Snake River Basin (1)
April 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 -- 120 percent (136% of last year at this time), Pacific Creek -- 129 percent (134% of last year at this time), Gros Ventre River -- 111 percent (120% of last year at this time), Hoback River - - 104 percent (118% of last year at this time), Greys River -- 102 percent (117% of last year at this time), Salt River -- 106 percent (114% of last year at this time). Snake River Basin above Palisades is 113 percent of average (126% 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 above average for last month. Monthly precipitation, for the basin, was 73 percent of average (59 percent of last March). March percentages range from 44 to 97 percent of average. Water- year-to-date precipitation is 104 percent of normal for the Snake River basin. This is 117 percent of last years total as of April 1. Year-to-date percentages range from 61 to 141 percent of average.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake -118 percent (13,200 acre feet compared to 7,600 last year), Jackson lake - 126 percent (596,700 acre feet compared to 648,600 acre feet last year), and Palisades Reservoir -70 percent (713,700 acre feet compared to 983,300 acre feet last year).

Streamflow
The most probable April through September runoff yield forecast is near or above average for the basin. The Snake near Moran is expected to yield 1,030,000 acre-feet (119 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 3,205,000 acre-feet (120 percent of normal). The 50 percent chance yield near Heise is expected to be 4,690,000 acre-feet (116 percent of normal). Pacific Creek at Moran is expected to yield about 195,000 acre-feet (118 percent of average). Greys River above Palisades Reservoir is estimated to yield 390,000 acre-feet (101 percent of normal). Salt River near Etna is estimated to have a yield of 400,000 acre-feet (100 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - April 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       889       986    |     1030       119    |      1074      1171            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      2934      3095    |     3205       120    |      3315      3526           2671
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      4220      4500    |     4690       116    |      4880      5160           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       165       183    |      195       118    |       207       225            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       336       368    |      390       101    |       412       444            388
                                                                    |                       |
SALT near Etna                       APR-SEP       322       369    |      400       100    |       431       478            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 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     13.2      7.6     11.2 |   SNAKE above Jackson Lake    9       136       120
                                                                        |
JACKSON LAKE                           847.0    596.7    648.6    473.2 |   PACIFIC CREEK               3       134       129
                                                                        |
PALISADES                             1400.0    713.7    983.3   1014.0 |   GROS VENTRE RIVER           3       120       111
                                                                        |
                                                                        |   HOBACK RIVER                6       118       104
                                                                        |
                                                                        |   GREYS RIVER                 4       117       102
                                                                        |
                                                                        |   SALT RIVER                  5       114       106
                                                                        |
                                                                        |   SNAKE above Palisades      30       126       113
                                                                        |
==================================================================================================================================
 * 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.
(D) - The operational measurement of flow at this point has been discountinued.
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