During breakup in the spring, or during winter thaws, an ice jam results from the accumulation of ice from the breakup of the upstream ice cover. A rise in water levels may result from the spring snowmelt, or common to the southern and Atlantic parts of Canada, a sudden midwinter thaw. Midwinter thaws are often accompanied by substantial rainfall, resulting in a rapid increase in water levels and severe ice jams. The ice cover may be broken in a major fashion due to uplift pressures exerted by rapidly rising water levels. The cover breaks down into smaller pieces after being set in motion by the flow and impacting with channel boundaries or other sections of ice cover.
Ice jams cause flooding because of two main features.
First, ice jam thickness can be considerable, amounting to several metres. Secondly, the underside of the ice cover is usually very rough. Under open water conditions, the only frictional resistance retarding the flow of the water is the streambed. The rougher the streambed, the greater the depth required to pass a given discharge. With an ice jam in place, the additional ice and very rough lower surface retard flow.