Upper Yellowstone and Madison River Basins
March 1997

Snow
Snow just keeps collecting at record levels in the Shoshone and Clarks Fork drainage this year. Average snow water equivalent (SWE) is about 68 percent above normal in the Madison drainage and 69 percent above average in the Yellowstone River drainage. The Madison drainage SWE is about 37 percent ahead of last year at this time. SWE in the Yellowstone drainage is nearly 22 percent more than last March. The graph below indicates the basin is well above prevoiously recorded snow levels. See the snow course basin summary at the beginning of this document for more details on specific sites.

Precipitation
February precipitation in the Madison and Yellowstone drainage was about 8 percent below average and 21 percent below last February. Water year-to-date precipitation is about 21 percent ahead of last year and 61 percent above average. Six stations were used to compute the averages.

Reservoir
Reservoir storage is near normal for the reporting reservoirs. Ennis Lake is about 17 percent below average (280,200 acre feet) and Hebgen Lake's current storage is about 1 percent below normal (245,700 acre feet). See reservoir storage on the following page for details.

Streamflow
The 50 percent chance runoff is expected to be 42 to 58 percent above average for the April through September runoff period. Yellowstone at Corwin Springs has a 50 percent chance of yielding about 3,0300,000 acre feet (56 percent above normal). Yellowstone near Livingston has a 50 percent chance of yielding about 3,540,000 acre feet (58 percent above normal). Madison River near Grayling has a 50 percent chance of yielding about 690,000 acre feet (42 percent above normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - March 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       994      1087    |     1150       145    |      1213      1306            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  APR-SEP      2811      2941    |     3030       156    |      3119      3249           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      3264      3428    |     3540       158    |      3652      3816           2241
                                                                    |                       |
MADISON RIVER near Grayling (2)      APR-SEP       610       658    |      690       142    |       722       770            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of February              |        Watershed Snowpack Analysis - March 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     28.2     27.1     34.1 |   MADISON RIVER in WY         9       137       168
                                                                        |
HEBGEN LAKE                            377.5    245.7    253.9    247.8 |   YELLOWSTONE RIVER in WY    11       122       169
                                                                        |
==================================================================================================================================
 * 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 March 1997 Basin Outlook Report Contents Page