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.

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. 

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).

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.

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.

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

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.

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…

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).

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.

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 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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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

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

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

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.

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.

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.

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

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.

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

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.

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

*****

References:

Armstrong, T., and B. Roberts. 1956. Illustrated ice glossary. Polar Record 8:4-32.

Ashton, G., editor. 2010. River Lake Ice Engineering. Water Resources Publications LLC, Highlands Ranch, Colorado, USA.

Bengtsson, L. 1986. Spatial Variability of Lake Ice Covers. Geografiska Annaler: Series A, Physical Geography 68:113-121.

Brown, L. C., and C. R. Duguay. 2011. A comparison of simulated and measured lake ice thickness using a Shallow Water Ice Profiler. Hydrological Processes 25:2932-2941.

Burn, C. R. 1990. Frost heave in lake-bottom sediments, Mackenzie Delta, Northwest Territories. Nordicana 54:103-109.

Cherepanov, N. 1974. Classification of ice of natural water bodies. Pages 97-101 in Ocean ’74 : IEEE International Conference on Engineering in the Ocean Environment Institute of Electrical and Electronic Engineers, New York, NY, USA.

Downing, John A. 2021. “Ice Formation is Not a Singular Phenomenon.” University of Minnesota Sea Grant. February 25, 2021.

Eisen, O., J. Freitag, C. Haas, W. Rack, G. Rotschky, and J. Schmitt. 2003. Bowling mermaids; or, how do beach ice balls form? Journal of Glaciology 49:605-606.

Fahnestock, R. K., D. J. Crowley, M. Wilson, and H. Schneider. 1973.Ice& volcanoes of the Lake Erie shore near Dunkirk, New York, USA. Journal of Glaciology 12:93-99.

Kavanaugh, J., R. Schultz, L. D. Andriashek, M. v. d. Baan, H. Ghofrani, G. Atkinson, and D. J. Utting. 2019. A New Year’s Day icebreaker: icequakes on lakes in Alberta, Canada. Canadian Journal of Earth Sciences 56:183-200.

Kempema, E. W., E. Reimnitz, and P. W. Barnes. 2001. Anchor-Ice Formation and Ice Rafting in Southwestern Lake Michigan, U.S.A. Journal of Sedimentary Research 71:346-354.

Knight, C. A. 1962. Studies of Arctic Lake Ice. Journal of Glaciology 4:319-335.

Michel, B. 1971. Winter regime of rivers and lakes. US Army Corps of Engineers, Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire USA.

Michel, B., and R. O. Ramseier. 1971. Classification of river and lake ice. Canadian Geotechnical Journal 8:36-45.

Muguruma, J., and K. Kikuchi. 1963. Lake Ice Investigation at Peters Lake, Alaska. Journal of Glaciology 4:689-708.

Pounder, E. 1965. Physics of ice. Pergammon Press, Oxford, UK.