Upper Yellowstone and Madison River Basins
May 1997

Snow
Snow just keeps collecting at record levels this year. Average snow water equivalent (SWE) is about 70 percent above normal in the Madison drainage and 70 percent above average in the Yellowstone River drainage. The Madison drainage SWE is about 21 percent ahead of last year at this time. SWE in the Yellowstone drainage is nearly 14 percent more than last April. The graph below indicates the basin is well above previously recorded snow levels. See the snow course basin summary at the beginning of this document for more details on specific sites.

Precipitation
March precipitation in the Madison and Yellowstone drainage was about 63 percent above average and 18 percent above last April. Water year-to-date precipitation is about 17 percent ahead of last year and 58 percent above average. Four stations were used to compute the averages.

Reservoir
Reservoir storage is below normal for the reporting reservoirs. Ennis Lake is about 7 percent below average (32,800 acre feet) and Hebgen Lake's current storage is about 3 percent below normal (137,600 acre feet). Ennis Lake is storing 80 percent of its capacity, while Hebgen Lake is storing 63 percent of capacity. See reservoir storage on the following page for details.

Streamflow
The 50 percent chance runoff is expected to be 48 to 59 percent above average for the May through September runoff period. Yellowstone at Corwin Springs has a 50 percent chance of yielding about 2,900,000 acre feet (57 percent above normal). Yellowstone near Livingston has a 50 percent chance of yielding about 3,380,000 acre feet (59 percent above normal). Madison River near Grayling has a 50 percent chance of yielding about 635,000 acre feet (48 percent above normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON 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)
====================================================================|=======================|=====================================
YELLOWSTONE at Lake Outlet           MAY-SEP      1013      1074    |     1115       148    |      1156      1217            756
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  MAY-SEP      2728      2830    |     2900       157    |      2970      3072           1844
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    MAY-SEP      3151      3287    |     3380       159    |      3473      3609           2123
                                                                    |                       |
MADISON RIVER near Grayling (2)      MAY-SEP       577       612    |      635       148    |       658       693            428
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON 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
========================================================================|=========================================================
ENNIS LAKE                              41.0     32.8     29.2     35.1 |   MADISON RIVER in WY         9       121       170
                                                                        |
HEBGEN LAKE                            377.5    237.6    271.5    246.1 |   YELLOWSTONE RIVER in WY    10       114       170
                                                                        |
==================================================================================================================================
 * 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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