Upper Yellowstone and Madison River Basins (2)
March 1999

Snow
Snowfall in the basin this year has been above average. Snow water equivalent (SWE) is about 141 percent of average (165 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 133 percent of average (153 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
February precipitation in the Madison and Yellowstone drainage was about 154 percent of average (307 percent of last February) for the 6 reporting stations. Percentages range from 122 to 246 percent. Water-year-to-date precipitation is about 129 percent of average (144 percent of last year's amount). Year to date percentages range from 102 to 156.

Reservoir
Ennis Lake is storing 28,500 acre-feet (89 percent of average and 74 percent of capacity). Hebgen Lake is storing about 307,800 acre-feet of water (117 percent of average and 77 percent of capacity). Hebgen 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 has is expected to yield about 990,000 acre feet (125 percent of normal). Yellowstone at Corwin Springs will yield about 2,450,00 acre-feet (127 percent of normal). Yellowstone near Livingston will yield about 2,850,000 acre-feet (127 percent of normal). Hebgen lake inflow is estimated to be 565,000 acre feet (116 percent of normal). See the following page for detailed runoff volumes.

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - March 1, 1999
==================================================================================================================================
                                              |  <<====== 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       803       914    |      990       125    |      1066      1177            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    APR-SEP      2201      2349    |     2450       127    |      2551      2699           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      2574      2738    |     2850       127    |      2962      3126           2241
                                                                    |                       |
HEBGEN LAKE Inflow                   APR-SEP       480       531    |      565       116    |       599       650            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of February              |        Watershed Snowpack Analysis - March 1, 1999
==================================================================================================================================
                                      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     30.3     29.2     34.1 |   MADISON RIVER in WY         9       167       144
                                                                        |
HEBGEN LAKE                            377.5    289.2    271.1    247.8 |   YELLOWSTONE RIVER in WY    12       154       134
                                                                        |
==================================================================================================================================
 * 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.
(D) - The operational measurement of flow at this point has been discountinued. 

To March 1999 Basin Outlook Report Contents Page