Some rivers in the high latitudes of the northern hemisphere do not ice over completely in winter to form a stable ice cover, usually due to their high water velocities. Ice may be formed due to low temperatures, but it breaks away and large quantities move downstream, tending to damage bridge structures, producing ice jams and consequent flooding, particularly when such jams break. Ice moving in this way may causes much damage to hydroelectric installation intakes.
Floods caused by ice jams and ice breaking up occur in the early spring. They often occur at constriction points such as at a sharp bend, gorge, bridge-crossing or any other physical obstacle. They may also occur where the gradient of a channel changes from steep to gentle, or at the point where a stream discharges into a lake. In Canada and Russia such floods typically occur when the ice and snow in the headwaters of northward flowing streams melt more rapidly than ice and snow in the lower reaches.
An analysis of the impact of climate warming on transportation in the MacKenzie Basin region of northern Canada found that there would be an increase of the open water (navigable) season on the MacKenzie River of 19-42 days, depending on the scenario used. This would favour an increase in shipping by barge, the least expensive method.