Frazil ice, as you see here in a recent picture of the Hudson River, forms in turbulent rivers (and the ocean and sometimes lakes) when the air is below about 18 degrees F. The river water itself has to be a fraction of a degree below 32 F (super-cooled).
For winter newcomers to the Upper Hudson, and you who have not read or understood my many attempts to explain this before: That brilliant white stuff that sometimes fills the Hudson from Thurman to The Glen is not snow, and has nothing to do with it. It is “frazil ice” (pronounced like frazzle), which is made of tiny, round, very thin disks of clear ice).
Frazil forms in turbulent rivers (and the ocean and sometimes lakes) when the air is below about 18 degrees F. The river water itself has to be a fraction of a degree below 32 F (super-cooled).
The nucleus of each frazil crystal is an ice crystal, often from the freezing of tiny drops from bursting bubbles. The frazil disks try to put out “dendrites” (like snowflakes) but they keep breaking off in the rough water, creating unending nuclei.
When the tons of frazil floating down the river hang up on gravel islands or get clogged in a curve, a floating cover of loose frazil can form across the river.
The inexorable frazil coming down the river gets pulled under the cover by the current, where it collects and builds “hanging dams”, which eventually block the current.
The river water is slowed down by the blockages. When it moves at less than two miles an hour, it collects on the surface.
The hanging dams cause the water level in the river to rise, loosening and spreading out the frazil cover until the edges are above the sloping cobble shores. (You can hear crackling of the frozen-over pools of water on the edge as they are cracked by the rising water.)
The pressure from the still collecting frazil and the loosening of the former cover cause the hanging dams to give way, lowering the water level quite quickly and leaving huge frazil banks on the shores.