Upper Yellowstone and Madison River Basins (2)
February 2001

Snow
Snowfall in these basins this year has been well below average for this time of the year. Snow water equivalent (SWE) is about 46 percent of average (60 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 50 percent of average (61 percent of last year at this time). 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 about 20 percent of average (20 percent of previous year) for the 6 reporting stations -- percentage range was from 8 to 38 percent of average. Water- year-to-date precipitation is about 60 percent of average (78 percent of last year's amount). Year to date percentage ranges from 62 to 84 percent.

Reservoir
Ennis Lake is storing 29,600 acre-feet (72 percent of capacity) - 87 percent of average. Hebgen Lake is storing about 298,200 acre-feet of water (79 percent of capacity) - 121 percent of average. Ennis Lake is storing about 105 percent and Hebgen Lake is storing about 92 percent of last year's volume.

Streamflow
All the following forecasts are the 50 percent chance runoff for the April through September runoff period. Yellowstone at Lake Outlet is expected to yield about 490,000 acre feet (62 percent of normal). Yellowstone at Corwin Springs will yield about 1250,000 acre-feet (65 percent of normal). Yellowstone near Livingston will yield about 1,435,000 acre feet (64 percent of normal). Hebgen lake inflow is estimated to be 360,000 acre feet (74 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - February 1, 2001
==================================================================================================================================
                                              |  <<====== 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       396       451    |      490        62    |       553       645            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    APR-SEP      1046      1162    |     1250        65    |      1375      1559           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1233      1367    |     1435        64    |      1557      1738           2241
                                                                    |                       |
HEBGEN Reservoir Inflow              APR-SEP       273       325    |      360        74    |       395       447            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 1, 2001
==================================================================================================================================
                                      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     29.6     28.2     34.0 |   MADISON RIVER in WY         9        60        46
                                                                        |
HEBGEN LAKE                            377.5    298.2    324.2    246.8 |   YELLOWSTONE RIVER in WY    11        62        50
                                                                        |
==================================================================================================================================
 * 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.
To February 2001 Basin Outlook Report Contents Page