Snake River Basin (1)
February 2002

Snow
The Snake River basin snow water equivalent (SWE) is below normal. Snake above Jackson Lake is 82 percent of average (164% of last year at this time). Pacific Creek is 86 percent of average (157% of last year at this time). Gros Ventre River is 87 percent of average (149% of last year at this time). Hoback River is 82 percent of average (139% of last year at this time), Greys River is 83 percent of average (138% of last year at this time). Salt River is 82 percent of average (131% of last year at this time). Snake River Basin above Palisades is 82 percent of average (149% 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 99 percent of average. Last months percentages range from 46 to 95 percent of average. Water-year-to-date precipitation is 87 percent of normal for the Snake River basin (143 percent of last year at this time) Year-to-date percentages range from 76 to 96 percent of average.

Reservoir.
Current reservoir storage compared to average for the three storage reservoirs in the basin is below average. Grassy Lake storage is about 80 percent of average (9,400 acre feet compared to 12,700 last year). Jackson Lake storage is 30 percent of average (146,300 acre feet compared to 635,200 acre feet last year). Palisades Reservoir storage is about 47 percent of average (489,200 acre feet compared to 638,700 acre feet last year).

Streamflow.
The most probable runoff, based on the 50 percent chance yield, for April through September runoff is forecast below average for the basin. The Snake near Moran is expected to yield 775,000 acre-feet (86 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,455,000 acre-feet (90 percent of normal). The 50 percent chance yield near Heise is expected to be 3,450,000 acre-feet (83 percent of normal). Pacific Creek at Moran is expected to yield about 149,000 acre-feet (84 percent of average). Greys River above Palisades Reservoir is estimated to yield 355,000 acre-feet (90 percent of normal). Salt River near Etna is estimated to have a yield of 370,000 acre-feet (88 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - February 1, 2002
==================================================================================================================================
                                              |  <<====== 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       584       715    |      775        86    |       835       966            905
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      2020      2279    |     2455        90    |      2631      2890           2730
                                                                    |                       |
PALISADES RESERVOIR INFLOW (1,2)     APR-SEP      2379      2978    |     3250        84    |      3522      4121           3870
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      2699      3146    |     3450        83    |      3754      4201           4160
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       115       135    |      149        84    |       163       183            178
                                                                    |                       |
GREYS above Palisades                APR-SEP       269       320    |      355        90    |       390       441            395
                                                                    |                       |
SALT near Etna                       APR-SEP       256       324    |      370        88    |       416       484            420
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 1, 2002
==================================================================================================================================
                                      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      9.4     12.7     11.8 |   SNAKE above Jackson Lake    9       164        82
                                                                        |
JACKSON LAKE                           847.0    146.3    635.2    490.1 |   PACIFIC CREEK               3       157        86
                                                                        |
PALISADES                             1400.0    489.2    638.7   1040.3 |   GROS VENTRE RIVER           4       149        85
                                                                        |
                                                                        |   HOBACK RIVER                6       139        82
                                                                        |
                                                                        |   GREYS RIVER                 5       138        83
                                                                        |
                                                                        |   SALT RIVER                  5       131        82
                                                                        |
                                                                        |   SNAKE above Palisades      30       149        82
                                                                        |
==================================================================================================================================
 * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual volume will exceed the volumes in the table.

(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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