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Mountain Snowpack Decreases Over March, Potentially Impacts Summer Streamflows

April 5, 2007

For More Information:
Roy Kaiser, 406-587-6991

“Lower than average mountain precipitation combined with higher than average temperatures resulted in an overall decrease in statewide snowpack in March and a potentially dismal forecast for streamflows,” said Roy Kaiser, Natural Resources Conservation Service (NRCS) water supply specialist.

March mountain precipitation statewide was 66 percent of average. Increases in mountain snowpack were well below average except in the Tongue and Powder River basins; where above average mountain snowpack increases occurred at the end of March. March mountain precipitation west of the Continental Divide was 82 percent of average and east of the Divide was 54 percent of average.

Of special concern to Kaiser was the fact that snow melt had already begun at lower elevations and on warmer exposures. “Luckily, cooler temperatures in late March and early April have slowed snow melt,” said Kaiser. The snowpack stored in the mountains makes a significant contribution to streamflows across the state during the spring and summer months.

Statewide, mountain snowpack decreased an average of 17 percent during March. However, data gathered by the automated SNOTEL system maintained by NRCS indicates there has been about a two to five percent gain in the mountain snowpack statewide since April 1 and the Lower Yellowstone River Basin gained about 16 percent.

On April 1, the mountain snowfall season is about 95 percent complete and the mountains will typically reach seasonal peaks during April. As of April 1, mountain snow water contents statewide were 70 percent of average and 70 percent of last year. West of the Continental Divide, snowpack was 75 percent of average and 75 percent of last year. East of the Divide, snowpack was 68 percent of average and 70 percent of last year.

Mountain Snowpack
River Basin Percent of Average Percent of Last Year March Percent Change
Columbia 75 75 -15
--Kootenai, Montana 85 80 -14
--Kootenai, Canada 105 119 -4
--Flathead, Montana 73 73 -16
--Flathead, Canada 75 69 -27
--Upper Clark Fork 71 75 -14
--Bitterroot 70 67 -15
--Lower Clark Fork 79 78 -14
Missouri 66 65 -21
--Missouri Headwaters 66 63 -16
----Jefferson 67 65 -16
----Madison 64 61 -15
----Gallatin 67 64 -13
--Missouri Mainstem 68 70 -30
----Headwaters Mainstem 75 77 -18
----Smith-Judith-Musselshell 69 64 -33
----Sun-Teton-Marias 67 75 -23
----Milk (Bearpaw Mtns.) 47 45 -68
St. Mary 65 67 -22
St. Mary & Milk 63 63 -39
Yellowstone 69 77 -10
--Upper Yellowstone 63 66 -18
--Lower Yellowstone 74 91 -3
Statewide 70 70 -17

“Statewide, streamflow forecasts average 64 to 77 percent, and rain will be necessary to keep rivers and streams from falling to critically low flows,” said Kaiser. Actual streamflow volumes and peaks will be dependent upon temperatures and the amount of rain each basin receives. West of the Continental Divide, streamflow forecasts average 74 to 83 percent and east of the Continental Divide, average between 57 and 73 percent.

Below are streamflow forecasts for April 1 through July 31. These forecasts assume near normal spring conditions and do not account for well below average (70 percent or less) or well above average (130 percent or more) snowmelt or spring rain. Specific forecast probabilities are available in individual River Basin Reports online at http://www.mt.nrcs.usda.gov/snow/.

Streamflow Forecasts
River Basin April-July This Year Percent of Average April-July Last Year Percent of Average
Columbia 74 to 83 87 to 97
--Kootenai 84 to 93 90 to 98
--Flathead 77 to 84 92 to 100
--Upper Clark Fork 70 to 82 76 to 90
--Bitterroot 71 to 79 102 to 111
--Lower Clark Fork 73 to 81 88 to 95
Missouri 54 to 72 78 to 97
--Jefferson 42 to 59 74 to 93
--Madison 71 to 77 103 to 110
--Gallatin 67 to 77 99 to 109
--Missouri Mainstem 55 to 68 85 to 100
--Smith-Judith-Musselshell 40 to 64 102 to 120
--Sun-Teton-Marias 65 to 82 63 to 82
--Milk 63 to 93 40 to 83
St. Mary 83 to 90 85 to 92
Yellowstone 63 to 75 83 to 94
--Upper Yellowstone 67 to 77 94 to 104
--Lower Yellowstone 59 to 73 67 to 81
Statewide 64 to 77 82 to 96

NOTE: The April-July Last Year Percent of Average column above is what was forecast last year, not what actually occurred.

The NRCS snow survey and water supply forecast program provides detailed snow water content, streamflow forecast, and climate data for Montana by evaluating data collected by a series of manual and automated snow survey sites located in the mountains of the state. Visit the NRCS website at http://www.mt.nrcs.usda.gov/snow/ for more information.

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