Upper Yellowstone and Madison River Basins (2)
April 1999

Snow
Snowfall in the basin this year has been well above average. Snow water equivalent (SWE) is about 123 percent of average (145 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 119 percent of average (137 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 80 percent of average (67 percent of last March) for the 7 reporting stations - - percentage range was from 22 to 96 percent of average. Water-year-to-date precipitation is about 121 percent of average (130 percent of last year's amount). Year to date percentages range from 91 to 146 for the 7 reporting stations.

Reservoir
Ennis Lake is storing 31, 000 acre-feet (93 percent of average and 76 percent of capacity). Hebgen Lake is storing about 258,300 acre-feet of water (105 percent of average and 68 percent of capacity). Hebgen Lake is storing about 99 percent and Ennis Lake is storing about 98 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 960,000 acre feet (121 percent of normal). Yellowstone at Corwin Springs will yield about 2,385,00 acre-feet (123 percent of normal). Yellowstone near Livingston will yield about 2,775,000 acre-feet (124 percent of normal). Hebgen lake inflow is estimated to be 580,000 acre feet (119 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - April 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       837       910    |      960       121    |      1010      1083            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    APR-SEP      2143      2287    |     2385       123    |      2483      2627           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      2531      2676    |     2775       124    |      2874      3019           2241
                                                                    |                       |
HEBGEN LAKE Inflow                   APR-SEP       503       549    |      580       119    |       611       657            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of March                 |        Watershed Snowpack Analysis - April 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     31.0     29.6     33.2 |   MADISON RIVER in WY         8       145       126
                                                                        |
HEBGEN LAKE                            377.5    258.3    262.4    246.6 |   YELLOWSTONE RIVER in WY     9       137       121
                                                                        |
==================================================================================================================================
 * 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 April 1999 Basin Outlook Report Contents Page