Ice jams are caused by melting snow and ice in the springtime. Warm temperatures and spring rains cause snow and ice to melt very rapidly. All this extra water causes frozen rivers and streams to swell up, and the layer of ice on top of the river begins to break up.
The rushing river carries large chunks of ice downstream, and sometimes a group of ice chunks get stuck in a narrow passage of the river. The ice chunks form an ice jam, which blocks the natural flow of the river.
Ice jams can be dangerous for people living in towns nearby. Because the river is blocked, the rushing water has nowhere to go and it can cause flooding in the surrounding area.
According to the National Weather Service:
Spring runoff causes the most common flood threats for Montana east of the divide, those threats being ice jamming and snowmelt flooding. Freeze-up jams occur when prolonged sub-freezing weather allows an ice cover to develop on a river or stream.
Break-up jams occur when the freezing weather is followed by significant warming, allowing the ice to break free and flow downstream. Jams typically form as ice accumulates at obstacles such as bends in the rivers or bridge supports. Water can quickly back up behind the jam causing 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.
When snowmelt, rain, or rain on snow occur in tandem with the breakup of river ice, the result is often more intense flooding.