Upper Yellowstone and Madison River Basins (2)
April 2000

Snow
Snowfall in the basin this year has been just below average for the year, and very much below the SWE last year at this time. For this time of the year, snow water equivalent (SWE) is about 89 percent of average (69 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 91 percent of average (74 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
March precipitation in the Madison and Yellowstone drainage was about 102 percent of average (129 percent of previous year) for the 7 reporting stations -- percentage range was from 50 to 144 percent of average. Water- year-to-date precipitation is about 88 percent of average (72 percent of last year's amount). Year to date percentage ranges from 69 to 113 percent

Reservoir
Ennis Lake is storing 29,200 acre-feet (88 percent of average and 71 percent of capacity). Hebgen Lake is storing about 304,700 acre-feet of water (124 percent of average and 81 percent of capacity). Hebgen Lake is storing about 118 percent and Ennis Lake is storing about 94 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 595,000 acre feet (75 percent of normal). Yellowstone at Corwin Springs will yield about 1,600,000 acre-feet (83 percent of normal). Yellowstone near Livingston will yield about 1,850,000 acre feet (83 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 - April 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       472       545    |      595        75    |       645       718            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    APR-SEP      1358      1502    |     1600        83    |      1698      1842           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1606      1751    |     1850        83    |      1949      2094           2241
                                                                    |                       |
HEBGEN Reservoir Inflow              APR-SEP       348       394    |      425        87    |       456       502            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 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     29.2     31.0     33.2 |   MADISON RIVER in WY         9        69        89
                                                                        |
HEBGEN LAKE                            377.5    304.7    258.3    246.6 |   YELLOWSTONE RIVER in WY    12        74        91
                                                                        |
==================================================================================================================================
 * 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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