Frazil Ice
From HydroWiki
Frazil ice is a whitish, slushy looking ice that is found in water. It is often called needle ice or frazil crystals. This ice occurs in waters that are below freezing, or supercooled.
Frazil ice forms in turbulent water, and often in fast flowing waters of rivers in the northern United States in the winter season. Normally ice will form over the surface of the river, but in the case of frazil the water is too turbulent to freeze over. Once frazil forms it will continue to form if temperatures permit. It will appear to look like sludge slowly moving in a river and it will freeze onto any object it may come in contact with. Frazil ice can freeze to rocks on the bottom of rivers and accumulate reducing the flow of the water, this is called anchor ice. It can also accumulate beneath other floating ice in the river. If accumulation continues frazil ice can take over a section of river and cause blockage.
The favorable conditions for the formation of frazil include a cold clear night when there is significant heat loss from the surface. This will enable the water to become supercooled in turbulent areas.
References:
River Ice Processes by Meteorology Education and Training (Meted)
Maidment, David R. (1993). Handbook of Hydrology. New York: McGraw-Hill, Inc.
