6.3.2: Other lakes
Although
levels of most other large lakes in Canada (e.g., Lakes Winnipeg, Athabasca,
and Great Slave Lake) are monitored, these lakes are influenced by human
regulation, making it difficult to assess past climate-related trends. An
exception is Great Bear Lake in the Northwest Territories, which is
unregulated. Figure 6.11 illustrates recurring high and low levels of this
lake, with no discernible long-term trend. The levels have varied, in part, due
to regional climatic conditions. In particular, the driest years were in the
late 1940s and early 1950s, when water levels reached an all-time low, with
another low recorded in the mid-1990s. The wettest years and highest levels
were in the early to mid-1960s, with another peak in the early 1970s (MacDonald
et al., 2004).
In the
Prairie region, glaciation and dry climate have resulted in numerous
closed-basin saline lakes, which drain internally and rarely spill runoff.
Water storage in these lakes is sensitive to climate, driven by precipitation,
local runoff, and evaporation. From 1910 to 2006, levels in several
closed-basin lakes across the Prairie region showed an overall decline of 4 to
10 m (see Figure 6.12), due, in part, to higher warm-season temperatures (and
resulting increased evaporation) and declining snowmelt runoff to the lakes.
However, climate variables alone did not explain the declines, and other
contributing factors, such as land-use changes (dams, ditches, wetland
drainage, and dugouts) and changes in agricultural practices, were also
involved (van der Kamp et al., 2008). From the late 2000s through 2016, there
has been an abrupt reversal in levels of many of these lakes (a rise of as much
as 6 to 8 m), reflecting the exceptionally wet conditions on the Prairies over
these years (e.g., Bonsal et al., 2017). The reversal has resulted in several
cases of overland flooding, exemplifying the natural hydroclimatic variability
in this region and the susceptibility of surface water bodies to precipitation
extremes, both dry and wet. Although no studies have investigated future
climate impacts on these lake levels, they will continue to be affected by dry
and wet extremes. However, given the projected higher temperatures and
resulting increased evaporation, future levels are expected to decline,
although the magnitude will depend on how much future precipitation increases
will offset evaporation.
Smaller lakes
and ponds are a characteristic feature of the Canadian Arctic, with large
numbers of permafrost thaw lakes found in northern Yukon and the Northwest
Territories. These water bodies are variable in size, with diameters of 10 to
10,000 m and depths of 1 to 20 m (Plug et al., 2008; Vincent et al., 2012).
Warming due to Arctic amplification at high latitudes can affect the size of
permafrost lakes. In particular, those in continuous permafrost may expand due
to acceleration of the permafrost thaw processes that formed them, whereas
those in discontinuous permafrost (i.e., patches of permafrost) may shrink and
even disappear due to rapid drainage as the underlying permafrost completely
thaws (e.g., Hinzman et al., 2005; Smith et al., 2005). Some evidence for these
processes has been observed in certain high-latitude regions, including Canada.
For example, total lake area in the Old Crow Flats (Yukon) declined by
approximately 6000 hectares between 1951 and 2007, with close to half of this
loss being caused by rapid and persistent drainage of 38 large lakes. This
drainage also resulted in the formation of numerous smaller residual ponds.
Catastrophic lake drainages in this region have become more than five times
more frequent in recent decades, and it has been suggested that these changes
are associated with increases in regional temperature and precipitation (Lantz
and Turner, 2015). This observation is consistent with local perceptions that
lakes in the Old Crow Flats are showing declining water levels (e.g., Wolfe et
al., 2011). However, other Canadian Arctic studies have revealed mixed results.
For example, aerial photographs and topographic maps showed that, in a 10,000
km2 region east of the Mackenzie delta in the Northwest Territories, 41 lakes
drained between 1950 and 2000, but the rate of drainage has decreased over time
(Marsh et al., 2009). Similarly, total lake area on the Tuktoyaktuk Peninsula
on the Arctic Ocean coast of the Northwest Territories from 1978 to 2001 ranged
from a 14% increase to an 11% decrease. The increases occurred primarily
between 1978 and 1992 and decreases between 1992 and 2001, depending strongly
on annual precipitation (Plug et al., 2008).
Future
warming and further permafrost thaw (see Chapter 5, Section 5.6.2) are
anticipated to have a substantial impact on surface water in the Arctic.
Permafrost thaw lakes currently have natural cycles of expansion, erosion,
drainage, and reformation (e.g., van Huissteden et al., 2011), which may
accelerate under warmer climate conditions. GCMs project increased
precipitation over the Canadian Arctic (see Chapter 4, Section 4.3.1.3);
however, these increases will be partially offset by greater evaporation due to
both warmer temperatures in summer and decreased duration of ice cover. In addition,
many high Arctic lakes depend on year-round snow and glaciers and are thus
vulnerable to the rapid warming of the cryosphere. As a result, the extent of
northern lakes is highly vulnerable to change as a result of increased water
loss from evaporation and/or drainage (e.g., Vincent et al., 2012).
6.3.3: Wetlands and deltas
Wetlands are
land saturated with water all or most of the time, with poorly drained soils
and vegetation adapted to wet environments. They are often associated with
standing surface water, and depths are generally less than 2 m. Canada has
approximately 1.5 million km2 of wetlands — commonly referred to as swamps,
marshes, bogs, muskegs, ponds, and sloughs — representing about 16% of the
country’s landmass (National Wetlands Working Group, 1988, 1997). The majority
of wetlands are peatlands in the Arctic, sub-Arctic, boreal, prairie, and
temperate regions (van der Kamp and Marsh, 2004). Canada also has several
deltas that form from sediments deposited by rivers entering a large lake or
ocean. The most prominent examples include the Mackenzie (with more than 25,000
shallow lakes and wetlands), Fraser, Peace–Athabasca, Slave, Saskatchewan, and
St. Clair river deltas. Critical to the resilience of delta ecosystems are
occasional low- and high-water events. High-water events can result in overland
flow (ice jam and open-water flooding) and are a crucial source of water
replenishment to disconnected water bodies perched above the main flow system
(see below; Peters et al., 2013).
By storing water
and releasing it slowly, wetlands and deltas are important to Canada’s
freshwater availability. Under certain conditions, wetlands can alleviate
floods, maintain groundwater levels and streamflow, filter sediments and
pollutants, cycle nutrients, and sequester carbon (Federal, Provincial and
Territorial Governments of Canada, 2010). They are closely linked with climate,
as they gain water from direct precipitation, runoff from surrounding uplands,
and groundwater inflow. They lose water via evapotranspiration and surface/
groundwater outflow. Some wetlands also owe their existence in part to cold
Canadian winters and resulting permafrost, snowmelt, and river ice jams. Thus,
both shorter winters and increased evaporation due to longer summers will increase
stress on wetland environments, unless increases in precipitation offset the
loss of water through evaporation (van der Kamp and Marsh, 2004).
Despite the
importance of wetlands, a comprehensive inventory or monitoring program for the
entire country does not exist (Fournier et al., 2007). However, since 1979,
Ducks Unlimited Canada has used aerial photography and satellite imagery to
inventory millions of hectares of wetlands across Canada. In addition, the US
Fish and Wildlife Service produces an annual report that summarizes the status
of North American waterfowl populations and their habitats, with input from
Canada (US Fish and Wildlife Service, 2017). Figure 6.13 shows Canadian prairie
pond counts during May from 1961 to 2017. The series shows substantial
multi-year variability and no long-term trends. The levels closely correspond
to long-term precipitation variability in the region. In many regions of
Canada, wetlands are being lost due to land conversion, water-level control,
and climate change (e.g., Watmough and Schmoll, 2007; Ducks Unlimited Canada,
2010).
Many small
lakes in freshwater delta systems are “perched basins,” located at a higher
elevation than the nearby rivers. These basins typically experience declines in
water levels during drier periods and replenishment during flood events in a
continuous cycle (e.g., Marsh and Lesack, 1996; Peters et al., 2006; Lesack and
Marsh, 2010). For example, in the Peace–Athabasca delta, evaporation exceeded
precipitation from 1900 to 1940; opposite conditions prevailed from 1940 to the
mid-1970s; and this was followed by a return to drier conditions that has
continued through 2009 (Peters et al., 2006; Peters, 2013). The Mackenzie,
Slave, and Saskatchewan river deltas had similar variability (e.g., Lesack and
Marsh, 2010; Peters, 2013). Under a warmer and wetter future climate
(2070–2099; ensemble of CMIP3 GCMs; high emission (A2) and medium emission (B2)
scenarios), a shorter ice season (by two to four weeks), thinner ice cover, and
depletion of the snowpack by mid-winter melt events are projected to lead to a
major reduction in the frequency of spring ice jam flooding in the
Peace–Athabasca delta (Beltaos et al., 2006). This reduction would have serious
ecological implications, including accelerated loss of aquatic habitat, unless
summer flood levels can reach the perched basins (Peters et al., 2006).
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