Snake River Basin (1)
May 1998

Snow
Snowfall has been close to average in the basin this year. Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 93 percent (58% of last year at this time), Pacific Creek -- 109 percent (55% of last year at this time), Gros Ventre River -- 99 percent (68% of last year at this time), Hoback River -- 97 percent (58% of last year at this time), Greys River -- 88 percent (51% of last year at this time), Salt River -- 83 percent (53% of last year at this time). Snake River Basin above Palisades is estimated to be 95 percent of average (59% 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 just below average last month. Monthly precipitation, for the basin, was 92 percent of average (62 percent of last year). Granite Creek SNOTEL reported the lowest percentage of average (46 percent) and Snake River Station SNOTEL reported the highest percentage of average (137 percent) for April. Water-year-to-date precipitation is 86 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 64 percent (7,500 acre feet compared to 13,700 last year), Jackson lake 145 percent (663,200 acre feet compared to 432,200 acre feet last year), and Palisades Reservoir 96% (910,600 acre feet compared to 259,900 acre feet last year).

Streamflow
The most probable May through September runoff yield forecast is near average for the basin. The Snake at Moran is expected to yield 775,000 acre feet (95 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,489,000 acre feet (101 percent of normal). The 50 percent chance yield near Heise is expected to be 3,616,000 acre feet (99 percent of normal). Pacific Creek at Moran is expected to yield about 151,000 acre feet (96 percent of average). Greys River above Palisades Reservoir is estimated to yield 322,000 acre feet (92 percent of normal). Salt River near Etna is estimated to have a yield of 310,000 acre feet (91 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - May 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)               MAY-SEP       648       735    |      775        95    |       815       902            814
                                                                    |                       |
SNAKE above Palisades (2)            MAY-SEP      2253      2394    |     2489       101    |      2584      2725           2475
                                                                    |                       |
SNAKE near Heise (2)                 MAY-SEP      3227      3458    |     3616        99    |      3774      4005           3672
                                                                    |                       |
PACIFIC CREEK at Moran               MAY-SEP       123       140    |      151        96    |       162       179            157
                                                                    |                       |
GREYS above Palisades                MAY-SEP       282       306    |      322        92    |       338       362            350
                                                                    |                       |
SALT near Etna                       MAY-SEP       247       285    |      310        91    |       335       373            340
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 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      7.5     13.7     11.7 |   SNAKE above Jackson Lake    5        58        93
                                                                        |
JACKSON LAKE                           847.0    663.2    432.2    456.5 |   PACIFIC CREEK               2        55       109
                                                                        |
PALISADES                             1400.0    910.6    259.9    950.0 |   GROS VENTRE RIVER           4        68        99
                                                                        |
                                                                        |   HOBACK RIVER                6        58        97
                                                                        |
                                                                        |   GREYS RIVER                 3        51        88
                                                                        |
                                                                        |   SALT RIVER                  5        53        83
                                                                        |
                                                                        |   SNAKE above Palisades      23        59        95
                                                                        |
==================================================================================================================================
 * 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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