NWS Reminder on Ice Jam Season

Temperatures will be warming up in the Upper Yellowstone and Upper Musselshell River basins with readings in the fifties to lower sixties by mid-week. That will allow tributaries to start sending water and ice into the main stem rivers. Generally, it takes about five to ten days to evacuate ice out from the Billings area all the way through the Lower Yellowstone River. Things will take a little longer in the Milk and Poplar River basins as temperatures are not as warm, but it won’t be too far behind the other two.

Two-thirds of Montana’s ice jams occur in February and March. Montana has the highest number of reported ice jams and unfortunately the highest number of ice jam related deaths in the lower 48 states. There are over 1700 ice jam events documented in the Cold Regions Research and Engineering Lab (CRREL) Ice Jam Database. That is approximately ten percent of the total for the continental United States. These ice events in Montana have been reported on 163 different streams and rivers and in 199 different locations. Of all these events, 21 percent have occurred in February with 45 percent occurring in March. Miles City has the highest number of reported ice jams in the state, with Bozeman, Nashua, Sidney, and Wolf Point all in the top five.

Ice jamming develops when prolonged subzero 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 rapid flooding. Jams can release quickly, and flash flooding often results as the water stored behind the ice jam rushes downstream.

No two ice jams are alike. They are unpredictable in where they will form, how long they will last or how high the water will rise. In Glendive in March 2014, the Yellowstone River rose eight feet in 15 minutes and caused significant flooding. Ice jams can cut off access routes and leave you trapped. Checking the river conditions should only be done during daylight hours by trained emergency response personnel. Six inches of moving water can knock a person off their feet, a foot of water can carry away a small car, and two feet of water can cause a larger vehicle to be carried away.  NEVER drive through flooded roads. 

If you notice ice jams occurring, contact your local National Weather Service office or county disaster and emergency services coordinator. While many ice jams develop and release before they cause significant flooding, some jams can produce extensive flooding.

If you have livestock, hay or equipment along a river in eastern Montana, now is a good time to move things out of the flood plain. For those who live near a river, having a go-kit ready with important papers and medications is recommended as well.

For more information on Ice Jams, visit the NWS page at: https://www.weather.gov/glasgow.