Snake River Basin (1)
April 2000

Snow
Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 87 percent (73% of last year at this time), Pacific Creek -- 95 percent (73% of last year at this time), Gros Ventre River -- 80 percent (75% of last year at this time), Hoback River -- 83 percent (79% of last year at this time), Greys River -- 89 percent (86% of last year at this time), Salt River -- 95 percent (90% of last year at this time). Snake River Basin above Palisades is 88 percent of average (78% 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 for last month. Monthly precipitation, for the basin, was 86 percent of average (118 percent of last year). March percentages range from 44 to 118 percent of average. Water-year-to-date precipitation is 76 percent of normal for the Snake River basin (75 percent of last year at this time) Year-to-date percentages range from 60 to 89 percent of average.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake -113 percent of average (12,700 acre feet compared to 13,200 last year), Jackson lake - 139 percent of average (657,400 acre feet compared to 596,700 acre feet last year), and Palisades Reservoir -117 percent of average (1,188,800 acre feet compared to 713,700 acre feet last year).

Streamflow
The most probable, 50 percent chance, April through September runoff yield forecast is below average for the basin. The Snake near Moran is expected to yield 745,000 acre-feet (86 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,325,000 acre-feet (87 percent of normal). The 50 percent chance yield near Heise is expected to be 3,450,000 acre-feet (85 percent of normal). Pacific Creek at Moran is expected to yield about 136,000 acre-feet (82 percent of average). Greys River above Palisades Reservoir is estimated to yield 310,000 acre-feet (80 percent of normal). Salt River near Etna is estimated to have a yield of 319,000 acre-feet (80 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - April 1, 2000
==================================================================================================================================
                                              |  <<====== 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       604       701    |      745        86    |       789       886            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      2054      2215    |     2325        87    |      2435      2596           2671
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      2980      3260    |     3450        85    |      3640      3920           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       106       124    |      136        82    |       148       166            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       256       288    |      310        80    |       332       364            388
                                                                    |                       |
SALT near Etna                       APR-SEP       241       288    |      319        80    |       350       397            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 1, 2000
==================================================================================================================================
                                      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     12.7     13.2     11.2 |   SNAKE above Jackson Lake    9        73        87
                                                                        |
JACKSON LAKE                           847.0    657.4    596.7    473.2 |   PACIFIC CREEK               3        73        95
                                                                        |
PALISADES                             1400.0   1188.8    713.7   1014.0 |   GROS VENTRE RIVER           4        76        80
                                                                        |
                                                                        |   HOBACK RIVER                6        79        83
                                                                        |
                                                                        |   GREYS RIVER                 5        86        89
                                                                        |
                                                                        |   SALT RIVER                  5        90        95
                                                                        |
                                                                        |   SNAKE above Palisades      30        78        88
                                                                        |
==================================================================================================================================
 * 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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