Shoshone and Clarks Fork River Basin (5)
May 1998

Snow
Snow Water Equivalent (SWE) is 90 percent of average (59 percent of last year) in the Shoshone River basin. Clarks Fork River basin SWE is 73 percent of average (47 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 57 percent of normal (46 percent of last April). The National Weather Service station Buffalo Bill Dam had the lowest percentage of average for the month (18 percent). Deaver had the highest percentage of average for the month (85 percent). The basin year-to-date precipitation is now 93 percent of average (63 percent of last year).

Reservoir
Current storage in Buffalo Bill Reservoir is 147 percent of average. Currently 492,100 acre feet is stored in the reservoir. This is 167 percent of last year's 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 near average this season. The fifty percent yield (May through September period) for North Fork Shoshone River at Wapiti is expected to be 470,000 acre feet (98 percent of average). South Fork of the Shoshone River near Valley is estimated to yield of 250,000 acre feet (97 percent of average), and South Fork above Buffalo Bill Reservoir is expected to be 215,000 acre feet (99 percent of average). Below Buffalo Bill Reservoir, the fifty percent chance yield for the Shoshone River is expected to be about 705,000 acre feet (94 percent of average). The fifty percent chance yield for the Clarks Fork of the Yellowstone near Belfry, Montana is expected to be about 460,000 acre feet (81 percent of average). 

==================================================================================================================================
                                               SHOSHONE & CLARKS FORK RIVER BASINS
                                               Streamflow Forecasts - May 1, 1998
==================================================================================================================================
                                              |  <<====== 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       387       436    |      470        98    |       504       553            480
                                                                    |                       |
SF SHOSHONE RIVER near Valley        MAY-SEP       214       235    |      250        97    |       265       286            259
                                                                    |                       |
SF SHOSHONE RIVER abv Buffalo Bill   MAY-SEP       146       187    |      215        99    |       243       284            218
                                                                    |                       |
SHOSHONE RIVER blw Buffalo Bill (2)  MAY-SEP       572       651    |      705        94    |       759       838            754
                                                                    |                       |
CLARKS FORK RIVER near Belfry        MAY-SEP       382       429    |      460        81    |       491       538            566
                                                                    |                       |
==================================================================================================================================
                   SHOSHONE & CLARKS FORK RIVER BASINS                  |           SHOSHONE & CLARKS FORK RIVER BASINS
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 1, 1998
==================================================================================================================================
                                      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    492.1    294.8    335.1 |   SHOSHONE RIVER              6        59        90
                                                                        |
                                                                        |   CLARKS FORK in WY           7        47        73
                                                                        |
==================================================================================================================================
 * 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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