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Lake and River Ice

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Title: Lake and River Ice


1
Lake and River Ice
Source City of Prince George
2
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.

3
  • 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.

4
  • 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.

5
  • 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.

6
Niagara River
7
Niagara River
8
Niagara Falls
9
For the adventurous ones
10
  • 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.

11
Clearwater River
Source Prowse and Ommanney, 1990
12
Low-flow frequency curves
Source Prowse and Ommanney, 1990
13
  • 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.

14
  • 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.

15
  • 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.

16
Peace-Athabasca Delta
Source Peters et al. (2006)
17
Peace-Athabasca Delta
18
Peace-Athabasca Delta
Source Peters et al. (2006)
19
Peace-Athabasca Delta
Source Peters et al. (2006)
20
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.

21
  • 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.

22
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.

23
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.

24
Source Prowse and Ommanney, 1990
25
Ice Jam on Nechako River
Prince George, BC (1957)
26
Source City of Prince George
27
Source City of Prince George
28
Ice Jam on Chena River
29
Ice Jam on St. John River
30
Current obs., Red River flooding
31
48-h forecast
32
MODIS, March 19, 2009
33
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.

34
  • 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
38
  • 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.

39
  • 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.

40
  • 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
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