Water – Here, There and Everywhere

How much water is there in the world? Scientists estimate over one billion cubic kilometres (one cubic kilometre of water would fill 300 Olympic-sized swimming pools). Water covers nearly three quarters of the earth's surface in oceans as well as rivers, lakes, snow and glaciers. There is water in the atmosphere and water underground. Water evaporates and returns to the land surface in what is known as the hydrologic cycle.

In the hydrologic cycle, water evaporates from the ocean into the atmosphere, from there it can precipitate back into the ocean, or onto the land surface. From the land, it can evaporate or transpire back into the atmosphere, or flow overland or percolate underground before flowing back into the ocean. The distribution of the water around the globe varies from season to season and year to year, but the total quantity of water on the earth's surface remains essentially constant. The hydrologic cycle is discussed in detail in Freshwater Series No. A-1, "Water – Nature's Magician".

Although water exists in other forms in the hydrologic cycle, this issue in the Freshwater Series focuses on surface water as it is this water which we see in our everyday lives. Most of the earth's water is salty or permanently frozen. Figure 1 illustrates the proportion of fresh water that is available to us from the world's water supply. 
Figure 2 lists quantities that scientists have estimated for the various types of water that make up the world's supply. These amounts should be regarded as indicators of the relative quantities of water on earth. Owing to the difficulties in estimating volumes of water on a global scale, especially water underground, estimates can vary considerably. What is important is the overall picture that these estimates give. 

Canada's freshwater is found in the form of rivers, lakes, groundwater, and ice and snow.

Rivers

A river's watershed or drainage basin – the area supplying it with water – is separated from the watersheds of neighbouring rivers by higher lands called drainage divides. The map shows Canada's continental watersheds, one to each surrounding ocean: the Pacific, the Arctic, and the Atlantic as well as to Hudson Bay and to the Gulf of Mexico. Small watersheds combine to make up regional watersheds, which in turn join others to form continental watersheds. 

The world's largest rivers are shown in this figure. 

Sculpting the earth

As a swiftly flowing river, water can erode the underlying terrain. Where the river slope is flatter, the river slows down and deposits materials. This usually occurs in the lower reaches and especially near the mouth of the river, either at a lake or an ocean. A river can carve steep valleys, especially in higher parts of the drainage basin. In the lower parts of the basin, deposits may create deltas at the river's mouth.

The volume of water flowing in a river, together with the speed and timing of the flows, determines how a river shapes the surrounding landscape and how people can use its waters. Rainfall, snowmelt, and groundwater all contribute to the volume of flow, producing variations from season to season and year to year.

In Canada, most high flows are caused by spring snowmelt. This is the season when floods are most likely to occur. Rainstorms can also cause high flows and floods, especially on small streams. The effects of floods and storms can be much less severe on rivers with large drainage basins. The lowest flows for rivers in Canada generally occur in late summer, when precipitation is low and evaporation along with consumption by plants is high, and in late winter, when rivers are ice covered and the precipitation is stored until spring in the form of ice and snow.

Lakes

Canada has more lake area than any other country in the world (Figure 4), with 563 lakes larger than 100 square kilometres. The Great Lakes, straddling the Canada-U.S. boundary, contain 18% of the world's fresh lake water. 

How is water measured?

The Water Survey of Canada, Environment Canada , along with many contributing agencies, measure the rate of flow (discharge) in rivers and record the levels of lakes and rivers at more than 2 600 locations in Canada. Typical river flows are listed in Figure 5. 

• Water levels are read manually by gauge readers or continuously recorded either digitally or on graph paper.
• Rate of flow (or discharge) requires multiple measurements of channel depth, width, and flow velocity to yield the average discharge in the stream crossing for a given water level. Measurements can be made from a bridge, by wading in a stream, by boat or from a cableway strung across the river. In winter, the measurements are made through the ice.
• With sufficient measurements of flow over a variety of water levels (including extreme lows and highs), a water level-discharge relationship is established at each location. The discharge rate can then be computed from measured water levels.
• Historical records from 5 000 active and discontinued sites permit the estimation of streamflow at ungauged locations.

Creating a balance – naturally

The importance of lakes lies in their ability to store water during times of plenty and release it gradually. Thus lakes perform an extremely valuable task in balancing the flow of the rivers on which they are located. For example, the Saskatchewan River, with few lakes, has a maximum recorded flow of 59 times its minimum flow. On the other hand, the St. Lawrence River, which drains the Great Lakes, has a maximum flow of only twice its minimum flow. The difference in flow patterns in these two rivers is partly due to precipitation differences, but results mainly from the vast storage provided by the Great Lakes for the St. Lawrence River compared with the negligible lake storage on the Saskatchewan River.

Water flow comparisons

1 cubic metre per second (m3/s) = 31 536 cubic decametres per year (dam3/yr) = 86 400 cubic metres per day (m3/d) fills 2000 backyard swimming pools per day fills 1000 rail tank cars per day

Creating a balance – artificially

Since ancient times, people have built dams to control the outflow from existing lakes or to create new lakes. Dams and their reservoirs have provided:

• A stable source of inexpensive energy
• A more dependable water supply throughout the year
• Flood control downstream
• Recreation

The underground reservoir

Beneath the surface of the earth is a huge reservoir of fresh water. Groundwater does not rest; it moves continuously, but at a snail's pace, from its point of entry to areas of natural discharge. Groundwater moves so slowly that its speed is measured in metres per day, and even per year. (Surface water velocities are described in metres per second.) Wells intercept some groundwater but most of it continues until it reappears naturally in a spring or a seepage area and joins a watercourse.

Groundwater contributes to Canada's water supply by:

• Feeding streams, producing the entire flow of some streams during dry periods
• Replenishing wells – a valuable source of supply for individuals, communities, industries, and irrigated farms
• Supporting important ecological systems such as wetlands
• Moderating the adverse impacts of acid rain on surface water systems

Additional information on groundwater can be found in: Freshwater Series No. A-5, "Groundwater – Nature's Hidden Treasure."

Nature's frozen rivers

A huge quantity of fresh water is frozen in polar ice caps and in high mountain glaciers. Snow that is packed down over many years at high elevations becomes glacial ice, which slowly proceeds downslope like a frozen river, under the pull of gravity, and eventually melts to become part of streamflow at lower elevations. If the rate of melting is greater than the rate of accumulation, the glacier recedes; if it is less, the glacier advances.

Glaciers exert a direct influence on the hydrologic cycle by slowing the passage of water through the cycle. Like lakes and groundwater reservoirs, glaciers are excellent natural storehouses, releasing water when it is needed most. Glaciers, however, can release water when you need it least. Glacier-outburst floods, called jökulhlaups, can be devastating. Glacier-fed rivers reach their peak during hot summer weather.

Snowfall

Much of Canada's annual precipitation comes as snow: in the North, 50%; in the Prairies, 25%; and on both coasts and in southern Ontario, as little as 5%. Snow exerts a marked effect on the distribution of streamflow throughout the year. Instead of immediately infiltrating the soil or running off into stream channels as rainfall does, this water is first stored in the snowpack for several months. 

The relatively quick melting of snow in spring causes peak flows, sometimes resulting in floods. Some of the worst and most unpredictable flooding occurs when ice that has not yet melted is carried along in the swollen rivers until it jams, blocking the flow of water and creating a lake behind the jam with attendant flooding. When the ice jam breaks, a tremendous amount of water is suddenly released downstream, and more flooding may result.

Freshwater Series A-2

Note: A resource guide, entitled Let's Not Take Water For Granted, is available to help classroom teachers of grades 5-7 use the information from the Water Fact Sheets.

Published by authority of the Minister of the Environment
© Her Majesty the Queen in Right of Canada, 2002