Upper Yellowstone and Madison River Basins
April 1997

Snow
Snow just keeps collecting at record levels this year. Average snow water equivalent (SWE) is about 63 percent above normal in the Madison drainage and 58 percent above average in the Yellowstone River drainage. The Madison drainage SWE is about 30 percent ahead of last year at this time. SWE in the Yellowstone drainage is nearly 14 percent more than last March. 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 40 percent above average and 3 percent below last March. Water year-to-date precipitation is about 17 percent ahead of last year and 57 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 15 percent below average (28,200 acre feet) and Hebgen Lake's current storage is about 5 percent below normal (234,400 acre feet). Ennis Lake is storing 69 percent of its capacity, while Hebgen Lake is storing 62 percent of capacity. See reservoir storage on the following page for details.

Streamflow
The 50 percent chance runoff is expected to be 42 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 690,000 acre feet (42 percent above normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - April 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      1026      1100    |     1150       145    |      1200      1274            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  APR-SEP      2786      2913    |     3000       155    |      3087      3214           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      3256      3401    |     3500       156    |      3599      3744           2241
                                                                    |                       |
MADISON RIVER near Grayling (2)      APR-SEP       626       664    |      690       142    |       716       754            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 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     29.6     33.2 |   MADISON RIVER in WY         9       130       163
                                                                        |
HEBGEN LAKE                            377.5    234.4    255.8    246.6 |   YELLOWSTONE RIVER in WY    11       114       158
                                                                        |
==================================================================================================================================
 * 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.
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