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slightly above freezing. This relatively warm water is transferred to the underside of the ice cover, where turbulence causes a wavy relief pattern, creating a series of crests and troughs. As ice thickness decreases, the troughs under the ice cover melt and create patches of elongated open water perpendicular to flow direction. With the introduction of warmer and higher magnitude flows into the river, particularly from tributaries or reservoir releases, the ice cover begins to melt along the shoreline. This leads to a failure of shoreline anchor points, where the ice cover is held against shear stresses of the underlying flow. This near shore melting is the first stage in deterioration of the ice cover, and makes it more vulnerable to mechanical destruction and further deterioration (Ashton 1980, Donchenko 1978). Donchenko (1978) reports that diurnal and weekly flow regulations in reservoir tailwaters changes hydraulic characteristics, resulting in a disruption of equilibrium forces that farm surface ice. Fluctuating water levels and increased velocity in the passage of discharge waves result in the appearance of shoreline, or "coastal" cracks and a disruption in continuity of the ice cover. Sharp oscillations in water level, during the first days of freeze-up, can lead to destruction of the ice cover and interruption of ice forming processes. The ice cover may buckle and reach critical deflection stresses, and ultimately break. Destruction of the upstream edge of the ice cover occurs when discharge waves crest beneath the ice cover and raise, break, and transport ice blocks downstream. It is possible to calculate the amount of critical rise in water level that would lead to the formation of coastal cracks (Donchenko 1978). The vertical rise in river level necessary to break an ice layer is approximately three times the thickness of that layer, i.e., a 3-cm rise in river level is needed to fracture a 1-cm layer of surface ice. Deterioration of the ice cover is also affected by increased air temperature and direct solar radiation: As snow cover melts, the exposed surface ice becomes isothermal at 0°C. This isothermal state leads to a general deterioration of ice cover strength, caused by melting at ice grain boundaries where impurities have collected during freezing. This deterioration greatly reduces internal strength, while ice thickness is not significantly diminished. This process is called "candling", and is characterized by a separation of vertically aligned columnar crystals. Candling has been observed on the surface and undersurface of ice cover, and leads to a greater susceptibility to mechanical destruction. River Ice Breakup and ice Jams Breakup of ice cover usually results from a complex set of circumstances, but can generally be attributed to increased flow, solar radiation, or thermal inputs. Major manifestations of breakup 35