Frazil ice was seen this morning in Yosemite Creek! Frazil ice forms when mist (for example, from a waterfall) freezes, then floats down the creek. This unique phenomenon 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. Since this phenomenon is entirely weather-dependent, it's difficult to predict in advance.
Have you witnessed this phenomenon in the Sierra before?