As the water surface continues to lose some of its heat to the atmosphere, these ice pans freeze together to form a continuous ice cover. The accumulation of frazil ice against an ice cover progresses in an upstream direction when the velocity at the upstream edge of the cover is low. At high velocities,the frazil is transported downstream under the ice cover where it adheres to the undersurface in a low velocity area. A hanging dam can cause extensive blockage of the flow area resulting in increased upstream water levels and potential flooding. Anchor ice generally forms in the evening when the rate of heat loss is the greatest and supercooling occurs.
Anchor ice can cause a significant blockage of the flow cross-section resulting in local flooding.
Little is known about anchor ice formation, growth and detachment. Once a thin ice sheet has formed, it begins to grow downward by freezing at the ice-water interface. Heat loss is retarded by the ice cover itself and by snow cover that may be present. Where freezing takes place in a frazil accumulation, the rate of thickening of the solid ice layer is inversely proportional to the porosity of the accumulation. In addition, longitudinal and transverse cracks, Plate 3, further reduce the strength of the cover.