Frazil ice

Frazil ice is a collection of loose, randomly oriented needle-shaped ice crystals in water. It resembles slush and has the appearance of being slightly oily when seen on the surface of water. It sporadically forms in open, turbulent, supercooled water, which means that it usually forms in rivers, lakes and oceans, on clear nights when the weather is colder, and air temperature reaches −6 °C (21 °F) or lower. Frazil ice is the first stage in the formation of sea ice.

Formation

When the water surface begins to lose heat rapidly, the water becomes supercooled. Turbulence, caused by strong winds or flow from a river, will mix the supercooled water throughout its entire depth. The supercooled water will already be encouraging the formation of small ice crystals (frazil ice) and the crystals get taken to the bottom of the water body. Ice generally floats, but due to frazil ice's small size relative to current speeds, it has an ineffective buoyancy and can be carried to the bottom very easily.