Shoshone and Clarks Fork River Basin (5)
May 1999

Snow
Snow Water Equivalent (SWE) is 133 percent of average (148 percent of last year) in the Shoshone River basin. The Clarks Fork River basin SWE is 111 percent of average (153 percent of last year). 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 149 percent of normal (257 percent of last April). Monthly percentages range from 71 to 389 percent of average. The basin year-to-date precipitation is now 128 percent of average (136 percent of last year). Year-to-date percentages range from 52 to 195 percent of average.

Reservoir
Current storage in Buffalo Bill Reservoir is 108 percent of average (73 percent of last year's storage and 56 percent of capacity. Currently, 360,800 acre-feet is stored in the reservoir compared to 492,100 acre feet last year. 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 drainages should be above average this season. The fifty percent yield (May through September period) for North Fork Shoshone River at Wapiti is expected to be 565,000 acre-feet (118 percent of average). South Fork of the Shoshone River near Valley is estimated to yield of 315,000 acre-feet (122 percent of average), and South Fork above Buffalo Bill Reservoir is expected to be 300,000 acre-feet (138 percent of average). At the Buffalo Bill Reservoir, the fifty percent chance yield for the Shoshone River is expected to be about 920,000 acre-feet (122 percent of average). The fifty-percent chance yield for the Clarks Fork of the Yellowstone near Belfry, Montana is expected to be about 595,000 acre-feet (105 percent of average). 

  
==================================================================================================================================
                                               SHOSHONE & CLARKS FORK RIVER BASINS
                                               Streamflow Forecasts - May 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)
====================================================================|=======================|=====================================
NF SHOSHONE RIVER at Wapiti          MAY-SEP       517       546    |      565       118    |       584       613            480
                                                                    |                       |
SF SHOSHONE RIVER nr Valley          MAY-SEP       279       301    |      315       122    |       329       351            259
                                                                    |                       |
SF SHOSHONE RIVER abv Buffalo Bill   MAY-SEP       243       277    |      300       138    |       323       357            218
                                                                    |                       |
BUFFALO BILL DAM Inflow (2)          MAY-SEP       787       866    |      920       122    |       974      1053            754
                                                                    |                       |
CLARKS FORK RIVER nr Belfry          MAY-SEP       511       561    |      595       105    |       629       679            566
                                                                    |                       |
==================================================================================================================================
                   SHOSHONE & CLARKS FORK RIVER BASINS                  |           SHOSHONE & CLARKS FORK RIVER BASINS
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 1, 1999
==================================================================================================================================
                                      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    360.8    492.1    335.1 |   SHOSHONE RIVER              6       148       133
                                                                        |
                                                                        |   CLARKS FORK in WY           7       153       111
                                                                        |
==================================================================================================================================
 * 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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