Upper Yellowstone and Madison River Basins (2)
March 1998

Snow
Snow water equivalent (SWE) is about 15 percent below average in the Madison drainage and 13 percent below average in the Yellowstone River drainage. The Madison drainage SWE is about 51 percent of last year at this time. SWE in the Yellowstone drainage is nearly 52 percent of last year at this time. Measured snow depths range from 27 to 44 inches in the Madison and from 29 to 41 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
February precipitation in the Madison and Yellowstone drainage was well below average (about 50 percent below average) and about 45 percent less than one year ago. All reporting stations were below average. Water year-to-date precipitation is about 8 percent below normal and about 55 percent of last year's amount. Averages were computed using 7 stations.

Reservoir
Current storage, compared to average for the two storage reservoirs in the basin, is as follows; Ennis Lake 86% (29,200 acre feet) and Hebgen Lake 109% (271,100 acre feet). Ennis Lake is about 7 percent below last year's storage amount, while Hebgen Lake is about 10 percent more than last year and Ennis Lake is about 4 percent more.

Streamflow
The 50 percent chance runoff is expected to be 87 to 90 percent of average for the April through September runoff period. Yellowstone at Lake Outlet has a 50 percent chance of yielding about 710,000 acre feet (90 percent of normal). Yellowstone at Corwin Springs has a 50 percent chance of yielding about 1,700,000 acre feet (88 percent of normal). Yellowstone near Livingston has a 50 percent chance of yielding about 2,000,000 acre feet (89 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 - March 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       554       647    |      710        90    |       773       866            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  APR-SEP      1481      1611    |     1700        88    |      1789      1919           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1724      1888    |     2000        89    |      2112      2276           2241
                                                                    |                       |
MADISON RIVER near Grayling (2)      APR-SEP       345       393    |      425        87    |       457       505            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of February              |        Watershed Snowpack Analysis - March 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     29.2     28.2     34.1 |   MADISON RIVER in WY         9        52        88
                                                                        |
HEBGEN LAKE                            377.5    271.1    245.7    247.8 |   YELLOWSTONE RIVER in WY    12        52        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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