Frazil ice are small crystals of frozen mist that end up getting washed into creeks and streams, turning them into piles of snow.
“In many ways, it flows like lava,” said Yosemite National park naturalist Pete Devine. “The ice will surge into place and with enough thickness, it stops, and the creek will flow in a different direction.”
It looks like snow, Devine says, but if you pay more attention you’ll notice that there are only piles of it in some areas and not others—and that’s because it’s not snow, but a semi-frozen body of water.
If the body of water was originally deep enough or big enough, it’s potentially dangerous. Someone stepping on what they think is snow could sink into the stream quite quickly.
And actually, the appearance of frazil ice means the opposite of what snowfall would—it means spring is here, and that snow-melt is speeding up.