Quantity of Freshwater

As flow patterns and water levels respond to the changing climate, our water supplies will be affected. Diminishing surface-water and groundwater supplies, coupled with increasing demands for these resources, would challenge all aspects of water resource management.

It is difficult to predict future changes in the availability of freshwater. While there is confidence that warmer temperatures will affect variables such as evaporation and snow cover, uncertainties concerning the nature of regional changes in precipitation patterns, as well as the complexity of natural ecosystems, limit our ability to project hydrological changes at the watershed scale. However, it is reasonable to generalize that, for many regions of Canada, climate change will likely result in decreased summer flows, warmer summer water temperatures and higher winter flows. This is particularly true for the snowmelt-dominated systems that are found across most of the country.⁽⁴⁾

Some of the most vulnerable regions of Canada with respect to the impact of climate change on water resources are those that are already under stress, with demand approaching or exceeding supply. This is most apparent in the driest regions of the southern Prairies, commonly referred to as the Palliser Triangle, where drought and severe annual soil moisture deficits are recurrent problems.⁽⁸⁾ Even Ontario, perceived to be an especially water-rich province, suffers from frequent freshwater shortages,⁽¹⁰⁾ and more than 17% of British Columbia's surface-water resources are at or near their supply capacity for extractive uses.⁽¹⁰⁾

For much of western Canada, snowmelt and glacier runoff from mountainous areas are primary sources of water supply for downstream regions. With warmer conditions, the seasonal and long-term storage capacity of alpine areas may decrease, due to thinner snowpacks, more rapid spring runoff, and decreased snow and ice coverage.⁽¹¹⁾ This, in turn, would result in lower summer river flows and therefore greater water shortages during the period of peak demand. Recent trends observed on the eastern slopes of the Canadian Rocky Mountains suggest that the impacts of diminishing glacier cover on downstream flows are already being felt (see Box 1). Across southern Canada, annual mean streamflow has decreased significantly over the last 30-50 years, with the greatest decrease observed during August and September.⁽¹²⁾ Continued decreases are projected to occur as a result of climate change.

Box 1: Diminishing flows in Prairie rivers ⁽¹³⁾

Glacial meltwater is a key source of water for rivers in western and northern Canada. Along the eastern slopes of the Canadian Rocky Mountains, glacier cover has decreased rapidly in recent years, and total cover is now approaching the lowest experienced in the past 10 000 years. As the glacial cover has decreased, so have the downstream flow volumes. This finding appears to contradict projections of the Intergovernmental Panel on Climate Change that warmer temperatures will cause glacial contributions to downstream flow regimes to increase in the short term. However, historical stream flow data indicate that this increased flow phase has already passed, and that the basins have entered a potentially long-term trend of declining flows. The continuation of this trend would exacerbate water shortages that are already apparent across many areas of Alberta and Saskatchewan owing to drought.

The Great Lakes basin is another region where there are significant concerns over the impact of climate change on water resources. More than 40 million people live within the basin, most of whom depend on the lakes for their water supply.⁽¹⁴⁾ Many studies have suggested that climate change will result in lower water levels for the Great Lakes, with consequences for municipal water supplies, navigation, hydroelectric power generation, recreation and natural ecosystems.

Although summer stream flows are generally expected to decline, many researchers project a corresponding increase in winter flows. This is because warmer winters would increase the frequency of mid-winter thaws and rain-on-snow events, a trend that is already evident on the upper Saint John River.⁽¹⁵⁾ This, in turn, would increase the risk of winter flooding in many regions as a result of high flows and severe ice jams.⁽¹⁶⁾ For example, on the Grand River of southern Ontario, researchers project that warmer temperatures and increased precipitation will extend the risk of severe flooding to the months of January and February.⁽¹⁷⁾ However, since snow accumulation will likely be reduced by frequent, small melt events throughout the winter, the magnitude of spring flooding will likely decline. Similar patterns are anticipated for snowmelt-dominated rivers across much of southern Canada.

Climate change affects not only the quantity of surface water but also that of groundwater. Every region of Canada is reliant, to some degree, on groundwater. For example, the entire population of Prince Edward Island relies on groundwater for potable water, while approximately 90% of the rural population in Ontario, Manitoba and Saskatchewan depend on groundwater resources.(18, 19) Despite groundwater's importance, recharge rates for groundwater across the country are virtually unknown, groundwater dynamics are poorly understood,⁽²⁰⁾ and research on the impacts of climate change remains limited.⁽⁶⁾

The depth and nature of groundwater affects its sensitivity to climate change. In general, shallow unconfined aquifers will be impacted most significantly. This is clearly demonstrated by historic variability, in which shallow wells in many parts of Canada run dry during drought periods. In many regions, unfortunately, these shallow aquifers also contain the highest quality groundwater and are a critical source of potable water and water for livestock. Although deeper aquifers are less sensitive to the direct impacts of climate change, the failure of shallow aquifers could encourage their exploitation. These deep aquifers can take decades to recover from pumping, due to slow recharge rates.⁽²⁰⁾

Local factors, such as the permeability of the material (e.g., soil, rock) above the aquifer, and the timing of precipitation, strongly affect the rate of groundwater recharge and therefore sensitivity to climate change.⁽¹⁸⁾ An increase in winter precipitation is expected to benefit groundwater levels more than an increase in summer precipitation. This is because snowmelt tends to recharge groundwater, whereas summer precipitation is primarily lost through evapotranspiration.⁽²⁰⁾

Quality of Freshwater

Water quality would suffer from the projected impacts of climate change. Poor water quality effectively diminishes the availability of potable water, and increases the costs associated with rendering water suitable for use.

Changes in water quantity and water quality are inextricably linked. Lower water levels tend to lead to higher pollutant concentrations, whereas high flow events and flooding increase turbidity and the flushing of contaminants into the water system. Box 2 lists some of the main water quality concerns facing different regions of the country.

Box 2: Main water quality concerns across Canada ⁽²¹⁾

Warmer air temperatures would result in increased surface-water temperatures, decreased duration of ice cover and, in some cases, lower water levels. These changes may contribute to decreased concentrations of dissolved oxygen, higher concentrations of nutrients such as phosphorus, and summer taste and odour problems (e.g., references 22, 23).

River flows are expected to become more variable in the future, with more flash floods and lower minimum flows. Both types of hydrological extreme have been shown to negatively affect water quality.

During low flow events, increased concentrations of toxins, bacterial contaminants and nuisance algae are common. For example, when flow dropped in the St. Lawrence and Ottawa rivers, noxious odours became a problem due to an increase in a particular type of phytoplankton.(24) Heavy flow events have been shown to increase soil erosion and chemical leaching, whereas intense rainfalls increase the risk of runoff of urban and livestock wastes and nutrients into source water systems.⁽²⁴⁾

Climate change may also affect the quality of groundwater. For example, reduced rates of groundwater recharge, flow and discharge may increase the concentrations of contaminants in groundwater. Saltwater intrusion into groundwater aquifers in coastal regions is another concern, although Canadian research on this topic is limited.⁽²⁶⁾ In southern Manitoba, future changes in precipitation and temperature may cause groundwater levels in some parts of the Red River basin to decline faster than others.⁽²⁷⁾ These changes would affect the flow in the aquifer, and possibly shift the saline-freshwater boundary beneath the Red River valley, so that the groundwater in some areas may no longer be drinkable.⁽²⁷⁾

Ecological Impacts

"Water is also a critical, limiting factor in the existence and distribution of our natural ecosystems." ⁽⁶⁾

Wetlands, important natural modifiers of water quality, are highly sensitive to climate change.⁽²⁸⁾ As water flows through a wetland, contaminants such as metals, nutrients and sulphates are often filtered out. Lower water table levels, however, decrease the assimilative and purification abilities of wetlands. Drier conditions have also been associated with acid pulses (which can cause fish kills) and the formation of highly toxic methyl- mercury.^{(29, 30)} In the Canadian Prairies, wetlands (sloughs) are of tremendous hydrological importance, and provide vital habitat for birds and aquatic species. The persistence of these wetlands depends on a complex interaction between climate, geology and land use patterns, and their extent is controlled by the balance between water inputs and outputs. ⁽³¹⁾ The greatest impact of future climate change on Prairie wetland coverage would result from changes in winter snowfall, whereas changes in evaporation would have a smaller impact.⁽³¹⁾ Coastal wetlands of the Great Lakes are likely to suffer from decreased lake water levels and from shifts in surface-water and groundwater flow patterns.⁽³²⁾

River ecosystems are also an important component of the Canadian landscape. Their sensitivity to climate change is influenced by the characteristics of the river and its location. Northern rivers may be impacted by permafrost degradation and changes in flood regimes.⁽³³⁾ Ice-jam flooding is a key dynamic of the Peace-Athabasca Delta in northern Alberta, particularly for rejuvenation of riverside ecosystems. A decrease in ice-jam flooding due to climate change would significantly impact this ecologically sensitive region.⁽³⁴⁾ In southern Canada, seasonal shifts in flow regimes projected for rivers could have major ecological impacts, including loss of habitat, species extinction, and increased water contamination. Drainage basins containing large lakes or glaciers are generally less sensitive to changes in climate, at least in the short term, as these features help buffer the impacts of climate change.

Forests cover almost half of Canada's landmass and are important regulators of the hydrological cycle. Changes in forest extent and distribution, due to climate change or other factors, impact the storage and flow of water. An increase in forest disturbances, such as fires and insect defoliation, would also affect the ability of the forest to store and filter water. The impacts of climate change on forest ecosystems are covered in greater detail in the forestry chapter.