Upper Yellowstone and Madison River Basins (2)
January 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 62 percent of average (84 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 67 percent of average (90 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
December precipitation in the Madison and Yellowstone drainage was about 77 percent of average (84 percent of previous year) for the 8 reporting stations -- percentage range was from 49 to 149 percent of average. Water- year-to-date precipitation is about 77 percent of average (118 percent of last year's amount). Year to date percentage ranges from 53 to 101 percent

Reservoir
Ennis Lake is storing 32,100 acre-feet (75 percent of capacity) - 91 percent of average. Hebgen Lake is storing about 355,500 acre-feet of water (79 percent of capacity) - 120 percent of average. Hebgen Lake is storing about 86 percent and Ennis Lake is storing about 106 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 630,000 acre feet (80 percent of normal). Yellowstone at Corwin Springs will yield about 1,550,000 acre-feet (80 percent of normal). Yellowstone near Livingston will yield about 1,790,000 acre feet (80 percent of normal). Hebgen lake inflow is estimated to be 425,000 acre feet (87 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - January 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       450       557    |      630        80    |       703       810            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    APR-SEP      1183      1401    |     1550        80    |      1699      1917           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1316      1598    |     1790        80    |      1982      2264           2241
                                                                    |                       |
HEBGEN Reservoir Inflow              APR-SEP       325       384    |      425        87    |       466       525            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of December              |        Watershed Snowpack Analysis - January 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     30.7     28.9     33.7 |   MADISON RIVER in WY         9        84        62
                                                                        |
HEBGEN LAKE                            377.5    297.0    344.8    246.8 |   YELLOWSTONE RIVER in WY    10        90        67
                                                                        |
==================================================================================================================================
 * 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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