Upper Yellowstone and Madison River Basins (2)
February 2000

Snow
Snowfall in the basin this year has been well below average. For this time of the year, snow water equivalent (SWE) is about 77 percent of average (60 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 82 percent of average (66 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 104 percent of average (78 percent of previous year) for the 7 reporting stations -- percentage range was from 75 percent of average at Canyon to 146 percent of average at Lake Yellowstone. Water-year-to-date precipitation is about 78 percent of average (64 percent of last year's amount). Year to date percentage ranges from 56 to 79 percent.

Reservoir
Ennis Lake is storing 28,900 acre-feet (83 percent of average and 69 percent of capacity). Hebgen Lake is storing about 344,800 acre-feet of water (131 percent of average and 86 percent of capacity). Hebgen Lake is storing about 105 percent and Ennis Lake is storing about 99 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 575,000 acre feet (73 percent of normal). Yellowstone at Corwin Springs will yield about 1,475,000 acre-feet (76 percent of normal). Yellowstone near Livingston will yield about 1,700,000 acre feet (76 percent of normal). Hebgen lake inflow is estimated to be 415,000 acre feet (85 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - February 1, 2000
==================================================================================================================================
                                              |  <<====== 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       420       512    |      575        73    |       638       730            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    APR-SEP      1166      1350    |     1475        76    |      1600      1784           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1397      1578    |     1700        76    |      1822      2003           2241
                                                                    |                       |
HEBGEN Reservoir Inflow              APR-SEP       328       380    |      415        85    |       450       502            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 1, 2000
==================================================================================================================================
                                      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     28.5     34.0 |   MADISON RIVER in WY         9        60        77
                                                                        |
HEBGEN LAKE                            377.5    324.2    307.8    246.8 |   YELLOWSTONE RIVER in WY    11        66        82
                                                                        |
==================================================================================================================================
 * 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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