Frazil Ice

an accumulation of loose, spongy ice within a body of water (deep frazil ice) or on its surface (surface frazil ice). Frazil ice forms from tiny crystals of deep ice (intermediate and bottom ice) and from grease ice and slush. It occurs before the freeze-up when water is supercooled below 0°C, primarily on mountain rivers and rivers with rapids, below polynyas and the lower pools of hydroengineering complexes, and, occasionally, on lowland rivers and lakes.

Frazil ice, carried downstream by the current, covers large areas and dams up the channel, causing ice jams. It also considerably impedes the operation of hydroengineering structures by clogging up the filters of water intakes and the aperture grills at hydroelectrical power plants.