Frazil is ice that forms on rivers with significant flow and under cold conditions. It is famous for plugging inlets to power dams and building up on the bottom of rivers (anchor ice). It forms mostly on rivers where there is enough tubulerance to mix the water. In cold weather the surface water supercools about 0.1 deg (C) and nucleates. The crystals are thin disks (kept round by the turbulance). On a lake with some waves and no snow falling, dendritic crystals form.
This LINK is to an exelent article by Evelyn Greene on the subject.
