Friday, April 2, 2010

Yosemite's Curious Frazil Ice

2 April 2010
This is the month when locals look for frazil ice flows in Yosemite Valley. They can occur below any of our waterfalls, but are most often seen on large scale below Lower Yosemite Falls, when deposits can cover acres with what looks like snow, and can persist for months. We've had frazil forming on several recent mornings in Bridalveil and Yosemite Creeks.

It's hard to describe this material and its origin if you haven't seen it for yourself. Look at in May for an NPS video that'll give you an idea. Imagine a creek turned into a slurry of suspended ice particles, or a lava flow made of ice, or spilling a million-gallon slushie drink into the forest.
It forms mostly in the spring when warm temperatures melt snow into high run-off and lots of mist in the waterfalls, then overnight freezing temperatures turn a portion of the spray into ice droplets, which flow along in the full creek. Because we get these conditions regularly, Yosemite is one of the best places to observe this 'waterfall spray ice' being created and then slowly melted away. There's a persistent myth that these deposits represent the sudden collapse of the Upper Yosemite Falls snow cone (a huge mound of ice that accumulates below the falls), but this is emphatically not the case. Frazil flows happen when it's quite cold, beneath falls with no snow cones, and they can happen repeatedly for weeks.

If you're lucky enough to be in the park this month, stroll by the base of the falls for a look.

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