Upper Yellowstone and Madison River Basins (2)
February 1998

Snow
Snow water equivalent (SWE) is about 8 percent below average in the Madison drainage and 5 percent below average in the Yellowstone River drainage. The Madison drainage SWE is about 48 percent of last year at this time. SWE in the Yellowstone drainage is nearly 52 percent of last year at this time. Measured snow depths range from 27 to 38 inches in the Madison and from 25 to 40 inches in the Yellowstone River drainage. 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 well above average (about 44 percent above average) and only 8 percent less than last January. Water year-to-date precipitation is normal and about 55 percent of last year's amount. Averages were computed using 8 stations.

Reservoir
Current storage, compared to average for the two storage reservoirs in the basin, is as follows; Ennis Lake no report and Hebgen Lake 114% (280,200 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 85 to 89 percent of average for the April through September runoff period. Yellowstone at Lake Outlet has a 50 percent chance of yielding about 690,000 acre feet (87 percent of normal). Yellowstone at Corwin Springs has a 50 percent chance of yielding about 1,655,000 acre feet (85 percent of normal). Yellowstone near Livingston has a 50 percent chance of yielding about 1,925,000 acre feet (86 percent of normal). Madison River near Grayling has a 50 percent chance of yielding about 430,000 acre feet (89 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - February 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       527       624    |      690        87    |       756       853            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  APR-SEP      1378      1543    |     1655        85    |      1767      1932           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1622      1803    |     1925        86    |      2047      2228           2241
                                                                    |                       |
MADISON RIVER near Grayling (2)      APR-SEP       351       398    |      430        89    |       462       509            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 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                                      NO REPORT               |   MADISON RIVER in WY         9        48        91
                                                                        |
HEBGEN LAKE                            377.5    280.2    259.1    246.8 |   YELLOWSTONE RIVER in WY    11        52        95
                                                                        |
==================================================================================================================================
 * 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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