In the old days, 30 years ago, frazil ice started floating down the Hudson River by late November, collecting and backing up from Warrensburg to The Glen by late December.

This year, 2020, it was February before it was cold enough for the “hanging dams” to do their thing.

Also called “slush ice” (and by some natives around here “anchor ice”), frazil is the white stuff that forms the brilliant white canyons in the river you used to see from The Glen bridge in early spring. This year you can see some low frazil banks along the Golf Course Rd. north of Warrensburg on the east side, and along the River Road on the west near where the RR track crosses the road.

I started watching frazil (a French Canadian word pronounced like frazzle), puzzled to see what looked like snow floating down the river on spectacularly clear, cold days. “Must be snowing in the mountains!”, I thought. On a natural history walk on the summer ice meadow shores along the Hudson one day, a native Adirondacker who had been watching it all his life asked me, “What is that white stuff that fills the river every winter?” That got me wondering too, so I started researching it, beginning with an ice engineer in Hanover, NH.

Frazil looks a lot like floating snow and can be found in northern, turbulent streams or rivers (and in the north and south polar oceans, and even on Lake Champlain shores.) Its tiny, thin disk-shaped crystals form only in “super-cooled” water (very slightly below 32 degrees F.) when the air temperature is below about 16 degrees. The turbulence of the river in our case mixes and cools the whole water column “entraining” some of the cold air and making bubbles which burst, their tiny droplets freezing in the cold air. These fall in the water and form the “nuclei” for making larger crystals, which try to grow projections around their edges (as on snowflakes). In the rough water, these break off continuously forming billions more nuclei.

By 1990 I knew about the ice meadows on the open cobbly, sloping shores of the Hudson  between The Glen and Thurman. Botanists, who saw the meadows only in growing season, said that the openness, sparse vegetation and rare plants were caused by the scouring of woody plants by ice floes coming down the river every spring. But the ice engineer at CRREL in Hanover told me that frazil cannot usually scour anything because it floats as single crystals, not as chunks of solid ice (which are more gray or blue instead of pure white).

Besides, five out of six years along the Hudson there were “thermal breakups,” rather than “:dynamic,” when the ten foot banks of ice melt in place on the shore where it had floated early in winter because of “hanging dams.” These form underneath the surface of stable frazil covering the river. Frazil that is moving faster than about two miles an hour gets sucked under the cover where it floats up and plates out, forming obstructions hanging down in huge tapering masses. The current is forced to slow down behind these dams and like any other dam, they raise the water level sometimes many feet in a few hours, breaking up the frazil cover and floating the loosened frazil sideways and out over the sloping cobble shores. You can hear when it is going to happen! The thin ice on the pools of water that have formed on the edges of the river start crackling and you can see the water seeping over the pools.

Periodically, usually in early winter, the force of the river breaks through the hanging dams, the water level goes down often very quickly, leaving the frazil sitting many feet deep on the shores. The river can be totally wide open again for a while or have just an open channel which during the next cold night will fill up with frazil again. You can watch frazil doing its interesting antics if you can find a safe place (safe from cars) at the upper active end of the frazil cover.

I’ve walked both shores between The Glen and the Thurman bridge looking for evidence of scouring by solid ice and have found almost none. But ten feet of flooding river water, which can occur for many weeks in a year, is extremely powerful and does moving a lot of rocks on the bottom and even up onto the shore. And there is flattening of flexible shrubs and tree species (and some breaking of brittle saplings) by the massive amounts of frazil that settle down onto the shores.

Once there is a continuous cover of ice, no more hanging dams can be made because the water needs to be open to the air to be able to be make frazil (to keep the water just below 32 degrees). Then the river can flow freely in channels below the frozen ice cover and there’s nothing to watch until spring.

In shallow areas of turbulent water such as along the road between North Creek and North River, on a very cold night, especially if there is wind to help cool the water, what ice engineers call “anchor ice” forms on the bottom of the river, visible as a greenish white layer. I think the shallowness allows the bottom of the river to cool below 32 degrees too so that frazil crystals stick to it, then build up a thicker layer. The next day, if the sun comes out and warms the water slightly, the anchor ice usually lets go its grip on the bottom, floats up to the surface, and sometimes floats down the river holding onto some gravel or small rocks.

Once in about every six years in spring there used to be a “dynamic break-up” when big rain storms raised the river quickly making the solid ice in the still water upriver near Newcomb break up and move fast downriver. The slug of ice solid ice and frazil usually takes only a few hours to go through North Creek. I’ve watched chunks of this hard ice riding up a bent sapling over and over, scraping off the bark, but this is rare. If the solid ice with mixed in frazil gets jammed behind a constriction like a bridge, when it lets loose it can act like a bulldozer just downstream, but this is rare.

Most years there were thermal break-ups when the ten foot high frazil banks at The Glen bridge were undercut by a low water level, sloughing off masses of frazil bank which dissolved in the river very quickly. This looked like miniature glaciers “calving” because frazil banks are just as vertical as glacier faces. In the last ten years however, we have often had warm spells in the middle of the winter when the whole frazil collection along the river washes out and there is usually not enough time and cold for the back-up to make it to The Glen again.

In mid-April at the Warrensburg marble bedrock area (at the Hudson River Recreation Area where DEC has a trail) often ten foot high deposits of frazil ice were left high and dry when the water level lowered even more. No damage to anything by ice would happen during these years but leaching of nutrients by floodwaters always cause the dwarfing of plants.

Floating frazil is loosely cohesive, like wet sand, which causes very interesting phenomena. You have to be lucky to catch the frazil at an exciting time – when the frazil is coagulating, forming quickly into pointed masses which shove into the downriver loose frazil cover, making it thicker, and rough chunks are shoved up into the air where they freeze. These masses of moving frazil can also be forced against stabilized or slower moving masses of frazil, or under shorefast solid ice.

Then huge, long, smooth-sided toothpaste-like squeezings form and freeze in place. Every really cold night you can see many narrow, parallel ridges along the shore in the morning from frazil “pans” bumping up against the newly collected frazil on the edge of the river. Coves also collect thick rounded pans of frazil which can bump together in the waves and make jigsaw-like puzzle pieces. Only once, after all the snow was gone in spring, a very cold night left wrinkles of frazil in eddies all along the shore, like a Bassett hound’s skin.

These frazil processes and scenes are hard to explain in words. But this rare winter phenomenon (also visible on the Sacandaga River above Northville in a much smaller version) is worth trying to see for yourself. If it has snowed since the last “shoving event” the surface of the frazil cover will be rounded, masking the jumbles of sharp frazil clumps. The floodplains south of the Thurman bridge also collect deep frazil around the big silver maples without damaging the trees, more evidence that it is not just frazil ice which causes open ice meadows.

Photos: Above two, by Bob Duncan; two below by Evelyn Greene.