Snake River Basin (1)
April 2001

Snow
Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 52 percent (59% of last year at this time), Pacific Creek -- 57 percent (60% of last year at this time), Gros Ventre River -- 63 percent (76% of last year at this time), Hoback River -- 60 percent (73% of last year at this time), Greys River - 58 percent (65% of last year at this time), Salt River -- 56 percent (59% of last year at this time). Snake River Basin above Palisades is 55 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 much below average for last month. Monthly precipitation, for the basin, was 85 percent of average (78 percent of last year). March percentages range from 29 to 120 percent of average. Water-year-to-date precipitation is 66 percent of normal for the Snake River basin (75 percent of last year at this time) Year-to-date percentages range from 49 to 90 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 (12,900 acre feet compared to 12,700 last year), Jackson lake - 136 percent of average (641,200 acre feet compared to 657,400 acre feet last year), and Palisades Reservoir -55 percent of average (773,000 acre feet compared to 1,188,800 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 540,000 acre-feet (62 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 1,710,000 acre-feet (64 percent of normal). The 50 percent chance yield near Heise is expected to be 2,380,000 acre-feet (59 percent of normal). Pacific Creek at Moran is expected to yield about 98,000 acre-feet (59 percent of average). Greys River above Palisades Reservoir is estimated to yield 197,000 acre-feet (51 percent of normal). Salt River near Etna is estimated to have a yield of 191,000 acre-feet (48 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - April 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)               APR-SEP       399       496    |      540        62    |       584       681            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      1439      1600    |     1710        64    |      1820      1981           2671
                                                                    |                       |
PALISADES RESERVOIR INFLOW (1,2)     APR-SEP      1669      2041    |     2210        59    |      2379      2751           3763
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      1910      2190    |     2380        59    |      2570      2850           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP        68        86    |       98        59    |       110       128            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       143       175    |      197        51    |       219       251            388
                                                                    |                       |
SALT near Etna                       APR-SEP       113       160    |      191        48    |       222       269            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 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     12.9     12.7     11.2 |   SNAKE above Jackson Lake    9        59        52
                                                                        |
JACKSON LAKE                           847.0    641.2    657.4    473.2 |   PACIFIC CREEK               3        60        57
                                                                        |
PALISADES                             1400.0    773.0   1188.8   1014.0 |   GROS VENTRE RIVER           3        76        63
                                                                        |
                                                                        |   HOBACK RIVER                6        73        60
                                                                        |
                                                                        |   GREYS RIVER                 5        65        58
                                                                        |
                                                                        |   SALT RIVER                  5        59        56
                                                                        |
                                                                        |   SNAKE above Palisades      29        63        55
                                                                        |
==================================================================================================================================
 * 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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