In the northern hemisphere , northerly flowing rivers tend to have more ice jams because the upper , more southerly reaches thaw first and the ice gets carried downstream into the still-frozen northerly part . There are three physical hazards of ice jams . The ice floe can form a dam that floods the areas upstream of the jam . This occurred during the 2009 Red River Flood and the 2009 Alaska floods . As the ice jam breaks apart , a sudden surge of water breaking through can then flood areas downstream of the jam . The third hazard is that the ice buildup and final drive may damage structures in or near the river and boats in the river
2009 Alaska floods
2009 Alaska floods
Ice jam