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Section 2: Canada’s water supply—stocks and flows

Canada is surrounded on three sides by the Pacific, Arctic and Atlantic oceans and has over 243,000 km of coastline. 1  This, combined with the characteristics of Canada’s topography and climate, results in abundant freshwater resources. These water resources however are not evenly distributed across the country—they are available in different amounts, and at different times throughout the year. This uneven distribution affects water availability in ecosystems, and water access and use by Canadians.

The total supply of water resources is dependent on the quantities of water accumulated in the environment, called the stocks, and the quantities of water that circulate in the system, called the flows. For water use to be sustainable it is necessary not to withdraw more water than is renewed over a given time period. The World Resources Institute defines renewable freshwater as “…water that is fully replaced in any given year through rain and snow that falls on continents and islands and flows through rivers and streams to the sea”. 2  Most water contained in lakes and reservoirs, or coming from receding glaciers, is not renewable considering the time frame used in this definition.

Stocks of water

Water stocks represent the quantities of water accumulated in the environment, and are found either on or below ground-level. A very small portion of this water is locked underground in confined aquifers, and does not flow through the hydrological cycle. The water in lakes and unconfined aquifers is turned over or renewed, but only over a period of time that can be very long. Water renewal in Lake Superior, for example, occurs over a period of 191 years, while the water in Lake Erie, which is much shallower, is refreshed every three years. 3  Additionally, while rivers may physically ‘flow,’ at any given point in time the water they contain is considered a stock.


Surface water

Surface water refers to the water found in water bodies such as lakes, rivers, and wetlands, and bound up in snow, ice and glaciers. Canada’s lakes and rivers cover about 12% of the country’s surface area (Table 2.1).

Lakes and rivers

Lakes and rivers are fed by runoff from precipitation, snowmelt, glacier melt (in summer) as well as by contributions from groundwater, known as baseflow.

With 563 lakes larger than 100 square kilometres, Canada has more lake area than any other country in the world. 4  The Great Lakes, which Canada shares with the United States, are the largest group of freshwater lakes in the world and contain roughly 18% of the global stock of fresh surface water. The Great Lakes have a volume of 22,684 km3 and cover a surface area of 244,160 km25 

Other large lakes include the Great Bear and Great Slave lakes in the Lower Mackenzie drainage region in the Northwest Territories and Lake Winnipeg in the Lower Saskatchewan–Nelson drainage region. These large lakes combined, however, hold only about one-fifth as much water as is contained in the Great Lakes. 6 

According to the Canadian Geographic Names Data Base, there are more than 8,500 named rivers in Canada. 7  The Mackenzie River in the Northwest Territories, Canada’s longest river, has a length of 4,241 km and flows to the Beaufort Sea. The St. Lawrence River, important for shipping in Canada, is 3,058 km long and flows into the Gulf of St. Lawrence. 8 


Canada’s glaciers are estimated to cover 200,000 km2. About three-quarters of the area of glaciers in Canada is located on the Arctic Islands, with a further 24% located in the interior ranges of the Rocky Mountains, and along the coast of the Pacific Ocean drainage area. 9  Melt from glaciers in the Rockies is an important source of water in summer months; however, many of these glaciers are receding and thinning. 10 , 11 



Wetlands include swamps, bogs, marshes, fens, and other areas where the soil is saturated either permanently or for part of the year. These areas cover approximately 14% of Canada’s total land area. More northerly regions contain a greater proportion of wetland area than those in the south. 12 , 13  Wetlands are biologically diverse areas that provide habitat for fish; mammals; birds, such as ducks, geese, cranes and sandpipers; amphibians, such as frogs and salamanders; reptiles, such as turtles; and invertebrates, including insects and shellfish. Wetlands provide many ecological benefits including filtering nutrients from water and controlling flooding.


Groundwater refers to water located under the soil surface—soil moisture and water stored in aquifers. Aquifers are geological formations of sand, gravel or permeable rock that can store and transmit water. Baseflow, water that flows from underground to the surface, is a reliable source of water for many rivers.

Most shallow aquifers contain freshwater that can be accessed through wells. The water table is the upper boundary that separates non-saturated soil from the saturated soil in shallow unconfined aquifers. In Southern Canada, the water table lies within 20 m of the surface. 14  Confined aquifers are bounded by layers of impervious rock. When tapped by artesian wells, the pressurized water in these aquifers can rise above the level of the water table.

Groundwater also includes non-renewable water resources in deep aquifers. This water is recharged over a very long time period, and is therefore not renewable over human time scales. Water in these deep aquifers often contains dissolved solids, becoming saline at depth, making it less fit for consumption. The portion of unconfined aquifers that are recharged every year in Canada contributes to Canada’s renewable water resources.

Overall it is estimated that 25% of Canadians rely on groundwater as a source of drinking water (Map 2.1). This includes Canadians supplied with water from drinking water plants that use groundwater sources and Canadians that rely on wells. This percentage varies depending on the region of the country. The population in the Saint John–St. Croix drainage region (drainage region 23) is the most reliant on groundwater, whereas the population in the South Saskatchewan drainage region (drainage region 11) is the least. The population of Prince Edward Island, which is part of the Maritime Coastal drainage region (drainage region 24) is 100% dependent on groundwater.

Flows of water

Most of Canada’s surface freshwater flows northward—39% of the total area of the country drains into Hudson Bay and a further 36% drains into the Arctic Ocean. Fifteen percent of the total area of Canada is within the Atlantic Ocean drainage area and 10% is in the Pacific Ocean drainage area. A small portion of southern Alberta and Saskatchewan, covering 0.3% of Canada’s total land area, is part of the Missouri drainage region, with waters flowing eventually to the Gulf of Mexico (Map 1.2). The direction and quantities of groundwater flows are not well understood at the national scale.

These flows can be considered the renewable portion of Canada’s freshwater resources. 15  These resources are replenished each year through approximately 5,500 km3 of precipitation, mainly composed of rain and snow (Table 2.1), and about 52 km3 of water that flows into the country from the United States. 16 

The geographic distribution of precipitation differs across the country. Generally, the Pacific and Atlantic coasts receive the most precipitation while the Prairies and the far North receive the least. The average annual amount of precipitation ranges from a high of 1,354 mm for the Pacific Coastal drainage region in British Columbia to a low of 189 mm for the Arctic Coast–Islands drainage region in the North (Table 2.1).

The timing of precipitation also varies across the country. Throughout the continental interior, maximum precipitation generally occurs in summer, while this is the driest time of year on the west and east coasts. 17  The Prairies and Arctic receive very little precipitation in winter, partly due to cold temperatures that limit the air’s capacity to hold water vapour. In comparison, in winter coastal British Columbia receives most precipitation as rain, and the east coast receives a mix of rain and snow, with more rain near the Atlantic Ocean and more snow further inland in southern Quebec and Labrador. 17 

Water yield in Canada

Water yield estimates are derived from the monthly amounts of unregulated flows of surface water in Canada’s rivers and streams. Measuring this part of the hydrological cycle over time provides insight into the status and trends of water resources in Canada, including monthly supplies and inter-annual changes. A complete discussion of the methodology, and the water yield results, is presented in a technical paper. 18 

The average annual water yield for Canada is 3,472 km3 (Table 2.2). To put this in perspective, this water yield amounts to almost as much water as there is in Lake Huron (which contains 3,540 km3), and is equivalent to a depth of 348 mm of water across the full extent of Canada’s landmass. 3  This abundance, however, is distributed unequally across the country (Table 2.2, Chart 2.1 and Map 2.2). Generally, drainage regions on the Pacific coast, northern Quebec and the Atlantic coast have the highest water yields. Drainage regions in the Prairies and north of the Prairies produce the least water. Furthermore, areas of abundant water yield do not correspond with the highly populated regions of the country—98% of Canadians live in the southern part of the country, but this area is responsible for only 38% of the water yield (Map 2.3). 19 

Variation in water yield amongst the 25 drainage regions in Canada is considerable (Map 1.1, Table 2.2, and Chart 2.1). The largest yields of renewable fresh water are on the two coasts. With an average annual water yield per unit area of 1.54 m3/m2, the Pacific Coastal drainage region in British Columbia has the highest renewable freshwater per unit area in the country. It is followed by the Newfoundland and Labrador and the Maritime coastal drainage regions which have average annual yields per unit area of 0.86 m3/m2 and 0.85 m3/m2 respectively (Table 2.2).

Differences amongst regions are most pronounced when water yields in the Prairies are compared to yields in other parts of the country. Drainage regions 9, 10, 11 and 12 comprise most of the Prairies and stretch across the southern part of Alberta, Saskatchewan and Manitoba. The average annual yield of renewable freshwater per unit area for this collection of drainage regions is 0.05 m3/m2. This is equivalent to 12% of the yield of the Great Lakes drainage region, 6% of the yield of the Maritime Coastal drainage region and only 3% of the Pacific Coastal drainage region (Table 2.2). When the water yields for regions 9, 10, 11 and 12 are combined, and divided by the total area of these four regions, the resulting renewable fresh water per unit area is less than that for either Australia or South Africa (Table 1.1).

These four drainage regions generally correspond to the Prairies ecozone, which had a population exceeding 4.5 million people in 2006. The population in this ecozone increased by 1.6 million people from 1971 to 2006. 20 

Regional disparity can also be quite pronounced even within a province. In British Columbia, the annual yield of water per unit area of the Pacific Coastal drainage region is 1.54 m3/m2, while the Fraser–Lower Mainland produces 36% of this volume, and the Okanagan–Similkameen, only 18%.

Water yield per capita

Examining population distribution in relation to water resources gives an indication of the pressures exerted on water resources. Calculating the water yield per capita is one way to show this relationship.

Dividing Canada’s total water yield by its population reveals that almost 110,000 m3 of renewable freshwater is produced per person each year (Table 2.3). Brazil, which has the highest water yield per unit area of any country in the world, provides 43,756 m3 of water per person per year, 40% of what is annually available per person in Canada. While total water yield is comparable between the United States and Canada, the amount of renewable freshwater per American is only 9.1% of that per Canadian because the United States has a much larger population (Table 1.1).

This fact reinforces the notion that Canada has an abundance of renewable freshwater available to its population. However, this assumption is misleading: the more populated regions of the country do not typically correspond with the regions that produce the bulk of renewable water in the country (Chart 2.1, Table 2.3). The top five drainage regions in terms of water yield, produce 55% of the water, but have only 8% of the population. The Great Lakes drainage region where 34% of the population of the country resides, produces only 4% of the national renewable water yield (Chart 2.1). In the South, where most of the population is located, there are 42,661 m3 of renewable freshwater available per capita, compared to 4,193,014 m3 per capita in the North: water availability per capita is 98 times greater above the North-line than below it (Map 2.3).

Similarly in British Columbia, almost 360,000 m3 of water are available per person in drainage region 1, while in the interior portion of the province only 4% of this amount is available per person in drainage region 3 (Table 2.3).

Changes in water yield over time

Trends in economic production, unemployment rates and temperature are often analyzed to better understand how the economy, society and the environment have changed during the period covered by the time series. Similarly, trends in water yield help understand changes in Canada’s renewable water supply from 1971 to 2004.

Trends in water yield in Southern Canada, 1971 to 2004

Trends in water yield from 1971 to 2004 in southern Canada were derived using annual estimated flow volumes for this time period. 21  Although a national trend could not be derived because of insufficient data in the North (Map 2.3), it was possible to estimate it for the area below the North-line. This is the portion of the country where most economic activity takes place, and it has an area of almost 2.6 million km2.

Chart 2.2 shows that in the southern portion of Canada, water yield decreased on average by 3.5 km3 per year from 1971 to 2004, which is equivalent to an overall loss of 8.5% of the water yield over this time period. This average annual decrease of 3.5 km3 is almost as much as the 3.8 km3 of water that is supplied to the residential population of Canada in a year (Table 3.1).

The smoothed trend line in Chart 2.2 shows a decrease in water yield from 1971 to 1987, followed by an increase until 1996, when it reverts back to a decrease until the end of the time period. Chart 2.3 compares the smoothed trends for selected drainage regions using a common scale, and shows how the water yield volumes of these drainage regions have changed over time.

The downward trend in Southern Canada was not distributed equally (Charts 2.3 and 2.4). Specifically, in British Columbia, water yield in the Columbia (drainage region 4) remained relatively constant over the 34-year period while volumes in the Fraser–Lower Mainland (drainage region 2) declined from 1971 to 1977, before levelling out. This resulted in a decrease of 9%, or 0.35 km3 of water per year in this drainage region from 1971 to 2004 (Chart 2.4).

While the volume of the water yield in the South Saskatchewan drainage region is roughly 8% that of the Fraser–Lower Mainland, both drainage regions showed a marked decrease in water yield in the early years of the period under study.

The sharpest decline occurred in eastern Canada: water yield in the Maritime Coastal drainage region decreased 19.6% from 1971 to 2004, and water yield in the St. John–St. Croix drainage region declined 21.5%.

Trends in water yield in the Prairies, 1971 to 2004

From 1971 to 2004 the highest variability in water yield in Canada was detected in the Prairies (Map 2.4). 18 , 22  This area includes drainage regions 9, 10, 11, and 12, that is to say, the Missouri, North Saskatchewan, South Saskatchewan, and Assiniboine–Red drainage regions, respectively; and part of drainage region 6, the Peace–Athabasca.

This variability in the flows of renewable water resources is of interest because the lack of predictability affects economic activities, including agriculture.

This variability of flows is illustrated by the severe floods and droughts that occur in this region. For example, the flood of the Red River in 1997, which brought about the worst flooding the region had seen since 1852, forced 75,000 people to abandon their homes, and caused $450 million in damages (Map 2.5). 23  At the other end of the spectrum, the drought of 2002 (Map 2.6) had adverse impacts on the agricultural community. In 2002, crop yields in Alberta declined compared to average yields for 1981 to 2000—the yield of spring wheat was down 29%, barley was down 27% and canola was down 13%. 24  Additionally, cattle inventories in Alberta declined by 10.4% (605,000 head) from January 2002 to January 2003. 25 

From 1971 to 2004, water yield for this area decreased by 0.56 km3/yr (Chart 2.5). To put this in perspective, this volume represents about 80% of the total volume of water that was produced by drinking water plants in these five drainage regions in 2005. 19  Over the 34-year period, this represents a total reduction of 20 km3 of water yield, equivalent to roughly one half of the long-term, average annual water yield.

Monthly water yield

Analysis of the annual water yield in Canada has shown that there was a decrease from 1971 to 2004, and that annual water yield was particularly variable in the Prairies, an area prone to floods and droughts. Monthly flows of renewable water can also be highly variable. Analysis of long-term monthly maximum and minimum water yields for 1971 to 2004 revealed that volumes can vary by more than 200% in both May and August. Therefore, analysis of the temporal distribution of water yield during the year is critical to understand the challenges that Canada faces in managing this resource.

For most of the country, the bulk of the water yield is produced in April, May and June, as snow and ice melt, and precipitation increases. These parts of the hydrological cycle generate more water yield than at any other time of the year. In the North this peak occurs in late spring and early summer. In the South, water yield is highest in the spring. As spring turns into summer, water yield declines and demand related to human activity increases.

By late summer the disparity between renewable supply and demand is typically at its greatest. In the interior and southern drainage regions of British Columbia (drainage region 2, 3 and 4), 56% of the water yield comes before July 1st in a typical year. In the Okanagan–Similkameen (drainage region 3) however, 80% comes before July 1st (Chart 2.6), and there is a 93% decline in water yield from a peak in May to the high demand month of August.

In the Prairies, spring freshets in the Assiniboine–Red and Missouri (drainage regions 12 and 9) bring a large quantity of water in spring with a sharp drop shortly thereafter, resulting in very dry summer months. In drainage region 12, the average year has a 96% decrease in water yield from the peak supply in April to the high-demand month of August (Chart 2.6).

The water yield in the North and South Saskatchewan drainage regions (drainage regions 10 and 11), is highest in June and July. Nevertheless, differences between supply and demand are often acute —in the South Saskatchewan drainage region there is a 48% decline in water yield from July to August (Chart 2.6). This decline can be larger in areas where glacial melt does not supply summer flows, or where there is a significant distance between the glacial melt and downstream locations. In these Prairie areas, evaporation removes a great deal of water from the land, rivers, streams, lakes and reservoirs over the hot and dry summer months.

Generally the pattern of higher spring water yield holds through Ontario, Quebec and Atlantic Canada. In the Great Lakes drainage region, water yields decline 88%, from a high in April to a low in August, when demand can be near its peak. In the St. Lawrence drainage region, home to both Montréal and Québec City, yield is at a high in April and declines by 82% to a low in August (Chart 2.6).

On the island of Newfoundland, and in the Maritime Coastal and Saint-John–St. Croix drainage regions, water yields peak in April and May, decline through the summer months and increase again to another smaller peak in November or December (Chart 2.6).

Understanding the temporal relationship between supply and demand provides insight into when pressure is exerted on water resources in specific regions. In some jurisdictions, to deal with immediate demands, water boards have been formed to licence and regulate water withdrawals and to inform the public of the need to conserve water. To manage longer-term water-supply demands, decisions have also been made to hold water that has been produced in spring for use during the summer months. This has been accomplished by creating dams, diversions and reservoirs such as Lake Diefenbaker in Saskatchewan.

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