Here's my annual frazil ice column, my apologies. But some of you are new to the area, and for oldtimers on whom I have inflicted this topic many times before, I have a wonderful film of active frazil you can google. "Youtube Yosemite frazil ice" should do it. If you don't have high speed internet, have your grandkid show it to you. The explanation by the naturalists is not totally adequate but it is wonderful to see someone else excited about this fascinating natural phenomenon. They don't mention fairies touching their wands to the water as a possible cause, so I think that is a fairy tale invented by a certain local character!
The white stuff that fills the Hudson in winter between The Glen and Warrensburg, and sometimes for 20 more miles, has nothing to do with snow. When it finally occurred to me that it is only when the sun is shining brightly that it floats down the river, I slapped my forehead and got to work researching the mystery.
When the temperature is below about 20 F., starting as early as late November and continuing till spring, a slush forms which, when the tops of the clumps drain, turns very white. Floating snow is gray, soggy looking and ugly.
The river water has to be slightly below 32 F. (supercooled) for frazil to form, which means cold air and wind need to make up for the heat that is released when ice forms, to keep the water supercooled and producing more frazil. (Physics is hard for us non-scientists to understand, but take my word for it!) Microscopic ice particles, which I think come mostly from bubbles bursting above and falling into the turbulent water, grow big enough to float, then collect in loose clumps which eventually bump each other into "pans" with raised white edges, a foot or so across and sometimes that thick. When these collect in a cove they can freeze together making a jigsaw puzzle effect. Google "youtube lull of frazil ice" for another short video. (I think that "lull" is a typo!)
The river full of frazil gets sluggish when there are islands in the middle or sharp curves. Eventually the frazil clogs and covers the whole width of the river a foot or so thick. Shoving from the tons of frazil still coming down the river forces big coagulated sections to wedge their way farther in to the cover, thickening it. This is the fun time to watch as all kinds of interesting things can happen.
What comes next is critical to the "ice meadow" shores having ten foot deep banks of frazil into April some places. The frazil floating down the river ducks under the cover and gets deposited on it, forming "hanging dams", unique formations which would look like pointed teeth in river cross-section. These dams block the river currents, cause the water level to rise several feet and float the saturated frazil out over the sloping shores. Sooner or later the hanging dams break and the water level goes down, leaving the frazil high, drained and beautifully white. This process can happen a few times before the cover gets frozen across the river and the water level stays below the cover. Unlike when a warm rain in late spring raises the water level quickly causing dangerous ice jams, damming from hanging dams is safe to watch from a bridge or high bank. Usually the frazil deposits on shore will stay in place until they melt in spring.
Ice engineers use the term "anchor ice" for the ice that in very cold weather covers the bottom of the river in shallow areas. (Odd, it always floats in your drink, right?) You will often see a greenish color under the water halfway between North Creek and North River. It turns out that the frazil, which in turbulent water tumbles around in the river water top to bottom, hits the rocks and sticks to them if they are cold enough. When the sun warms the anchor ice enough later in the day (and it will be sunny), it releases its hold and floats down the river, sometimes carrying gravel and even rocks with it. Read Caperton Tissot's new book, "Ice", about the history, natural and otherwise, of ice for more of my ideas about frazil.
This year much of the river between North Creek and The Glen has snow-covered sheet ice covering it instead of being open. As no frazil can be made if the water is not open to the air, unless we have a major melt which opens the river the frazil watching may be over this year.
On cold nights frazil collects in eddies below boulders and along the shore making long points and beaver tails. Ridges of frazil parallel to the shore also form. How are they made? They appear overnight and I've never caught them being made.