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My document Snowmelt and Ice Jam Flooding
National Flood Safety Awareness Week
March 17-21, 2014

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National Flood Safety Awareness Week continues today with the theme of Flood Hazards. Here we will discuss ice jam and snowmelt flooding.

Spring runoff causes the most common flood threats in jam 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 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 National Weather Service offices in Montana...

Month of Occurrence of Reported Ice Events
West and East of the Divide

Percent by Month West of Divide Percent by Month East of Divide

Locations of Reported Ice Events West of the Divide

Streams West of Divide 2 or more Cities West of Divide 2 or more

Locations of Reported Ice Events East of the Divide
Streams East of Divide 10 or more Cities East of Divide 10 or more

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