Upper Yellowstone and Madison River Basins
February 1997

Snow
Snow is collecting at record levels in the Shoshone and Clarks Fork drainage this year. Average snow water equivalent (SWE) for the SNOTEL sites is about 92 percent above normal in the Madison drainage and 86 percent above average in the Yellowstone River drainage. The Madison drainage SWE is about 54 percent ahead of last year at this time. SWE in the Yellowstone drainage is nearly 29 percent more than last February. See the snow course basin summary at the beginning of this document for more details on specific sites.

Precipitation
January precipitation in the Madison and Yellowstone drainage was about 66 percent above average and 24 percent above last January. Water year-to-date precipitation is about 30 percent ahead of last year and 77 percent above average. Four stations were used to compute the averages.

Reservoir
Reservoir storage is near normal for the reporting reservoirs. Ennis Lake is about 15 percent below average (289,900 acre feet) and Hebgen Lake's current storage is about 5 percent above normal (259,100 acre feet). See reservoir storage on the following page for details.

Streamflow
The 50 percent chance runoff is expected to be 51 to 56 percent above average for the April through September runoff period. Yellowstone at Corwin Springs has a 50 percent chance of yielding about 3,000,000 acre feet (55 percent above normal). Yellowstone near Livingston has a 50 percent chance of yielding about 3,500,000 acre feet (56 percent above normal). Madison River near Grayling has a 50 percent chance of yielding about 735,000 acre feet (51 percent above normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - February 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           APR-SEP       997      1094    |     1160       147    |      1226      1323            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  APR-SEP      2623      2788    |     2900       150    |      3012      3177           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      3087      3268    |     3390       151    |      3512      3693           2241
                                                                    |                       |
MADISON RIVER near Grayling (2)      APR-SEP       596       643    |      675       139    |       707       754            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 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     28.9     30.3     34.0 |   MADISON RIVER in WY         9       154       192
                                                                        |
HEBGEN LAKE                            377.5    259.1    252.0    246.8 |   YELLOWSTONE RIVER in WY    10       129       186
                                                                        |
==================================================================================================================================
 * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual flow 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 flow - actual flow may be affected by upstream water management.
To February 1997 Basin Outlook Report Contents Page