Brain Post: What Is Frazil Ice?

Guest Author | | BrainsBrains

Frazil ice (pronounced “frazzle”) isn’t quite ice and it’s not quite snow. It’s the slushy mass that forms in your local creeks and rivers in the winter….sometimes causing the waterway to flood or change course.

Frazil ice, an ice crystal formation, is the first stage in the forming of sea ice, but also commonly occurs in mountain streams and rivers. Frazil forms on clear, cold evenings when the air temperature reaches –6°C/ 20°F or lower or a very cold front moves into the area rapidly cooling the water temperatures.

It’s the primary culprit in causing ice jams and winter flooding in mountain rivers and streams and wreaks havoc on familiar locales like downtown Jackson, Wy and Yosemite creek.

Frazil accumulates as the crystals grow in number and size and starts to move somewhat like flowing lava.Because of the small size of the ice crystals, frazil tends to sink, rather than float like typical ice, so it can quickly overwhelm a small river or creek from the bottom up before the issue is recognized.

Frazil ice yosemite
Frazil ice forming within the falling water on Yosemite Falls, CA.

If the frazil is able to bridge the waterway, it can block the flow and cause flooding. This type of flooding is particularly dangerous as the frazil can often look like a solid ice sheet, although it’s generally weak and easily breakable at this stage and can mask very deep, frigid water below.

Once frazil has started to accumulate and block waterways, it can be controlled in a few different ways; Here in Jackson, both mechanical and thermal methods are used to break up the frazil.

Mechanical generally involves using a back ho or bucket loader to manually break up the ice sheet along the river banks or simply adding sandbags to hold back the water. Thermal control entails drawing warmer water from various thermal wells and pumping into the waterway to break up the ice downstream.

In addition the benevolent sounding but more exciting ‘vibration control’ is utilized elsewhere, which really means blasting the ice with dynamite to break it up.


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