Climate Change Canada

 

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).