March 2001 Water Supply Outlook - Snake River Basin (1)

Snake River Basin (1)
March 2001

Snow
Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 57 percent (65% of last year at this time), Pacific Creek -- 63 percent (65% of last year at this time), Gros Ventre River -- 62 percent (80% of last year at this time), Hoback River -- 62 percent (71% of last year at this time), Greys River - 61 percent (66% of last year at this time), Salt River -- 65 percent (66% of last year at this time). Snake River Basin above Palisades is 60 percent of average (68% of last year at this time). See the Basin Summary of Snow Courses at the beginning of this report for a detailed listing of snow course information.

Precipitation
Precipitation across the basin was much below average for last month. Monthly precipitation, for the basin, was 70 percent of average (63 percent of last year). February percentages range from 49 to 108 percent of average. Water-year-to-date precipitation is 58 percent of normal for the Snake River basin (77 percent of last year at this time) Year-to-date percentages range from 43 to 81 percent of average.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake -116 percent of average (12,800 acre feet compared to 12,500 last year), Jackson lake - 133 percent of average (638,300 acre feet compared to 653,500 acre feet last year), and Palisades Reservoir -65 percent of average (695,600 acre feet compared to 1,247,100 acre feet last year).

Streamflow
The most probable, 50 percent chance April through September runoff yield forecast is below average for the basin. The Snake near Moran is expected to yield 590,000 acre-feet (68 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 1,820,000 acre-feet (68 percent of normal). The 50 percent chance yield near Heise is expected to be 2,550,000 acre-feet (63 percent of normal). Pacific Creek at Moran is expected to yield about 111,000 acre-feet (67 percent of average). Greys River above Palisades Reservoir is estimated to yield 235,000 acre-feet (61 percent of normal). Salt River near Etna is estimated to have a yield of 235,000 acre-feet (59 percent of normal).

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - March 1, 2001
==================================================================================================================================
                                              |  <<====== 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)
====================================================================|=======================|=====================================
SNAKE near Moran (1,2)               APR-SEP       411       534    |      590        68    |       646       769            869
                                                                    |                       |
SNAKE above Palisades (2)            APR-SEP      1459      1674    |     1820        68    |      1966      2181           2671
                                                                    |                       |
PALISADES RESERVOIR INFLOW (1,2)     APR-SEP      1677      2167    |     2390        64    |      2613      3103           3763
                                                                    |                       |
SNAKE near Heise (2)                 APR-SEP      1934      2301    |     2550        63    |      2799      3166           4049
                                                                    |                       |
PACIFIC CREEK at Moran               APR-SEP        76        97    |      111        67    |       125       146            166
                                                                    |                       |
GREYS above Palisades                APR-SEP       161       205    |      235        61    |       265       309            388
                                                                    |                       |
SALT near Etna                       APR-SEP       128       192    |      235        59    |       278       342            399
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of February              |        Watershed Snowpack Analysis - March 1, 2001
==================================================================================================================================
                                      Usable |  *** Usable Storage ***  |                            Number     This Year as % of
Reservoir                            Capacity|  This      Last          |   Watershed                  of       =================
                                             |  Year      Year     Avg  |                          Data Sites   Last Yr   Average
========================================================================|=========================================================
GRASSY LAKE                             15.2     12.8     12.5     11.0 |   SNAKE above Jackson Lake    9        65        57
                                                                        |
JACKSON LAKE                           847.0    638.3    653.5    481.0 |   PACIFIC CREEK               3        65        63
                                                                        |
PALISADES                             1400.0    695.6   1247.1   1063.1 |   GROS VENTRE RIVER           3        80        62
                                                                        |
                                                                        |   HOBACK RIVER                6        71        62
                                                                        |
                                                                        |   GREYS RIVER                 5        66        61
                                                                        |
                                                                        |   SALT RIVER                  5        66        65
                                                                        |
                                                                        |   SNAKE above Palisades      29        68        60
                                                                        |
==================================================================================================================================
 * 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.

To March 2001 Basin Outlook Report Contents Page

Snake River Basin (1) March 2001

Snake River Basin (1)
March 2001