In late winter, as air temperatures rise above the freezing point, river ice begins to melt owing to heat transfer from above and to the action of the slightly warm water flowing beneath. As occurs in lake ice, river ice also may deteriorate and rot because of absorption of solar radiation. On the undersurface, the action of the turbulent flowing water causes a melt pattern in the form of a wavy relief, with the waves oriented crosswise to the current direction. Eventually, if the ice cover is not subjected to a suddenly increased flow, it may melt in place with little jamming or significant rise in water level. More likely; however, the ice may be moved and form ice jams.
During the spring in very northern areas, and during periods of midwinter thaw in more temperate areas, additional runoff from snowmelt and rain increases the flow in the river. The increased flow raises the water level and may break ice loose from the banks. It also increases the forces exerted on the ice cover. If these forces exceed the strength of the ice, the cover will move and break up and be transported downstream. At some places the quantity of ice will exceed the transport capacity of the river, and an ice jam will form. The jam may then build to thicknesses great enough to raise the water level and cause flooding. Typically, jams form where the slope of the river changes from steeper to milder or where the moving ice meets an intact ice cover—as in a large pool or at the point of outflow into a lake.
Spring breakup jams are usually more destructive than freeze-up jams because of the larger quantities of ice present. Besides causing sudden flooding, the ice itself may collide with structures and cause damage, even to the point of taking out bridges. Sometimes a jam forms, water builds up above it, and the jam breaks loose and moves downstream only to form again. This process may repeat itself several times. In northerly flowing rivers such behaviour is typical, since the upstream ice is freed first and moves toward colder, more stable ice covers.
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