Upper Yellowstone and Madison River Basins (2)
June 2001

Snow
Snow in these basins this year has been well below average. All the SNOTEL sites in this basin have melted out. See the "Snow Course Basin Summary" at the beginning of this document for more details on specific sites.

Precipitation
May precipitation in the Madison and Yellowstone drainage was about 74 percent of average (49 percent of previous year) for the 6 reporting stations -- percentage range was from 47 to 126 percent of average. Water-year-to-date precipitation is about 76 percent of average (79 percent of last year's amount). Year to date percentage ranges from 57 to 94 percent

Reservoir
Usable reservoir storage for Ennis Lake is 36,600 acre-feet (89 percent of capacity) – 102 percent of average. Hebgen Lake usable storage is about 349,100 acre-feet of water (92 percent of capacity) – 113 percent of average. Ennis Lake is storing about 114 percent and Hebgen Lake is storing about 98 percent of last year's volume.

Streamflow
Runoff in this basin will be well below average. Irrigation water shortages can be expected from all direct diversions. All the following forecasts are the 50 percent chance runoff for the June through September runoff period. Yellowstone at Lake Outlet is expected to yield about 275,000 acre feet (40 percent of normal). Yellowstone at Corwin Springs will yield about 700,000 acre-feet (47 percent of normal). Yellowstone near Livingston will yield about 800,000 acre feet (47 percent of normal). Hebgen lake inflow is estimated to be 120,000 acre feet (39 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - June 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)
====================================================================|=======================|=====================================
YELLOWSTONE at Lake Outlet           JUN-SEP       263       270    |      275        40    |       287       306            691
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    JUN-SEP       625       670    |      700        47    |       765       860           1484
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    JUN-SEP       601       719    |      800        47    |       881       999           1721
                                                                    |                       |
HEBGEN Reservoir Inflow              JUN-SEP       109       115    |      120        39    |       132       149            307
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of May                   |        Watershed Snowpack Analysis - June 1, 2001
==================================================================================================================================
                                      Usable |  *** Usable Storage ***  |                            Number     This Year as % of
Reservoir                            Capacity|  This      Last          |   Watershed                  of       =================
                                             |  Year      Year     Avg  |                          Data Sites   Last Yr   Average
========================================================================|=========================================================
ENNIS LAKE                              41.0     36.6     32.1     35.8 |   MADISON RIVER in WY         6         0         0
                                                                        |
HEBGEN LAKE                            377.5    349.1    355.5    309.8 |   YELLOWSTONE RIVER in WY     8        16         8
                                                                        |
==================================================================================================================================
 * 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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