Snake River Basin (1)
February 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 -- 118 percent (115% of last year at this time), Pacific Creek -- 124 percent (109% of last year at this time), Gros Ventre River -- 100 percent (100% of last year at this time), Hoback River - - 95 percent (102% of last year at this time), Greys River -- 94 percent (98% of last year at this time), Salt River -- 95 percent (93% of last year at this time). Snake River Basin above Palisades is 108 percent of average (105% 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 near average for last month. Monthly precipitation, for the basin, was 108 percent of average (75 percent of last January). January percentages range from 57 to 251 percent. Water-year- to-date precipitation is 97 percent of normal for the Snake River basin. This is 108 percent of last years total as of February 1. Year to date percentages range from 48 to 138 percent.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake -119 percent (12,900 acre feet compared to 8200 last year), Jackson lake - 127 percent (606,900 acre feet compared to 655,900 acre feet last year), and Palisades Reservoir -115 percent (1,196,000 acre feet compared to 1284,200 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 950,000 acre-feet (109 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,940,000 acre-feet (110 percent of normal). The 50 percent chance yield near Heise is expected to be 4,270,000 acre-feet (106 percent of normal). Pacific Creek at Moran is expected to yield about 204,000 acre-feet (123 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 385,000 acre- feet (97 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - February 1, 1999
==================================================================================================================================
                                              |  <<====== 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       759       890    |      950       109    |      1010      1141            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      2505      2764    |     2940       110    |      3116      3375           2671
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      3519      3966    |     4270       106    |      4574      5021           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       170       190    |      204       123    |       218       238            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       289       340    |      375        97    |       410       461            388
                                                                    |                       |
SALT near Etna                       APR-SEP       271       339    |      385        97    |       431       499            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 1, 1999
==================================================================================================================================
                                      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     12.9      8.1     10.8 |   SNAKE above Jackson Lake    9       115       118
                                                                        |
JACKSON LAKE                           847.0    606.9    655.9    479.6 |   PACIFIC CREEK               3       109       124
                                                                        |
PALISADES                             1400.0   1196.0   1284.2   1044.0 |   GROS VENTRE RIVER           4       100       102
                                                                        |
                                                                        |   HOBACK RIVER                6       102        95
                                                                        |
                                                                        |   GREYS RIVER                 3        98        94
                                                                        |
                                                                        |   SALT RIVER                  5        93        95
                                                                        |
                                                                        |   SNAKE above Palisades      29       105       108
                                                                        |
==================================================================================================================================
 * 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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