Upper Yellowstone and Madison River Basins (2)
January 1998

Snow
Average snow water equivalent (SWE) is about 35 percent below normal in the Madison drainage and 26 percent below average in the Yellowstone River drainage. The Madison drainage SWE is about 72 percent behind last year at this time. SWE in the Yellowstone drainage is nearly 64 percent less than last year at this time.. The graph below indicates SWE is below normal; El Nino effects are being felt in this basin. See the "snow course basin summary" at the beginning of this document for more details on specific sites.

Precipitation
December precipitation in the Madison and Yellowstone drainage was about 45 percent below average and 79 percent less than last December. Water year-to-date precipitation is about 56 percent behind last year and 20 percent below average. Averages were computed using seven stations.

Reservoir
Current storage, compared to average for the two storage reservoirs in the basin, is as follows; Ennis Lake 89% (29,900 acre feet and Hebgen Lake 116% (286,100 acre feet). Ennis Lake is about 7 percent below last year's storage amount, while Hebgen Lake is about 8 percent more than last year at this time.

Streamflow The 50 percent chance runoff is expected to be 79 to 80 percent of average for the April through September runoff period. Yellowstone at Lake Outlet has a 50 percent chance of yielding about 635,000 acre feet (80 percent of normal). Yellowstone at Corwin Springs has a 50 percent chance of yielding about 1,525,000 acre feet (79 percent of normal). Yellowstone near Livingston has a 50 percent chance of yielding about 1,760,000 acre feet (79 percent of normal). Madison River near Grayling has a 50 percent chance of yielding about 385,000 acre feet (79 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - January 1, 1998
==================================================================================================================================
                                              |  <<====== 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       419       547    |      635        80    |       723       851            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  APR-SEP      1126      1364    |     1525        79    |      1686      1924           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1286      1568    |     1760        79    |      1952      2234           2241
                                                                    |                       |
MADISON RIVER near Grayling (2)      APR-SEP       297       349    |      385        79    |       421       473            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of December              |        Watershed Snowpack Analysis - January 1, 1998
==================================================================================================================================
                                      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     29.9     32.1     33.7 |   MADISON RIVER in WY         9        28        65
                                                                        |
HEBGEN LAKE                            377.5    286.1    263.8    246.8 |   YELLOWSTONE RIVER in WY    11        36        74
                                                                        |
==================================================================================================================================
 * 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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