Snake River Basin (1)
May 2001

Snow
Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 46 percent (85% of last year at this time), Pacific Creek -- 55 percent (86% of last year at this time), Gros Ventre River -- 63 percent (84% of last year at this time), Hoback River -- 54 percent (87% of last year at this time), Greys River - 54 percent (80% of last year at this time), Salt River -- 36 percent (66% of last year at this time). Snake River Basin above Palisades is 48 percent of average (81% 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 much below average for last month. Monthly precipitation, for the basin, was 150 percent of average (162 percent of last year). April percentages range from 83 to 177 percent of average. Water-year-to-date precipitation is 76 percent of normal for the Snake River basin (85 percent of last year at this time). Year-to-date percentages range from 58 to 102 percent of average.

Reservoir
Current usable storage compared to average for the three reservoirs in the basin is as follows: Grassy Lake -115 percent of average (13,400 acre feet compared to 13,000 last year), Jackson lake - 145 percent of average (663,400 acre feet compared to 716,600 acre feet last year), and Palisades Reservoir - 90 percent of average (858,800 acre feet compared to 1,161,100 acre feet last year).

Streamflow
The most probable, 50 percent chance May through September runoff yield forecast is much below average for the basin. The Snake near Moran is expected to yield 475,000 acre-feet (58 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 1,585,000 acre-feet (64 percent of normal). The 50 percent chance yield near Heise is expected to be 2,055,000 acre-feet (60 percent of normal). Pacific Creek at Moran is expected to yield about 88,000 acre-feet (56 percent of average). Greys River above Palisades Reservoir is estimated to yield 182,000 acre-feet (52 percent of normal). Salt River near Etna is estimated to have a yield of 156,000 acre-feet (46 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - May 1, 2001
==================================================================================================================================
                                              |  <<====== 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       348       435    |      475        58    |       515       602            814
                                                                    |                       |
SNAKE above Palisades (2)            MAY-SEP      1349      1490    |     1585        64    |      1680      1821           2475
                                                                    |                       |
PALISADES RESERVOIR INFLOW (1,2)     MAY-SEP      1592      1910    |     2055        60    |      2200      2518           3428
                                                                    |                       |
SNAKE near Heise (2)                 MAY-SEP      1811      2042    |     2200        60    |      2358      2589           3672
                                                                    |                       |
PACIFIC CREEK at Moran               MAY-SEP        60        77    |       88        56    |        99       116            157
                                                                    |                       |
GREYS above Palisades                MAY-SEP       142       166    |      182        52    |       198       222            350
                                                                    |                       |
SALT near Etna                       MAY-SEP        93       131    |      156        46    |       181       219            339
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 1, 2001
==================================================================================================================================
                                      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     13.4     13.0     11.7 |   SNAKE above Jackson Lake    6        85        46
                                                                        |
JACKSON LAKE                           847.0    663.4    716.6    456.5 |   PACIFIC CREEK               2        86        55
                                                                        |
PALISADES                             1400.0    858.8   1161.1    950.0 |   GROS VENTRE RIVER           2        84        63
                                                                        |
                                                                        |   HOBACK RIVER                6        91        57
                                                                        |
                                                                        |   GREYS RIVER                 5        80        54
                                                                        |
                                                                        |   SALT RIVER                  5        66        36
                                                                        |
                                                                        |   SNAKE above Palisades      23        82        48
                                                                        |
==================================================================================================================================
 * 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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