Shoshone and Clarks Fork River Basin
May 1997

Snow
Snowfall in the basin has been very good this year. SWE is 153 percent of average (2 percent above last year) in the Shoshone River basin, and 154 percent of average (7 percent above last year) in the Clarks Fork drainage. All sites are reporting well above average snowpack as of May 1. For more information see the Basin Summary of Snow Course Data at the beginning of this report.

Precipitation
Precipitation for the month of April was 33 percent above normal. Parkers Peak had the highest percentage of average for the month (212%). Year-to-date precipitation figures for the basin are also very high. They range from 44 percent at Heart Mountain to 186 percent of average at Parkers Peak. The basin year-to-date precipitation is now 153 percent of average (3 percent more than last year).

Reservoir
Current storage in Buffalo Bill Reservoir is about 18 percent above average. Currently 294,800 acre feet is stored in Buffalo Bill Reservoir (about 88 percent of average). This is 79 percent of last years storage at this time. Detailed reservoir data is shown on the following page and on the reservoir storage summary at the beginning of this report.

Streamflow Runoff from all drainage's should be much above average this season. The fifty percent yield (May through September period) for North Fork at Wapiti is expected to be 785,000 acre feet (64 percent more than normal). South Fork of the Shoshone near Valley has an estimated yield of about 390,000 acre feet (51 percent above normal) and South Fork above Buffalo Bill Reservoir the fifty percent chance yield it is expected to be 345,000 (58 percent more than normal). Below Buffalo Bill Reservoir, the fifty percent chance yield for the Shoshone River is expected to be about 1,145,000 acre feet (52 percent above average). The fifty percent chance yield for the Clarks Fork of the Yellowstone near Belfry, Montana is expected to be about 835,000 acre feet (48 percent above average). 

==================================================================================================================================
                                               SHOSHONE & CLARKS FORK RIVER BASINS
                                               Streamflow Forecasts - May 1, 1997
==================================================================================================================================
                                              |  <<====== 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)
====================================================================|=======================|=====================================
NF SHOSHONE RIVER at Wapiti          MAY-SEP       702       751    |      785       164    |       819       868            480
                                                                    |                       |
SF SHOSHONE RIVER near Valley        MAY-SEP       354       375    |      390       151    |       405       426            259
                                                                    |                       |
SF SHOSHONE RIVER abv Buffalo Bill   MAY-SEP       276       317    |      345       158    |       373       414            218
                                                                    |                       |
SHOSHONE RIVER blw Buffalo Bill (2)  MAY-SEP      1012      1091    |     1145       152    |      1199      1278            754
                                                                    |                       |
CLARKS FORK RIVER near Belfry        MAY-SEP       757       804    |      835       148    |       866       913            566
                                                                    |                       |
==================================================================================================================================
                   SHOSHONE & CLARKS FORK RIVER BASINS                  |           SHOSHONE & CLARKS FORK RIVER BASINS
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 1, 1997
==================================================================================================================================
                                      Usable |  *** Usable Storage ***  |                            Number     This Year as % of
Reservoir                            Capacity|  This      Last          |   Watershed                  of       =================
                                             |  Year      Year     Avg  |                          Data Sites   Last Yr   Average
========================================================================|=========================================================
BUFFALO BILL                           646.6    294.8    371.2    335.1 |   SHOSHONE RIVER              6       102       153
                                                                        |
                                                                        |   CLARKS FORK in WY           7       107       154
                                                                        |
==================================================================================================================================
 * 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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