What causes an ice jam to form in a river?

An ice jam forms in a river when the quantity of moving ice exceeds the river's transport capacity, causing an accumulation that blocks the flow. This typically occurs in two specific scenarios: where the slope of the river changes from steeper to milder, or where moving ice encounters an intact ice cover, such as in a large pool or at the point of outflow into a lake.12

During the spring or midwinter thaws, increased runoff from rain and snowmelt raises water levels, breaking ice loose from riverbanks and exerting forces that exceed the strength of the ice cover. This causes the ice to break up and move downstream. If the ice encounters a restriction or a change in river geometry, it clusters and stacks, creating a blockage. These jams can significantly raise water levels upstream, leading to flooding, and may cause structural damage to bridges or other infrastructure as the ice moves or the jam fails.12