When Ice Plays the Frazil Jam

Various types of ice form an ice jam above the Douro dam of the Otonabee River, ON; left, looking downstream toward the Douro dam; right, looking upstream at the Sawer dam (photos by Nina Munteanu)

I’m a limnologist (someone who studies water and water systems); I’m also a Canadian, living in the north. That means that the water and waterways I study are often covered in ice and snow.

Since moving to Peterborough a few years ago, I’ve been walking daily along the shores of the Otonabee River, through riparian forest and marsh and small tributaries. The Otonabee River is a regulated river, with several dams and locks, forming part of the Trent-Severn Waterway in the Great Lakes Basin. The Otonabee River, which provides Peterborough its drinking water, receives water from Katchewanooka Lake in Lakefield and flows south through Peterborough into Rice Lake and from there water flows via the Trent River into Lake Ontario.

Flow regulation of the Otonabee begins with several dams and lock in Lakefield then passes through several dams associated with four more locks (Lock 25, Sawer Creek; Lock 24, Douro; Lock 23, Otonabee; and Lock 22, Nassau Mills). The Otonabee continues south along Armour Road where it passes over several hydroelectricity dams and generating stations into the centre of the city before reaching Little Lake.

Summer image showing two dams on the Otonabee River off Armour Road in Peterborough, ON

I’ve been enjoying the seasonal changes of the river, along with the ostensible water level changes imposed throughout the seasons by the various dams and diversions. This has been particularly interesting for me during the onset and duration of winter, when ice and snow play a role in the river’s character. When it’s sufficiently cold (at zero degrees Celsius or 32 degrees Fahrenheit), ice forms. It can form as a solid sheet on lakes and rivers and on land (as a glacier). Ice can also occur as frost, snow, sleet and hail.

Limnologists talk about the ice-up of lakes and rivers, often making it sound like a singular phenomenon. But it isn’t. The characteristic ice sheet of a fully frozen lake or river goes through several stages and will vary from year to year. The cyclic nature or phenology of ice-up determines the quality and nature of the ice that forms and the under-ice environment. In a regulated river it gets even more complicated.

But it all starts with young ice crystals, frazil ice, that grow and evolve into something bigger. 

Frazil and grease ice (essentially collections of frazil ice) drift down-river at sunset on the Otonabee River, ON (photo by Nina Munteanu)

When Water Freezes & Ice Grows

Two things determine how ice forms: temperature and turbulence. The Otonabee experiences below freezing air temperatures for close to five months of the year and is both turbulent and calm in various places and times based on its level changes. This makes for some varied and interesting ice phenomena.

As early as November, when it’s freezing cold and water supercools, ice crystals (frazil ice) form and mix into the waterbody’s upper layer. The process can be accelerated when snow falls on the waterbody, contributing ice crystals and often creating grease ice. The ice molecules expand into an organized latticework that is less dense and lighter than liquid water, allowing it to float. Frazil ice often develops into slushy clumps of white ice a few centimeters across (grease ice or slushy, spongy grease ice called shuga).

Grease ice forms during a cold snowy day, creating a mat appearance, Otonabee River, ON (photo by Nina Munteanu)
Grease ice forming a lumpy surface mat from snow that doesn’t melt in supercooled water during a snowy day in December, Otonabee River, ON (photo by Nina Munteanu)
Various opaque loose mats of grease ice form during a heavy snowfall on supercooled river water in December, Otonabee River, ON (photo by Nina Munteanu)
Swirling films of grease ice form on the Otonabee River during a heavy snowfall, collecting as they float downstream, ON (photo by Nina Munteanu)
Frazil (dark) and shuga ice drifting down the Otonabee River in January, ON (photo by Nina Munteanu)

On a quiet surface with little wind, such as a protected bay or pond, clear ice forms in very cold weather. Transparent ice may resemble Goethe glass and reflect light like clear water. John A. Downing, director of The University of Minnesota Sea Grant tells us that this “primary ice” is crystalline and composed of hexagonal plates, needles, or sheath-like structures with large crystals oriented up and down. It can be transparent, like glass, or slightly cloudy, reflecting a deep or aqua-turquoise blue, depending on the materials the crystals nucleate on. When the ice cover expands from the shore to the entire river or lake, it’s called fast ice because it’s held fast by the shore.

Clear ice forming in a protected bay of Jackson Creek, Peterborough (photo by Nina Munteanu)
Clear ice sheet in quiet bay of Jackson Creek reflects trees of the riparian forest, ON (photo by Nina Munteanu)
Clear ice forms on the shore of Jackson Creek as water freely flows along the thalweg of the creek, ON (photo by Nina Munteanu)
Ice shows a milky/cloudy appearance as it nucleates on different particles during its formation in the Thompson Creek marsh, ON (photo by Nina Munteanu)

In rougher moving water, ice forms in a less orderly and transparent way. It forms first as frazil: fine crystals suspended in water, usually smaller than 3 cm in length and shaped like sharp-pointed objects or small disks (see my discussion below on frazil ice).

In the calm waters of shorelines and inlets, frazil ice and grease ice may form skim ice that may look like a film of grease just like grease ice but is more consolidated. Frazil and grease ice may also create nilas ice, an up to 10 cm thick elastic ice crust with a mat surface (basically a larger version of skim ice). Ice rind, a brittle, often shiny crust up to about 5 cm thick may form along protected shores around marsh reeds or on exposed rocks.

Skim ice forming in the shallow protected marsh of Thompson Creek in early winter, ON (photo by Nina Munteanu)
Ice rind forms beautiful sculptures on exposed rocks of Jackson Creek, ON (photo by Nina Munteanu)

Ice crystals need a nucleating agent to form in supercooled surface water. Examples include snow and ice fog, or already existing ice (e.g. frazil). Sediment and bacteria in lake and river water can also act as nucleating agents. In moderately cold and calm water with no snow falling, large crystals can form what is called unseeded ice; the nucleation sites are most likely particulates in the water. When snow falls, tiny ice crystals form on the water surface (seeded ice).

Frazil ice piling up in frozen waves of brash ice above the Douro dam of the Otonabee River, ON (photo by Nina Munteanu)
Frozen waves of frazil ice form on the turbulent narrow stretch of the Otonabee River above the Trent dam, ON (photo by Nina Munteanu)

Frazil Ice

Frazil Ice is a collection of loose, randomly oriented, plate or discoid ice crystals that form in supercooled turbulent water. This young ice forms quickly and may congeal into slush, shuga ice, or grease ice that forms a soupy layer on the surface of a waterway. When the air temperature reaches  minus 6 °C (21 °F) dropping surface water temperature below minus 0.1 °C, frazil will form.

Frazil floc along with small pancake ice collides and merges into nilas ice and brash ice above Douro dam on the Otonabee River (photo by Nina Munteanu)

On a minus twenty C° January day, I followed the frazil or floating slush as it drifted downstream below the Sawer Creek dam until the frazil ran into an ice jam that was piling up behind the Douro dam. Much of the frazil had organized into hundreds of small circular 4-cm diameter wide ice pancakes in the turbulent flow. The tiny pancakes collided into one another and jammed up against the established but moving nilas ice sheet, creating a frazil floc and eventually cementing into the larger ice jam. The small ice pancakes foamed up with a milky froth, sliding on top or below each other and crowding into the ice jam.

The ice pancakes make a distinct fizzing high pitched ‘shhh’-sound, just like soda pop when it’s first opened. They are frazilling

Close up of small pancake ice foaming as they collide and develop into nilas ice above Douro dam on the Otonabee River (photos by Nina Munteanu)
Fizzing, foaming ice pancake in the Otonabee River, ON (photo by Nina Munteanu)

In an ice jam just upstream of Trent University and Lock 22 (Nassau Mills), frozen waves of ice fraziling had formed. Thin shards of broken ice rind had rafted over each other to form rows of frozen waves and hummocks as the ice jam grew upstream from the dam. It resembled a freshwater version of stamukhi ice (Russian word for sea ice rubble that develops along the boundary between fast ice and drifting pack ice).

Frozen waves of broken shards of frazil ice rind grow the ice jam above the Trent dam on the Otonabee River (photo by Nina Munteanu)

Ice Rind

Ice rind is a brittle shiny crust of ice that forms on a quiet surface by direct freezing or from grease ice. Ice rind is quite thin, from 1 to 5 cm thick, and easily breaks with wind action or water swells, usually in rectangular pieces. I saw several larger pieces drifting between dams, colliding and rafting over each other and eventually finding themselves consolidated into a downstream ice jam above another dam. Rind ice is best seen suspended in a crispy film around marsh reeds when the water level of the river goes down. Then the brittle ice breaks off into pieces, leaving various river vegetation and rocks with ice collars. I’ve also seen beautifully sculpted ice shapes that have formed on exposed rocks in Jackson Creek as the turbulent creek flows around them. ice sculptures resembling clear pearls or steps into heaven shone like jewels in the sunlight.

Ice rind formations on exposed rocks of Jackson Creek, ON (photo by Nina Munteanu)
The turbulent Jackson Creek forms bubbles of ice rind on exposed rocks, ON (photo by Nina Munteanu)

Pancake Ice

Pancake Ice is ice that spins around in waves and thickens into free-floating ice disks. It forms particularly where the turbulence of rough water and rapids affect slush or ice rind, just downstream of a dam. This is exactly where I’ve seen pancake ice of varying sizes on the Otonabee River (pancakes from as small as 4-centimetres to as large as 3-metres wide and up to 10 cm thick). These were particularly noticeable below the dam associated with Lock 23.

Pancake ice circle some quieter waters just below the dam associated with Lock 23 on the Otonabee River, ON–note upturned rims (photo by Nina Munteanu)
Ice pancakes in early stage formation from frazil on the Otonabee River (photo by Nina Munteanu)
Pancake ice collecting along shore and ‘stitching’ to form brash ice that adds to fast ice, ON (photo by Nina Munteanu)
Pancake ice forming from grease ice, shuga, and fragments that congeal into fast ice adjacent to shore of river, ON (photo by Nina Munteanu)

Pancake ice forms in two ways: 1) on water covered by slush, shuga or grease ice that, when it becomes sufficiently dense, congeals to form a pancake, or 2) from breaking ice rind, nilas or even gray ice in agitated conditions. When the floating ice rinds of grease ice break up, pancake ice forms from the pieces. I’ve seen pancakes raft over each other, creating an uneven top and bottom surface on an ice jam. I saw good examples of pancake-frazil formation below the Sawer dam at Lock 25 that created an ice jam behind the Lock 24 dam at Douro.

Ice pancakes raft together to form irregular ice jam as they raft over and under each other on the Otonabee River (photo by Nina Munteanu)
Ice pancakes form from frazil, grease, and shuga in the turbulent Otonabee River, ON (photo by Nina Munteanu)
Pancake ice with copious ice-foam close to shore of Otonabee River, ON (photo by Nina Munteanu)

The rims of pancake ice are often turned up; when the pancakes collide into each other like bumper cars, frazil ice or slush piles onto their edges. According to Alberto Reyes (Northern landscape and climate change expert with the University of Alberta), ice pancakes “also like to glom onto infrastructure,” such as intakes to hydroelectric facilities; these affect power generation and outflow pipes that potentially block discharge pipes and cause flooding. The City of Peterborough has often had to issue a flood watch due to frazil ice accumulation.

Ice breccia below Sawer dam of the Otonabee River, composed of differing stages of moving ice including frazil, fragments, pancakes, and nilas–note the clear deep blue ice fragments with older ice below (photo by Nina Munteanu)

As frazil ice flows downstream it will eventually come to rest against obstructions (e.g., islands, bridge piers and abutments), in low velocity areas (bends and slope reductions) or in areas of channel constrictions. Where it comes to rest, it will accumulate. Frazil ice may also anchor itself to the bottom of a watercourse and accumulate. “Where frazil ice accumulates, it is likely to cause a restriction of water flow downstream, thereby resulting in a rise of water, and possibly flooding, behind the frazil ice jam,” said Gord Earle to the Canadian News, flood forecaster and warning service officer in Peterborough. Earle advises residents and businesses along the banks of the Otonabee River and Jackson Creek to keep a close eye on ice formation, accumulation and trapping.

When Ice Flowers

During my walks along the Otonabee and Thompson Creek, its tributary stream/marsh, I often witness the clear ice formations in the calm bay leading to the larger river. Some of the most beautiful ice formations arise when water vapor condenses directly into frost or rime when it’s cold enough during a freezing fog.

Fast ice at Thompson Creek outlet forms leaf-petaled frost flowers after a freezing fog in January (photo by Nina Munteanu)
Frost forming leaf-shapes as it grows on the Thompson Creek ice (photo by Nina Munteanu)

Frost crystals that form directly on cold surfaces such as a snow surface or an ice sheet from freezing water vapor grow larger and flower into beautiful leaf-like shapes or hexagonal snowflakes. The different frost shapes and patterns arise from the topography of the ice sheet, often specks of dust, salt, or residue, twigs and other organic material that lies on the surface.

Hoar frost forms on older ice of Thompson Creek with newly formed ice (to the right) yet to be colonized (photo by Nina Munteanu)
Hoar frost forms intricate patterns of ‘leaves’ and ‘ferns’ on iced over Thompson Creek, ON (photo by Nina Munteanu)

The ice crystals nucleate on the particles and branch out, forming unique fractal patterns that resemble leaves, ferns, feathers and more. This ground frost closely resembles snowflakes; given that their formation and structure are similar only the nucleation particles for frost are surface objects.

Graupel snow settles in the vortex swirls of Otonabee River ice (photo by Nina Munteanu)

On a particularly cold and windy day last winter, after a spate of light snow, the ice on the Otonabee was dark with swirling vortexes that resembled the Kármán vortex street patterns of Jupiter or Saturn or a ‘muse on ice’ by Van Gogh. The frozen swirls sang with the tiny dobs of graupel that had fallen earlier.

Animal tracks and wind and snow create interesting patterns on river ice, Otonabee River, ON (photo by Nina Munteanu)

Glossary of Ice Terms (Environment Canada):

ADVECTION FROST: A collection of small ice crystals in the shape of spikes that form when a cold wind blows over branches of trees, poles, and other surfaces.

BRASH ICE: Accumulations of floating ice made up of fragments not more than 2m across; wreckage of other forms of ice.

Brash ice, made up of large ice fragments jammed up against each other, on the Otonabee River (photo by Nina Munteanu)

FAST ICE: Ice that forms and remains fast along the shore, where it is attached to the shore, an ice wall, or ice front.

Various stages of fast ice forming along the shore of the Otonabee River in January, ON (photo by Nina Munteanu)
Fast ice forms solid connection with river bank, Otonabee River, ON (photo by Nina Munteanu)

FERN FROST: Frost that appears on windows and ice as moist air comes into contact with the freezing surface. The tiny water droplets freeze into patterns that resemble leaves or ferns.

Fern frost formed on the inside of my car windshield on a cold night in January (photo by Nina Munteanu)

FRACTURING: Pressure process whereby ice is permanently deformed, and rupture occurs.

FRAZIL ICE: Fine spicules or plates or disks of ice (ice crystals), suspended in water and associated with early ice formation, particularly in more turbulent moving water.

Frazil ice floats downstream in the Otonabee River, ON (photo by Nina Munteanu)

FROST: Deposit of small white ice crystals formed on the ground or other surface when the temperature falls below freezing. A more extensive and larger crystalline structure of frost formed with more moisture, such as a freezing fog, is called hoarfrost. When frost forms rapidly, often during windy weather, it is called rime.

Frost flakes glint in the sun as they form on fast ice of the Otonabee River in January, ON (photo by Nina Munteanu)

GRAY ICE: Young ice 10-15 cm thick, less elastic than nilas and breaks on swell. Usually rafts under pressure.

GRAUPEL: Heavily rimed snow particles or pellets, typically white, soft and crumbly.

Clear ice of the Otonabee River, dusted with light graupel that has fallen the previous night (photo by Nina Munteanu)

GREASE ICE: A later stage of freezing than frazil ice. It occurs when the crystals have coagulated to form a soup layer on the water surface. Grease ice reflects little light, giving the water a mat appearance. Forms shuga.

Grease ice forms a soupy opaque film as frazil ice crystals collect during a snowy December day, Otonabee River, ON (photo by Nina Munteanu)

HOARFROST: a grayish-white crystalline deposit of frozen water vapor in clear still weather on vegetation, fences, ice and snow, and other surfaces. It often resembles spiky hairs. The word ‘hoar’ comes from ‘ancient’ given it resembles an old man’s bushy, white beard. Also called radiation frost.

Hoar frost crystals forming on branches of a small bush in mid winter, ON (photo by Nina Munteanu)

HUMMOCKED ICE: ice piled haphazardly one piece over another to form an uneven surface. When weathered, it has the appearance of smooth hillocks.

Ice hummocks on Otonabee River above Trent dam, ON (photo by Nina Munteanu)

ICE BRECCIA: Ice of different stages of development frozen together.

Ice breccia, showing several stages and types of ice, including frazil, fragments, and nilas (photo by Nina Munteanu)
Ice breccia, showing several stages of ice formation, from clear ice to pancake ice, fragments, and nilas (photo by Nina Munteanu)

ICE JAM: An accumulation of broken river ice caught in a narrow channel.

Ice jam forms in January of fragments, frazil and pancake ice above Douro Dam on Otonabee River, ON (photo by Nina Munteanu)

ICE FRAGMENT: an often irregularly shaped piece of ice that has broken off from nilas or gray ice, often floating downstream in a river upon ice breakup or a thaw event in spring.

Large ice fragment held fast from drifting downstream by newly forming ice during early spring thaw of river, ON (photo by Nina Munteanu)
Large ice fragments form by cracking ice nilas, leaving gaps that are icing over, Otonabee River, ON (photo by Nina Munteanu)

ICE RIND: A brittle shiny crust of ice formed on a quiet surface by direct freezing or from grease ice. Thickness to about 5 cm. Easily broken by wind or swell, commonly breaking in rectangular pieces.

Ice rind forms on exposed pilings on shore of Lake Ontario, ON (photo by Nina Munteanu)

NILAS: A thin elastic crust of ice, bending easily on waves and swell. Up to 10 cm thick with a mat surface. Under pressure it thrusts into a pattern of interlocking fingers.

PANCAKE ICE: Mostly circular pieces of ice from 30 cm to 3 m in diameter and up to 10 cm thick, with raised rims due to the pieces striking against one another. May form on a slight swell from grease ice, shuga, or slush, or from the breaking of ice rind, nilas or gray ice.

Ice pancakes on the Otonabee River in January (photo by Nina Munteanu)

POLYNYA: Any nonlinear-shaped opening in the water enclosed by ice. Some polynya recur annually in the same position.

Small polynya establish in the quiet outlet bay of Thompson Creek where it empties into the Otonabee River, ON (photo by Nina Munteanu)
Large polynya forms amid waves of brash ice upstream of dam on Otonabee River, ON (photo by Nina Munteanu)

RAFTED ICE: Type of deformed ice formed by one piece of ice overriding another.

RAFTING: Pressure processes whereby one piece of ice overrides another. Most common in new and young ice.

Ice rafts formed on the shore of Otonabee River above the Trent dam (photo by Nina Munteanu)

RIME FROST: Rime frost resembles sugar sprinkled on the edges of leaves and flower petals. It occurs whenever damp winds conspire with extremely low temperatures. Rime frost forms rapidly; the word ‘rime’ means ‘crust.’ Rime forms from moisture that comes from freezing fog water droplets that turn directly from a liquid state to a solid state, or through direct freezing. In contrast, hoar frost occurs on a clear, cold night where water vapor sublimates, transitioning immediately from a gaseous state to a solid state

Rime frost forming on poplar leave edges, conifer leaves and small twigs on the ground in early winter from a freezing fog, ON (photo by Nina Munteanu)

SHUGA: An accumulation of spongy white ice lumps, several centimeters across; formed from grease ice or slush and sometimes from ice rising to the surface.

Clumps of shuga ice form from frazil slush and grease ice on the river (photo by Nina Munteanu)

THERMAL CRACK: Cracks in ice caused by thermal contraction of the ice.

Thermal cracks in ice of the Otonabee River in late winter, ON (photo by Nina Munteanu)


Flood waters ice-up in the shallows around the cedars and rocks of Jackson Creek, ON (photo by Nina Munteanu)
Brash ice forming on shore from wreckage of various ice forms including pancake, shuga, nilas and gray ice. Note how gaps are already turning into clear blue ice as part of brash ice (photo by Nina Munteanu)
Large ice fragments play jigsaw on the Otonabee River during a brief thaw, ON (photo by Nina Munteanu)


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Frozen waves of glassy ice form on Jackson Creek, ON (photo by Nina Munteanu)

Nina Munteanu is a Canadian ecologist / limnologist and novelist. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit www.ninamunteanu.ca for the latest on her books. Nina’s bilingual “La natura dell’acqua / The Way of Water” was published by Mincione Edizioni in Rome. Her non-fiction book “Water Is…” by Pixl Press(Vancouver) was selected by Margaret Atwood in the New York Times ‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” was released by Inanna Publications (Toronto) in June 2020.

3 thoughts on “When Ice Plays the Frazil Jam

  1. Ice is so cool, isn’t it? Me too, Merridy. In fact, I’m heading out now… Got my tea in my thermos and my snow pants in case I need to wade through knee-high snow for that quintessential shot and, of course, my orange for quick sustenance…


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