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May Snowpack Below Average, Streamflow Forecasts Affected by Rapid Snowmelt

May 4, 2007

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

Bozeman-- Statewide mountain snowpack decreased during April to about 68 percent of average as recorded by the Natural Resources Conservation Service (NRCS) on May 1. This is only 70 percent of the snowpack that was recorded on this date last year.

“These conditions are due mostly to the variable April weather,” said Roy Kaiser, NRCS water supply specialist. “There were both record high temperatures and storms that yielded significant snow water increases.”

Mountain precipitation was below average at 83 percent during April and 96 percent of average for the water year. Mountain precipitation varied widely across the state. Precipitation levels swung from 74 percent of average in the Yellowstone River Basin to 78 percent of average in the Columbia River Basin to 86 percent of average in the Missouri River Basin.

The unseasonably warm temperatures during April and the beginning of May contributed to the above average snowmelt occurring across the state. This rapid snowmelt has resulted in decreased mountain snowpack. Snowpack decreased by one to three percent west of the Continental Divide and decreased by five to seven percent east of the Divide. Mountain snowpack was 74 percent of average and 75 percent of last year in the Columbia River Basin; 63 percent of average and 64 percent of last year in the Missouri River Basin; and 63 percent of average and 90 percent of last year in the Yellowstone River Basin.

Mountain Snowpack
River Basin Percent of Average Percent of Last Year April Percent Change
Columbia 74 75 -1
--Kootenai, Montana 81 77 -4
--Kootenai, Canada 108 114 +3
--Flathead, Montana 77 79 +4
--Upper Clark Fork 72 75 +1
--Bitterroot 57 56 -13
--Lower Clark Fork 69 65 -10
Missouri 63 64 -3
--Missouri Headwaters 62 59 -4
----Jefferson 66 61 -1
----Madison 58 57 -6
----Gallatin 63 63 -4
--Missouri Mainstem 67 76 -1
----Headwaters Mainstem 63 63 -12
----Smith-Judith-Musselshell 74 77 +5
----Sun-Teton-Marias 61 79 -6
----Milk (Bearpaw Mountains) 173 -- --
St. Mary 75 89 -10
St. Mary & Milk 76 93 +13
Yellowstone 63 90 -6
--Upper Yellowstone 62 75 -3
--Lower Yellowstone 64 108 -10
Statewide 68 70 -2

Statewide, streamflows are forecast to average between 62 and 74 percent. Streamflows west of the Divide are forecast to average between 72 and 87 percent, while streamflows are forecast to average between 54 and 69 percent of average east of the Divide.

“Below average May 1 snowpack and higher than average snowmelt may mean that streams could reach their lowest flows in mid summer,” said Kaiser. “Streams and rivers without storage could be affected earlier than those with lake and reservoir storage.”

Below are averaged river basin streamflow forecast summaries for the period May 1 through July 31. These forecasts assume near normal spring conditions and do not account for well below average (70 percent of less) or well above average (130 percent or more) snowmelt or spring rain. Specific forecast probabilities are available in each individual River Basin Report.

Streamflows
River Basin May-July This Year Percent of Average May-July Last Year Percent of Average
Columbia 72 to 81 85 to 95
--Kootenai 84 to 92 85 to 94
--Flathead 77 to 84 83 to 92
--Upper Clark Fork 65 to 78 81 to 96
--Bitterroot 67 to 74 96 to 105
--Lower Clark Fork 70 to 77 86 to 93
Missouri 50 to 66 75 to 94
--Jefferson 37 to 55 75 to 97
--Madison 67 to 74 101 to 108
--Gallatin 70 to 78 98 to 106
--Missouri Mainstem 54 to 68 89 to 103
--Smith-Judith-Musselshell 35 to 57 81 to 101
--Sun-Teton-Marias 59 to 76 57 to 76
St. Mary 82 to 88 85 to 91
Yellowstone 63 to 75 74 to 85
--Upper Yellowstone 67 to 78 87 to 97
--Lower Yellowstone 56 to 70 54 to 68
Statewide 62 to 74 78 to 92

NOTE: The May-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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