What is frazil ice?

Frazil ice is a kind of slush ice. It can form in watercourses when cold air temperatures and wind chill combine to cause surface water temperature to be super-cooled (i.e. cooled below the freezing point).

The super-cooled water is unable to form a solid ice cover because the water is moving or flowing fast.

As the slushy frazil ice flows downstream, it will eventually come to rest against obstructions like islands, bridge piers, and abutments. It also comes to rest in low velocity areas around bends and over slope reductions and in areas where the channel constricts.

Where it comes to rest, frazil ice will accumulate. It may also anchor itself to the bottom of a watercourse and accumulate. Where frazil ice accumulates, it is likely to cause a restriction of water flow downstream, resulting in increased water levels and possibly flooding.