Snake River Basin (1)
April 1998

Snow
Snowfall has been close to average in the basin this year. Percent of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 88 percent (60% of last year at this time), Pacific Creek -- 96 percent (59% of last year at this time), Gros Ventre River -- 91 percent (50% of last year at this time), Hoback River -- 84 percent (57% of last year at this time), Greys River -- 91 percent (63% of last year at this time), Salt River -- 93 percent (71% of last year at this time). Snake River Basin above Palisades is estimated to be 90 percent of average (63% 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 last month. Monthly precipitation, for the basin, was 121 percent of average (108 percent of last year). Phillips Bench SNOTEL reported the lowest percentage of average (78 percent) and East Rim Divide SNOTEL reported the highest percentage of average (193 percent) for March. Water-year-to-date precipitation is 88 percent of normal for the Snake River basin. Year-to-date precipitation is 63 percent of last year.

Reservoir.
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake 68% (7,600 acre feet compared to 13,400 last year), Jackson lake 137% (648,600 acre feet compared to 552,300 acre feet last year), and Palisades Reservoir 97% (983,300 acre feet compared to 397,000 acre feet last year).

Streamflow.
The most probable April through September runoff yield forecast is near average for the basin. The Snake at Moran is expected to yield 825,000 acre feet (95 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,585,000 acre feet (97 percent of normal). The 50 percent chance yield near Heise is expected to be 3,790,000 acre feet (94 percent of normal). Pacific Creek at Moran is expected to yield about 164,000 acre feet (99 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 395,000 acre feet (99 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - April 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       684       781    |      825        95    |       869       966            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      2314      2475    |     2585        97    |      2695      2856           2671
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      3320      3600    |     3790        94    |      3980      4260           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       134       152    |      164        99    |       176       194            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       321       353    |      375        97    |       397       429            388
                                                                    |                       |
SALT near Etna                       APR-SEP       317       364    |      395        99    |       426       473            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 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      7.6     13.4     11.2 |   SNAKE above Jackson Lake    9        60        88
                                                                        |
JACKSON LAKE                           847.0    648.6    552.3    473.2 |   PACIFIC CREEK               3        59        96
                                                                        |
PALISADES                             1400.0    983.3    397.0   1013.5 |   GROS VENTRE RIVER           4        50        91
                                                                        |
                                                                        |   HOBACK RIVER                6        57        84
                                                                        |
                                                                        |   GREYS RIVER                 3        63        91
                                                                        |
                                                                        |   SALT RIVER                  5        71        93
                                                                        |
                                                                        |   SNAKE above Palisades      29        60        90
                                                                        |
==================================================================================================================================
 * 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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