This seems to be Anchor Ice.
Anchor Ice is defined by the World Meteorological
Organization as "submerged ice attached or anchored to the bottom,
irrespective of the nature of its formation." It is usually seen in rivers
or lakes during extremely cold weather conditions and in the Antarctic and the Arctic
regions as well.
Submerged rocks are clearly visible in these photographs.
The essential conditions for anchor ice formation
are that the temperature of the water is at its freezing point, and that
of the air in contact to be below that point; the surface of the water must be
exposed to the air, and there must be a current in the water.
In places where water is moving, ice formation
may not be uniform, but ice platelets can form quickly in the water column
overlying submerged objects like rocks, metals etc. Anchor Ice tends to be
composed of numerous crystals attached to each other as flocculent masses. The constant agitation prevents a continuous sheet of ice to be formed.
Anchor Ice formation can seriously hamper the
functioning of Hydro Electric power plants by impeding the flow or stopping the
turbines.
