Upper Yellowstone and Madison River Basins (2)
June 1997

Snow
Snow just keeps collecting at record levels this year. Average snow water equivalent (SWE) is about 76 percent above normal in the Madison drainage and 42 percent above average in the Yellowstone River drainage. The Madison drainage SWE is about 25 percent ahead of last year at this time. SWE in the Yellowstone drainage is nearly 23 percent more than last May. The graph below indicates SWE is well above normal, but less than the previous record level. See the snow course basin summary at the beginning of this document for more details on specific sites.

Precipitation
May precipitation in the Madison and Yellowstone drainage was about 14 percent below average and 61 percent below last May. Water year-to-date precipitation is about 2 percent ahead of last year and 49 percent above average. Five stations were used to compute the averages.

Reservoir
Reservoir storage was not reported this month.

Streamflow
The 50 percent chance runoff is expected to be 48 to 59 percent above average for the June through September runoff period. Yellowstone at Lake Outlet has a 50 percent chance of yielding about 895,000 acre feet (30 percent above normal). Yellowstone at Corwin Springs has a 50 percent chance of yielding about 2,050,000 acre feet (38 percent above normal). Yellowstone near Livingston has a 50 percent chance of yielding about 2,405,000 acre feet (40 percent above normal). Madison River near Grayling has a 50 percent chance of yielding about 440,000 acre feet (43 percent above normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - June 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           JUN-SEP       803       858    |      895       130    |       932       987            691
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  JUN-SEP      1889      1985    |     2050       138    |      2115      2211           1484
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    JUN-SEP      2206      2324    |     2405       140    |      2486      2604           1721
                                                                    |                       |
MADISON RIVER near Grayling (2)      JUN-SEP       401       424    |      440       143    |       456       479            307
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of May                   |        Watershed Snowpack Analysis - June 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     35.1     35.8 |   MADISON RIVER in WY         6        75       176
                                                                        |
HEBGEN LAKE                            377.5    330.5    312.0    309.8 |   YELLOWSTONE RIVER in WY     7        77       142
                                                                        |
==================================================================================================================================
 * 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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