Upper Yellowstone and Madison River Basins (2)
April 1998

Snow
Snow water equivalent (SWE) is about 87 percent of average (54 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is 87 percent of average (56 percent of last year at this time). Measured snow depths range from 28 to 40 inches in the Madison and from 24 to 37 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
March precipitation in the Madison and Yellowstone drainage was about 112 percent of average (90 percent of last year) for the 7 reporting stations. Old Faithful had the lowest percentage of average (48 percent) and Sylvan Lake SNOTEL had the highest percentage of average (133 percent). Water year-to-date precipitation is about 94 percent of average (59 percent of last year's amount).

Reservoir
Reservoir storage is near normal. Ennis lake is currently storing about 29,600 acre feet of water (89 percent of average). Hebgen Lake is storing about 262,400 acre feet of water (106 percent of average). Ennis Lake is storing 105 percent of last year at this time and Hebgen Lake is storing about 112 percent of last year's volume.

Streamflow
The 50 percent chance runoff is expected to be 82 to 87 percent of average for the April through September runoff period. Yellowstone at Lake Outlet has a is expected to yield about 690,000 acre feet (87 percent of normal). Yellowstone at Corwin Springs will yield about 1,590,00 acre feet (82 percent of normal). Yellowstone near Livingston will yield about 1,840,000 acre feet (82 percent of normal). Madison River near Grayling has a 50 percent chance of yielding about 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, 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       566       640    |      690        87    |       740       814            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  APR-SEP      1376      1503    |     1590        82    |      1677      1804           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1596      1741    |     1840        82    |      1939      2084           2241
                                                                    |                       |
MADISON RIVER near Grayling (2)      APR-SEP       361       399    |      425        87    |       451       489            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 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                              41.0     29.6     28.2     33.2 |   MADISON RIVER in WY         9        54        87
                                                                        |
HEBGEN LAKE                            377.5    262.4    234.4    246.6 |   YELLOWSTONE RIVER in WY    12        56        87
                                                                        |
==================================================================================================================================
 * 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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