Lake and River Ice

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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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For the adventurous ones
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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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Low-flow frequency curves
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
  lake-dominated 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).
• Autumn snow falling on land can remain until
  spring.

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• As indicated, they 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
  ever-present 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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Ice Jam on Chena River
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Ice Jam on St. John River
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Current obs., Red River flooding
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48-h forecast
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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
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• 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
  7-10 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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Lake Temiskaming
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Lake and River Ice Monitoring

• IceWatch Assessments
• River Ice Reports - Alberta Environment
• State of the Canadian Cryosphere