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.
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.