Upper Yellowstone and Madison River Basins (2)
May 2001

Snow
Snowfall in these basins this year has been well below average for this time of the year. Snow water equivalent (SWE) is about 49 percent of average (83 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 54 percent of average (78 percent of last year at this time). See the "Snow Course Basin Summary" at the beginning of this document for more details on specific sites.

Precipitation
April precipitation in the Madison and Yellowstone drainage was about 154 percent of average (166 percent of previous year) for the 6 reporting stations -- percentage range was from 83 to 177 percent of average. Water- year-to-date precipitation is about 76 percent of average (85 percent of last year's amount). Year to date percentage ranges from 60 to 102 percent

Reservoir
Usable reservoir storage for Ennis Lake is 31,000 acre-feet (76 percent of capacity) - 88 percent of average. Hebgen Lake usable storage is about 290,000 acre-feet of water (77 percent of capacity) - 118 percent of average. Ennis Lake is storing about 97 percent and Hebgen Lake is storing about 95 percent of last year's volume.

Streamflow
All the following forecasts are the 50 percent chance runoff for the April through September runoff period. Yellowstone at Lake Outlet is expected to yield about 440,000 acre feet (56 percent of normal). Yellowstone at Corwin Springs will yield about 1,200,000 acre-feet (62 percent of normal). Yellowstone near Livingston will yield about 1,375,000 acre feet (61 percent of normal). Hebgen lake inflow is estimated to be 355,000 acre feet (69 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - May 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           MAY-SEP       343       398    |      435        58    |       472       527            756
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    MAY-SEP      1042      1184    |     1280        69    |      1376      1518           1844
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    MAY-SEP      1226      1362    |     1455        69    |      1548      1684           2123
                                                                    |                       |
HEBGEN Reservoir Inflow              MAY-SEP       218       258    |      285        67    |       312       352            428
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 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     31.0     32.1     35.1 |   MADISON RIVER in WY         9        83        49
                                                                        |
HEBGEN LAKE                            377.5    290.0    305.7    246.1 |   YELLOWSTONE RIVER in WY    11        78        54
                                                                        |
==================================================================================================================================
 * 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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