Snake River Basin (1)
March 1999

Snow
Snowfall has been close to average in the basin so far this year. Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 118 percent (115% of last year at this time), Pacific Creek -- 124 percent (109% of last year at this time), Gros Ventre River -- 100 percent (100% of last year at this time), Hoback River - - 95 percent (102% of last year at this time), Greys River -- 94 percent (98% of last year at this time), Salt River -- 95 percent (93% of last year at this time). Snake River Basin above Palisades is 108 percent of average (105% 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 for last month. Monthly precipitation, for the basin, was 156 percent of average (291 percent of last February). February percentages range from 111 to 244 percent. Water-year-to-date precipitation is 107 percent of normal for the Snake River basin. This is 131 percent of last years total as of March 1. Year to date percentages range from 59 to 126 percent.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake -119 percent (12,900 acre feet compared to 8200 last year), Jackson lake - 127 percent (606,900 acre feet compared to 655,900 acre feet last year), and Palisades Reservoir -115 percent (1,196,000 acre feet compared to 1284,200 acre feet last year).

Streamflow
The most probable April through September runoff yield forecast is near average for the basin. The Snake near Moran is expected to yield 1,080,000 acre-feet (124 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 3,390,000 acre-feet (127 percent of normal). The 50 percent chance yield near Heise is expected to be 5,010,000 acre-feet (124 percent of normal). Pacific Creek at Moran is expected to yield about 231,000 acre-feet (139 percent of average). Greys River above Palisades Reservoir is estimated to yield 445,000 acre-feet (115 percent of normal). Salt River near Etna is estimated to have a yield of 450,000 acre- feet (113 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - March 1, 1999
==================================================================================================================================
                                              |  <<====== 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       901      1024    |     1080       124    |      1136      1259            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      3029      3244    |     3390       127    |      3536      3751           2671
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      4394      4761    |     5010       124    |      5259      5626           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP       196       217    |      231       139    |       245       266            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       371       415    |      445       115    |       475       519            388
                                                                    |                       |
SALT near Etna                       APR-SEP       343       407    |      450       113    |       493       557            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of February              |        Watershed Snowpack Analysis - March 1, 1999
==================================================================================================================================
                                      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     13.1      7.8     11.0 |   SNAKE above Jackson Lake    9       144       132
                                                                        |
JACKSON LAKE                           847.0    623.4    649.6    481.0 |   PACIFIC CREEK               3       143       143
                                                                        |
PALISADES                             1400.0   1039.3   1204.5   1063.1 |   GROS VENTRE RIVER           4       118       118
                                                                        |
                                                                        |   HOBACK RIVER                6       132       113
                                                                        |
                                                                        |   GREYS RIVER                 3       123       113
                                                                        |
                                                                        |   SALT RIVER                  5       119       116
                                                                        |
                                                                        |   SNAKE above Palisades      29       133       124
                                                                        |
==================================================================================================================================
 * 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.
(D) - The operational measurement of flow at this point has been discountinued.
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