Snake River Basin (1)
January 1998

Snow
Snowfall has been well below average in the basin this year. Percent of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake - 75% (36% of last year at this time), Pacific Creek - 80% (37% of last year at this time), Gros Ventre River - 63% (33% of last year at this time), Hoback River -51% (22% of last year at this time), Greys River - 54% (27% of last year at this time), Salt River - 64% (31% of last year at this time). Snake River Basin above Palisades is estimated to be 68 percent of average (32% 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. Bondurant was the lowest reporting site (27 percent of average) and Bedford was highest (84 percent of average) for December. Monthly precipitation for the basin was 43 percent of average (16 percent of last year). Water-year-to-date precipitation is 64 percent of normal for the Snake River basin. Year-to-date precipitation is 63 percent behind last year.

Reservoir.
Current storage compared to average for the three storage reservoirs in the basin is as follows; Grassy Lake 78% (8,200 acre feet compared to 12,700 last year), Jackson lake 137% (644,300 acre feet compared to 680,000 acre feet last year), and Palisades Reservoir 126% (1,308,400 acre feet compared to 1,244,900 acre feet last year).

Streamflow.
Streamflow is forecast to be below average for the basin. The 50 percent chance yields for the April to September period are forecast to be 14 to 24 percent below normal in the Snake River basin. The 50 percent chance yield near Heise is expected to be 3,280,000 acre feet (81 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,296,000 acre feet (14 percent less than normal). The Snake at Moran is expected to yield 695,000 acre feet (80 percent of normal). Pacific Creek at Moran will probably yield 127,000 acre feet (77 percent of average). Greys River near Palisades Reservoir is estimated to yield 300,000 acre feet (77 percent of normal). Salt River near Etna is estimated to have a yield of 305,000 acre feet (76 percent of normal). 
==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - January 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       459       621    |      695        80    |       769       931            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      1723      2064    |     2296        86    |      2528      2869           2671
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      2367      2911    |     3280        81    |      3649      4193           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP        83       109    |      127        77    |       145       171            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       190       255    |      300        77    |       345       410            388
                                                                    |                       |
SALT near Etna                       APR-SEP       171       251    |      305        76    |       359       439            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of December              |        Watershed Snowpack Analysis - January 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      8.2     12.7     10.5 |   SNAKE above Jackson Lake    9        36        75
                                                                        |
JACKSON LAKE                           847.0    644.3    680.0    470.2 |   PACIFIC CREEK               3        37        80
                                                                        |
PALISADES                             1400.0   1308.4   1244.9   1035.6 |   GROS VENTRE RIVER           2        33        63
                                                                        |
                                                                        |   HOBACK RIVER                5        22        51
                                                                        |
                                                                        |   GREYS RIVER                 2        27        54
                                                                        |
                                                                        |   SALT RIVER                  4        31        64
                                                                        |
                                                                        |   SNAKE above Palisades      21        32        68
                                                                        |
==================================================================================================================================
 * 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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