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.
The Otonabee is regulated through a series of locks and dams with generating stations for electricity. 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 cold enough (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 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, sharp pointed discs of ice crystals (frazil ice) form and mix into the waterbody’s upper layer. 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). Frazil and grease ice may also create nilas ice, an up to 10 cm thick elastic ice crust with a mat surface.
On a quiet surface with little wind, such as a protected bay or pond, clear ice forms under very cold weather. Transparent ice may resemble Goethe glass and reflect light like clear wate or it can be 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, first forming frazil.
In more calm waters of shorelines and inlets, frazil ice may form skim ice that may look like a film of grease. Ice rind, a brittle 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 falling snow, large crystals form 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).
On a minus twenty C° January day, I followed the frazil or floating slush as it drifted downstream below a dam until the frazil ran into an ice jam that was piling up behind the next 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 frazil 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. They made a distinct fizzing high pitched ‘shhh’-sound, just like soda pop when it’s first opened. They were frazilling. Frozen waves of ice fraziling formed and thin shards of broken ice rind had rafted over each other to form rows of hummocks as the ice jam grew upstream from the dam.
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, such as 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).
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 one dam and these formed an ice jam behind a downstream dam.
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.
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.
FRACTURING: Pressure process whereby ice is permanently deformed, and rupture occurs.
FRAZIL ICE: Fine spicules or plates of ice (ice crystals), suspended in water.
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.
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 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 but 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.
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.
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