            104
                                                                    |                       |
GREYS above Palisades                JUN-SEP       169       192    |      207        86    |       222       245            241
                                                                    |                       |
SALT near Etna                       JUN-SEP       173       200    |      219        96    |       238       265            228
                                                                    |                       |
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                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of May                   |        Watershed Snowpack Analysis - June 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     14.9     15.1     13.6 |   SNAKE above Jackson Lake    4        58        90
                                                                        |
JACKSON LAKE                           847.0    757.7    717.9    540.5 |   PACIFIC CREEK               2        60       106
                                                                        |
PALISADES                             1400.0   1121.1    921.6   1055.2 |   GROS VENTRE RIVER           2        69        86
                                                                        |
                                                                        |   HOBACK RIVER                5        33        58
                                                                        |
                                                                        |   GREYS RIVER                 2        38        60
                                                                        |
                                                                        |   SALT RIVER                  3        26        35
                                                                        |
                                                                        |   SNAKE above Palisades      15        52        76
                                                                        |
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 * 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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