Ice jams are a major cause of flooding in Canada.
In fact, for most Canadian rivers, the annual peak water levels are due to ice jams. These jams result from the accumulation of ice fragments that build up to restrict the flow of water and then act as a temporary obstruction. It is very common in rapids. The ice crystals tend to coalesce and accumulate, and may become attached to the underside of the ice cover or to the river bed as anchor ice. The coalescing frazil can also form flocs, which float to the surface when stream turbulence is reduced or when the buoyancy of a growing mass exceeds downward turbulence.