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Lake and River Ice
Source: City of Prince George
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Lake and River Ice
• An obvious and notable feature of lakes and rivers
in the North is that they are ice-covered for
portions of the year.
• Its significant hydrological influence arises through
its effect on the flow and water level in a stream,
the water level in a lake, and through seasonal
storage represented by the ice itself, the
snowcover it carries, and the channel and lake
storage it induces.
• Indeed it can be argued the hydrological extremes
of common interest, floods and low flows, are as
much a function of stream processes through the
action of ice, as they are of the catchment
processes of traditional concern.
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• While the peak discharge is primarily a
function of catchment processes such as
snowmelt, the peak water level (the cause of
the flooding), is very much a function of the
ice conditions on the stream.
• This is particularly so for the North where
the snowmelt peak is the peak discharge
event of the year and can occur while the
stream is still ice-covered or otherwise
influenced by ice in the channel.
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• For example, in the period 1983-87, ice
jams were involved in some 30% of the
flood events across Canada.
• In New Brunswick ice-jam floods are
responsible for more flood damage than
open-water floods.
• The 1987 ice jams on the St. John River
alone caused $30 million damages.
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• On the other side of the country, in
northwestern Canada, the flood threat at
almost all riverside communities is
primarily due to ice jams, not summer
floods.
• At the other extreme, low flow at a site on
a cold-region stream can also depend
heavily on ice processes.
• A striking example of this is the fact that
the discharge over Niagara Falls was
halted on 29 March 1848 by ice
obstructing the outlet of Lake Erie.
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Niagara River
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Niagara River
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Niagara Falls
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• A more common circumstance is the minimum
discharge that occurs in October discharge in
the Clearwater River due to ice formation
upstream, rather than in late winter discharge
from the catchment.
• The low flow frequency curves for several rivers
in northern Alberta show marked “abnormalities”
in the curves for smaller streams that are
explained by ice effects.
• As well as influencing the extremes, ice effects
can have a major influence on the winter
hydrograph of cold-region streams in general.
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Clearwater River
Source: Prowse and
Ommanney, 1990
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• In streams the volume of water stored as ice,
and as channel storage due to the increase in
water level caused by the ice, can represent a
significant portion of winter flow which does not
become available until spring.
• This may be particularly so for the lakedominated rivers of the Canadian Shield where
slight changes in the resistance to flow from the
outlet due to changes in the ice cover can trigger
enormous changes in lake storage.
• Snowfall on lake ice can cause an increase in
flow from a lake.
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• The weight of water displaced from the lake
must equal the weight of the snowfall on the lake
ice (if the latter is simply floating, with little
restraint from the shore, as is often the case).
• Hence a 0.3 m snowfall will displace ≈30 mm of
water from the lake, a flow that can be very
significant in a stream in mid-winter in a
catchment with a large proportion of lakes.
• Therefore, unlike on land, a water equivalent of
snow falling on lake ice is made immediately
available as flow (while a similar amount will be
made available in the spring when the snow
melts, it should not be counted twice when
evaluating the catchment yield).
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• As indicated, lake and river ice are a major cause
of floods in Canada, but these floods are not just
significant because of the damages and loss of
life they may cause.
• In other circumstances they can be beneficial.
• For example, the multitude of lakes in the vast
and environmentally important Mackenzie and
Peace-Athabasca Deltas in western Canada
depend on periodic flooding caused by ice jams
to refill and refresh them.
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Peace-Athabasca Delta
Source: Peters et al. (2006)
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Peace-Athabasca Delta
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Peace-Athabasca Delta
Source: Peters et al. (2006)
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Peace-Athabasca Delta
Source: Peters et al. (2006)
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Lake Ice Formation
• Freeze-up of a small, well-mixed lake in
calm weather occurs in a straightforward
manner (as discussed in the previous
lecture).
• When the lake has cooled sufficiently that
the surface water temperature falls to a
little below freezing during the diurnal
minimum, a thin and fragile ice sheet will
form over the lake surface.
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• While the water temperature at the under-ice
surface in a lake is at freezing, that just below be
significantly above freezing due to the winter
“inversion” caused by the fact that water reaches
its maximum density at 4oC.
• Because of this “warm” water within the lake, the
flow at the outlet of the lake is above freezing.
• The outlet can therefore remain open long after
the remainder of the lake is ice covered.
• This can have significant repercussions on the
variation in flow from the lake, and the winter
hydrology of the outlet stream.
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River Ice Formation
• The situation at freeze-up in a river is somewhat
similar to that of a large lake, with two major
differences: the turbulence in a river is
generated by its own flow, and is therefore everpresent except in pools above rapids, bars,
weirs, or dams.
• It is sufficient to prevent any thermal stratification
of the flow so that the water temperature
remains within a few hundredths of a degree
throughout the flow depth.
• Again the first ice to form is sheet ice over the
quiet water of the shallows along the banks.
• Out in the central region of the stream, the flow
and turbulence is usually sufficient to prevent the
formation of sheet ice on the surface.
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Ice Jams
• When the ice run stalls an ice jam has formed
and the water level will increase substantially.
• Eventually the ice jam will fail or move, possibly
releasing another surge that will trigger an ice
run again if any ice remains downstream.
• This process is repeated, not necessarily
sequentially, until the whole river is finally free of
ice.
• On a lake the process of ice decay and melt
begins as on a river.
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Source: Prowse and
Ommanney, 1990
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Ice Jam on Nechako River
Prince George, BC (1957)
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Source: City of Prince George
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Source: City of Prince George
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Flooding on the Red River
• Manitobans should brace for “major flooding” in some
regions as recent heavy snowfall and a prolonged winter
have wiped out early predictions of only mild to moderate
water swells for the province.
• In issuing its spring flood outlook on Tuesday, provincial
flood forecaster Phillip Mutulu said precipitation over the
Prairies was 200 per cent greater than normal in March,
while freezing temperatures have delayed the spring
thaw, which could mean a rapid snowmelt as the
mercury rises. It’s a situation made worse, he added,
when the frozen soil, which reaches down more than a
metre in some places, cannot absorb all the excess
water.
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MODIS, March 19, 2009
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Ice Break-Up
• On a large lake, wind can assist break-up by
blowing large ice floes about the lake once they
have been freed from shore by melt.
• However, on more moderate-sized lakes the ice
more-or-less decays and melts in place, only
disturbed by wind when it is in a very frail state.
• The above events are typical of a truly cold
region, so that the water body experiences only
one freeze-up and one break-up each year.
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• In more temperate regions there may be more
than one freeze-up and break-up cycle in a
given year, whereas other years there may be
none at all. In such situations events become a
strong function of the quantity of ice that can be
generated in each cold spell.
• In North America such a situation is typical of the
Maritimes, southern Ontario and New England,
and of British Columbia and the northern Pacific
States of the USA. Inland and north of these
locations the former scenario is more typical.
• On lakes in the High Arctic the situation can be
such that there may be no break-up at all in a
particular year.
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Climate Change & Lake/River Ice
• Chronologies of river and lake ice formation and
disappearance provide broad indicators of
climate change over extensive lowland areas.
• Broad scale patterns of freeze-up are available
for Russia from 1893 to 1985.
• In general, freeze-up in western Russia is 2-3
weeks later now than at the turn of the century,
whereas further east there is a slight trend
toward earlier freeze-up.
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• Similar patterns are available for ice break-up
dates, with western Russia rivers breaking up 710 days earlier now than in the 19th century.
• In North America, records from 1823 to 1994 at
six sites on the Great Lakes show that freeze-up
came later and break-up was earlier until the
1890s, but they have remained constant during
the 20th century.
• Freeze-up and break-up dates of ice on lakes
and rivers provide consistent evidence of later
freeze-up and earlier break-up in the northern
hemisphere from 1846 to 1995.
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• Under conditions of overall annual warming, the
duration of river ice cover can be expected to be
reduced.
• Many rivers within temperate regions would tend
to become ice-free, whereas in colder regions
the present ice season could be shortened by up
to one month by 2050.
• Warmer winters would cause more mid-winter
break-ups as rapid snowmelt becomes more
common.
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Summary of Trends in Canada
• Statistically-significant trends toward
earlier river ice freeze-up, particularly in
eastern Canada, and earlier river ice
break-up in British Columbia (1967-1996)
• Increased river ice cover duration over the
Maritimes, variable response elsewhere
• Western Canada shows the most
consistent trends toward earlier break-up
of lake ice.
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