Montana receives precipitation in the form of rain and snow. Runoff from rainwater and snowmelt may be captured and temporarily stored in one of Montana’s many lakes and reservoirs.
West of the Continental Divide, the Clark Fork and Kootenai Basins have a Pacific Northwest climate, which is generally wetter and more temperate than the rest of the state. Higher elevations receive a heavy winter snowpack, and much of the area receives more rainfall than lands to the east.
East of the divide, Montana is generally drier, windier, and experiences more extreme seasonal temperature fluctuations. Valley and prairie lands are arid to semi-arid, some receiving less than 10 inches of moisture a year. High elevations east of the divide can accumulate a heavy snowpack and also receive more rainfall than the lower elevations.
Montana’s mountain snowpack serves as a natural reservoir for water that is released over the spring and summer months. The heaviest snowfall occurs between November and March, but heavy snowstorms can occur as early as mid-September or as late as early May. High elevation snowmelt runoff generally peaks in mid-June and trails off over the summer months. In the lower elevations, snow accumulation may runoff during February and March.
Periods of drought may result in less winter snowfall and less water stored in the mountain snowpack. Periods of warmer temperatures may or may not decrease the overall amount of precipitation Montana receives. Warm temperatures may result in more of the state’s precipitation falling as rain rather than snow. Warmer temperatures may also cause the snowpack to melt sooner and faster shifting the peak period of runoff to earlier in the season. See Chapter 7 – A Changing Climate for more information.
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