During some mornings in Yosemite Valley, when streamflow is relatively high but temperatures are below freezing, some creeks may seem to be full of slush rather than full of water. This is frazil ice, which forms when mist (for example, from a waterfall) freezes, then floats down the creek.
When can I see Frazil Ice?
Frazil ice can occur in fall, winter, or spring, as long as there is relatively high flow over waterfalls and Yosemite Valley has overnight lows below freezing. This most commonly occurs in spring--especially in April, but sometimes in March and May, as well. Since this phenomenon is entirely weather-dependent, it's difficult to predict in advance. The flowing frazil ice usually occurs in the morning, before 9 am.
While working on Yosemite Nature Notes' Frazil Ice episode, the producer (Steven M. Bumgardner) noted how often frazil ice occurred on Yosemite Creek. In 2009, he witnessed about seven frazil ice events on Yosemite Creek, mostly in April. In 2010, there were 20 or 30 events in April, May, and even early June.
Where can I see Frazil Ice?
Frazil ice is most famously seen at Yosemite Creek just below Lower Yosemite Fall, but also occurs in Yosemite Valley along Ribbon Creek, and Sentinel Creek, among other places.
A similar form of frazil ice commonly occurs elsewhere in the US and Canada, for example, in the upper Midwest and along the Hudson River in upstate New York.