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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 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).

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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 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)

24Source: City of Prince George

25Source: 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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MO

DIS

, 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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• 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.

Climate Change & Lake/River Ice

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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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