| Spring runoff causes the most common flood threats for Montana east of the divide, those threats being ice jamming and snowmelt flooding. Ice jamming develops when prolonged sub zero weather is followed by significant warming, allowing the ice on rivers to break free and flow downstream. Ice jams typically form as ice accumulates at bends in rivers or at other obstacles such as bridge supports. Water quickly backs up behind the jam and can cause localized flooding. Jams can release very quickly, and flash flooding is often the result as the water stored behind the ice jam then rushes downstream.
Ice jams and ice formation in rivers and creeks can lead to flooding in western Montana starting as early as December. Freeze-up and break-up ice jams have been documented in the months of December, January and February. More extensive flooding results from a combination of ice jams and rain on snow runoff.
Snowmelt flooding occurs as warmer spring temperatures melt snow over the mountains and the runoff overfills creeks and rivers. Snowmelt flooding may be worsened by spring rains falling over the mountain snow pack adding to the water flowing into creeks and rivers.
Snowmelt and the breakup of river ice often occur in tandem. Ice jams respond to fluctuating river flows associated with snowmelt and, as a result, can intensify snowmelt flooding. The National Weather Service provides extensive information on snowpack conditions across the country. A wide array of ground, airborne and satellite observations are used to monitor snow conditions. Your local National Weather Service office routinely provides flood forecasts, whether or not they are related to snowmelt or ice jams.
More information may be obtained from your local National Weather Service Office...
weather.gov/greatfalls
weather.gov/billings
weather.gov/glasgow
weather.gov/missoula
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www.weather.gov 
