Saturday, March 14, 2020

The Hudson River’s Mysterious Frazil Ice

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.

Frazel ice Floating - Evelyn Greene PhotoFrazil 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.

Frazel Ice - Bob Duncan PhotoPeriodically, 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.

Frazel Ice 2 - Bob Duncan PhotoFloating 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.

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Evelyn Greene is a self-taught naturalist who has lived near and in North Creek since 1976. She writes about the flora, fauna and other native and natural features of the Adirondacks and leads trips for friends and relatives to see them. These rambles often involve lightweight, solo Hornbeck canoes which are ideal for exploring wild backcountry waters, especially bog ponds. Greene has been a board member of Adirondack environmental groups since 1990 and has been active in invasive plant control since 1998. She participated in a total of 11 years of the NYS Breeding Bird Atlas, helped with the Huntington Ecological Center bird survey for many years, locates interesting plants for botanists, photographers and field guide authors, and is a local contact for the identification of puzzling sightings. Greene is 46er #110, having finished climbing the 46 Adirondack High Peaks in 1956 with her mother and three siblings during week-long backpack trips, most of them shortly after the 1950 blowdown. She is happy to stay in the valley trails now, walking at a pace at which she and her companions do not miss anything new, puzzling, or amusing.

15 Responses

  1. Phil Terrie says:


  2. Ed Zahniser says:

    Great to have your long-term frazil fascination documented, Evelyn. Thanks so much! Having been to those ice meadows with you in summer, I can appreciate the unusual plant community there, but, being born south of the Mason-Dixon line and a weather-wimp to boot, I have yet to see the winter phenomenon itself.

  3. Frances Gaffney says:

    Ever since the first time I heard Evelyn talk about the frazil ice I have made it a point to go to Warrensburg to see it. The colors in the layers of ice are remarkable!

  4. Charlie S says:

    “Ever since the first time I heard Evelyn talk about the frazil ice I have made it a point to go to Warrensburg to see it.”

    When i began reading this immediately Warrensburg came to mind. In the winters along River Road in Warrensburg the ice in the Hudson heaps up into giant masses and collects in the water.I have always found this very interesting and would stop and just look at those giant masses in that part of the Hudson. I never knew this was called frazil ice. Always something new to learn!

  5. Jim Fox says:

    Wow, what a thorough treatise on frazil! Thank you for sharing it, Evelyn.

    Growing up on the West Branch of the Delaware, I only knew it as slush ice. The only thing I knew about it, was that when it was flowing, it was the best time to go sucker hooking. The slush ice supposedly drove the suckers into the binnicals (backwaters out of the river’s main current). One or two guys with sucker hooks kneel at holes chopped in the ice at the upper end of the binnical, while the other guy starts at the lower end and pounds the ice with the axe and drives the suckers up to be snatched up and onto the ice. That’s how it was supposed to work.

    Until I read your article, Evelyn, I believed that slush ice drove the suckers out of the river’s current. I know now that it’s hogwash, but if anybody tells me different, I’ll tell them that Evelyn’s extensive observations about frazil ice was on the Hudson, and I only know about slush ice on the Delaware.

  6. Lorraine Duvall says:

    Thanks for this informative article. A fresh look at another winter gem.

  7. Ellen says:

    Great piece, Evelyn! Love your photo, too! 🙂

  8. James M Schaefer says:

    Another gem by cousin Evy…she has all the traits of a good naturalist: keen observation, patience to revisit phenomena under varying conditions, appreciation for physical dynamics, documentation by pencil and film, activity despite challenging weather, access by land and water, cultivating the wisdom and experience of others, writing simply and clearly, and humble appreciation of things wild.
    Like her Schaefer elders, she has been like wet cement, everything makes a deep impression on her.

  9. Evelyn Greene says:

    From Evelyn:

    Today I drove the loop from The Glen to Warrensburg and back up the west side of the Hudson to The Glen. The frazil cover has been washed away but the frazil banks should be on the shores for another couple weeks. The best area to see them is about three miles down from The Glen on the west side. All the white on the islands is frazil too. Again this year the backing up started five miles below the 418 bridge just above 1000 Acre Ranch.

  10. Thomas Keating says:

    I wish I took a picture on this road 1980-1984.

    Amazing blocks of ice – anybody take a picture?

    Please post a photo – if you took one. Thank you.

    * Before the internet & “smartphones”.

  11. Tom Kligerman says:

    Yes thank you Evelyn! What a great treatise on the scientific explanation behind one of the most spectacular North country phenomena of all! Now I also recall the classic ice days as I was a student at Clarkson 1976-1980, and during our drive home and back in April, we would marvel at the gargantuan ice blocks lining the river. Funny how few of us carried cameras then and now no one doesnt.

  12. Bonnie Vicki says:

    Great explanations, Evelyn! And the photos show the phenomenon very well.

  13. Pat B says:

    I have pictures from late March 2013 taken along River Road. Ice along the road near the rail crossing was piled higher than the roof of my Forester. As we were taking photos, a gent stopped & we chatted briefly. He said he had lived along this stretch of road for 30 years and never had seen anything like it. I’d like to share the pictures but don’t see where to do so in this comment format.

  14. Reuben says:

    Thank you for this article. My crew and I at the boatshop on 28 get to watch this happen every winter. I would love to see time-lapse footage from along the Hudson in this area documenting the changes in the ice. It’s beautiful–and a worrisome when it jambs up!

  15. My pal Evelyn Greene sure knows what she’s talking about when she talks about frazil ice. She taught me everything I know about it, and I have featured posts about this amazing phenomenon many times on my blog Saratoga Woods and Waterways. To see some photos of amazing accumulations you can check out this post from 2013:

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