Rangers from Yosemite National Park have shared a video to help hikers recognize dangerous frazil ice, which can be a serious hazard to winter hikers.
The video, which you can watch below, was shot at Yosemite Creek, and shows a stream of slushy ice flowing along the ground. The ice has filled the creek's drainage under Yosemite Fall, and overtaken the bridge and trail.
"While its lava-like flow may mesmerize the eye, frazil ice is deceptively dangerous," explained Rangers in a Facebook post. "It’s not a solid surface despite its appearance, so when its slush hides the boundaries of sure footing, falling in and becoming trapped beneath is a serious hazard."
Rangers explained that the affected area is temporarily closed, and will need to be assessed for potential damage once the ice has melted.
What is frazil ice?
Frazil ice is made up of ice crystals measuring 1mm or less in diameter, of various different shapes. It forms in areas of moving water, such as rivers and even oceans, where the water temperature drops below freezing.
Water on the surface becomes supercooled, creating ice crystals, and the turbulence distributes these throughout the whole body of water. The number of crystals rapidly increases, creating the slush, which can accumulate at points and cause flooding where the normal flow of water is blocked.
As the National Park Service explains, frazil ice can occur at Yosemite in fall, winter, or spring provided there's relatively high flow over waterfalls and temperatures drop below freezing at night. It's hard to predict, so it's well worth being aware of what it looks like, and the potential dangers.
If you want to see the phenomenon yourself, you're most likely to spot flowing frazil ice at Yosemite Creek before 9am, just below Lower Yosemite Fall.
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Cat is the editor of Advnture, She’s been a journalist for 15 years, and was fitness and wellbeing editor on TechRadar before joining the Advnture team in 2022. She’s a UK Athletics qualified run leader, and in her spare time enjoys nothing more than lacing up her shoes and hitting the roads and trails (the muddier, the better), usually wearing at least two sports watches.