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 stadiums). 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 leaves and returns to the earth in what is known as the hydrologic cycle.

In the hydrologic cycle, water precipitates from the atmosphere to the oceans or to the land surface (where it flows overland and underground back to the oceans), and then evaporates or transpires back to the atmosphere from the land and the oceans. 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".

When we think of water, we tend to think of surface water, i.e., water in lakes and rivers because it is this water which we are most conscious of in our everyday lives. Although water in other parts of the hydrologic cycle makes up a significant proportion of Canada's water supply, the focus of this issue in the Freshwater Series is surface water. 

Most of the earth's water is salty or permanently frozen. This figure illustrates the proportion of fresh water that is available to us from the world's water supply. It also lists the range of 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 disagree 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. Small watersheds combine to make up regional watersheds, which in turn join others to form continental watersheds. 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. 

The world's largest river drainage basins are listed in this figure. 

Sculpting the earth

As a swiftly flowing river, water erodes the underlying terrain to varying degrees. 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 snowmelt and occur in spring. 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 (see next section). The lowest flows on 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 probably has more lake area than any other country in the world, with 565 lakes larger than 100 square kilometres. The Great Lakes, straddling the Canada-U.S. boundary, contain 25% (22 700 cubic kilometres) of the world's freshwater in lakes (sharing "first place" with Lake Baykal, Russia). 

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 3 100 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 be computed from measured water levels.
• Historical records from 7700 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.

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

Canada, which is one of the major dam-building countries in the world, uses dams mainly for the production of hydroelectric power. Canada also diverts more water between river basins than any other country, again mainly for the purpose of hydroelectric power production.

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 10%. 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 for several months.

The relatively quick thawing 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

Published by authority of the Minister of the Environment
©Minister of Supply and Services Canada, 1992
Cat. No. En 37-81/2-1992E
ISBN 0-662-18069-0