From AMS Glossary
Ice attached to the beds of streams, lakes, and shallow seas, irrespective of its nature of formation.
On clear, cold nights in relatively still water, anchor ice may form directly on submerged objects. It also develops in supercooled water if turbulence is sufficient to maintain uniform temperature at all depths, in which case a spongy mass of frazil accumulates on objects exposed to rapid flow, and later deposition fills in the pores and creates solid ice. When the water temperature increases to above 0°C, the ice rises to the surface, often carrying with it the object on which it had accumulated. Sometimes anchor ice is erroneously called ground ice, a term which should be reserved for bodies of more or less clear ice in frozen ground.