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Reference.org
Ice jam
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<p>Ice jams occur when the ice that is drifting down-current in a river comes to a stop, for instance, at a river bend, when it contacts the river bed in a shallow area, or against bridge piers. Doing so increases the resistance to flow, thereby inducing an increase in water level upstream of the jam (referred to as <i>backwater</i>). Ice jams are thus a main cause for flooding during the winter. In addition, when the jam is released, depending on the conditions under which this happens, the amount of water that was retained behind the jam can also lead to flooding <i>downstream</i> of where the jam occurred. Ice jam floods are generally less predictable and can also be faster than open-water floods. </p>