The easiest way to describe the pattern of rivers
and lakes on a small-scale map is to show the pattern of drainage
basins. In Canada, there is a detailed hierarchy of drainage basins,
ranging from the largest (drainage into oceans and their equivalents),
down to very detailed patterns. When a user sees the initial view
map, it will show the ocean drainage areas, and then will expand
to a more detailed network as he or she zooms in.
The notes below describe the main drainage
basins, major lakes,
and rivers
in Canada.
Drainage Basins
A drainage basin is an area that drains all precipitation received
as either runoff or base flow (groundwater sources) into a particular
river or set of rivers. The boundary of a drainage basin is defined
as the ridge beyond which water flows in the opposite direction.
A drainage basin is also known as a catchment area or a watershed.
Canada's major drainage regions are as follows:
Table 1. Ocean Drainage Areas of Canada
Ocean Drainage Areas of Canada
Atlantic Ocean |
1
520 000 |
Hudson Bay |
3 861 400 |
Arctic Ocean |
3
583 300 |
Pacific Ocean |
1
009 100 |
Gulf of Mexico |
21
600 |
|
Source: Canada. Natural Resources Canada.
The Atlas of Canada. Facts About Canada: Fresh Water. Ottawa, 1999.
The location and main component rivers of each of these regions
is as follows:
- The Pacific Ocean drainage area drains the
area west of the Rocky Mountains. The Fraser, Yukon and Columbia
rivers are the largest rivers draining this region. It is separated
from all other drainage areas by the continental divide.
This is defined as the north-south line along the western Cordillera
separating rivers flowing ultimately into the Pacific Ocean from
those flowing into other oceans.
- The Arctic Ocean drainage area is the area
flowing directly into the Arctic Ocean or into the channels of
the Arctic Islands. Hudson, James and Ungava bays are considered
to be part of the Arctic Ocean, but for most purposes their drainage
area is usually considered as a separate entity. The Mackenzie
River dominates the Arctic Ocean drainage area.
- The Hudson Bay drainage area is a huge area
that captures about 30% of total Canadian runoff. Many of its
river systems such as the Nelson and Churchill River (of Manitoba)
drain eastwards from the continental divide to Hudson Bay. As
well, many large rivers drain from the south and east into Hudson
Bay or James Bay. The extensive area of drainage into Ungava Bay
is also considered to be part of the Hudson Bay drainage area.
- The Atlantic Ocean drainage area is dominated
by the Great Lakes-St. Lawrence system but there are other significant
drainage basins such as those of the Churchill River (of Labrador)
and the Saint John River in New Brunswick.
- Gulf of Mexico drainage area is a small portion
of southern Alberta and Saskatchewan drains south into the Mississippi
system which ultimately drains into the Gulf of Mexico. (The Gulf
is part of the Atlantic Ocean, but because of the Mississippi,
it is often studied as a separate entity).
- Parts of Alberta and Saskatchewan have areas of internal
drainage: these are river systems that do not drain into
any ocean. Maps often assign these areas to one or other of the
drainage areas noted above.
More detailed data on Canadian drainage basins is given in the
Rivers
section of Facts About Canada.
Lakes
A lake may be defined as any inland body of standing
water, usually freshwater, larger than a pool or pond. Canada is
famous for its number of lakes. There are as many as two million
lakes covering, in total, 8.9% of the country.
In order to discuss the pattern of lake distributions, users may
find it useful to turn on the layer called Geological Provinces.
This layer shows all 17 geological provinces of Canada. However,
for ease of use as a base layer, the layer colour-codes them into
a small number of types: thus, all seven of the provinces making
up the Canadian Shield units are in the same colour, as are all
platform areas, and all mountainous (orogen) areas.
Most of the larger lakes are found either within the Canadian Shield,
or in the line of contact between the Canadian Shield and the two
platform areas to its south, the Interior Plains, and the St. Lawrence
Lowlands. Other than on the Shield, nearly all the lakes in Canada
owe their origin to glacial activity. Lakes such as Great Bear,
Great Slave, Athabasca, Winnipeg, and the Great Lakes are all found
along the line separating the Shield from platform areas to its
south and west. These lake depressions were formed by glaciers from
the Shield carrying hard granitic debris and gouging deeply onto
the softer sedimentary rock. Lakes in the highly resistant rocks
of the Shield tend to be clear and long-lived. By contrast, Prairie
lakes, which are often formed by melted-out glacial deposits, tend
to be shallower and contain more sediment. Lakes in the mountainous
areas of British Columbia and the Yukon are typically confined to
deep glaciated valleys.
Canada probably has more lakes than any other country in the world.
Canada has 563 lakes having an area greater than 100 square kilometres.
A complete list is given in the Facts
about Canada section of The Atlas of Canada. The largest set
of lakes, the Great Lakes, straddle the Canada-US boundary and contain
18% of the world's freshwater in lakes.
Table 2 indicates that many of the largest lakes in the world are
either wholly or partly within Canada.
Table 2. The World's Largest Lakes
The World's Largest Lakes
1 |
Caspian
Sea |
Russia/
Kazakhstan/
Uzbekistan /
Iran/
Azerbaijan |
378
400 |
1025 |
2 |
Superior |
USA/Ontario |
83 300 |
401 |
3 |
Victoria |
Kenya/Uganda/
Tanzania |
69
900 |
92 |
4 |
Huron |
Ontario/USA |
59
800 |
229 |
5 |
Michigan |
USA |
58
100 |
285 |
6 |
Tanganyika |
Tanzania/
Zambia/
Democratic
Republic
of the Congo/
Burundi |
34
000 |
1470 |
7 |
Baikal |
Russia |
31
500 |
1741 |
8 |
Great
Bear |
Northwest
Territories |
31
792 |
445 |
9 |
Great
Slave |
Northwest
Territories |
28
570 |
614 |
10 |
Erie |
Ontario/USA |
25
720 |
64 |
11 |
Winnipeg |
Manitoba |
24
600 |
28 |
13 |
Ontario |
Ontario/USA |
19
480 |
273 |
22 |
Athabasca |
Saskatchewan/
Alberta |
8
080 |
124 |
31 |
Winnipegosis |
Manitoba |
5
470 |
124 |
|
Source: Adapted from Peter H. Gleick. Water
in Crisis. New York: Oxford University Press, 1993.
Note: The Aral Sea is not listed as its area has shrunk drastically
in recent years.
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.
The Great Lakes
The Great Lakes basin (the lakes plus the area of land draining
into the lakes) is home to 8.5 million Canadians and 30.7 million
Americans. As well as providing drinking water, these lakes have
played a major role in the development of both countries. They allow
goods to be shipped to and from the heart of North America; they
are a source of hydroelectricity; and they are the site of immense
industrial, commercial, agricultural, and urban development. The
Great Lakes also provide an array of recreational opportunities.
The overall management of many aspects of the Great Lakes is handled
by the International Joint Commission, which is based in Detroit.
Figure 1 shows the elevations and depth profiles of the Great Lakes,
and route of the St. Lawrence Seaway.
[D] Click for larger version, 12 KB Figure 1. Cross-sectional Profile of the Great Lakes and the St. Lawrence Seaway
Rivers and Runoff
Aside from glaciers, running water has had the most impact on shaping
the earth's landscape. Most Canadian rivers have developed since
the last ice age. Almost 75% of the Canadian landmass contains water
that drains northward into either the Arctic Ocean or into Hudson
and James bays. This flow involves almost half (47.9%) of the total
flow of Canadian rivers.
Most rivers in Canada have a substantial seasonal variation in
runoff. There are different patterns for flow variations:
- Most high flows in Canada are caused by snowmelt, which usually
peaks in the spring. Consequently, this is the season when floods
due to rivers are most likely to occur in Canada.
- Rainstorms can also cause sudden high flows and floods in almost
any season, especially on small streams.
- Glacier meltwater provides the high-water flow in mid-summer
for rivers that drain from mountainous regions of BC and Alberta.
Fortunately for these regions, this causes these rivers to have
a peak flow in mid-summer.
- In many other parts of Canada, natural water storage in lakes,
wetlands and aquifers, provides more consistent sources of water
for rivers throughout the year. A good example of this is the
St. Lawrence River, which has a relatively even flow year-round
due to the storage capacity of the Great Lakes.
- The lowest flows on rivers in Canada generally occur at two
times of the year: in late summer, when precipitation is low,
and evaporation along with water 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.
A map in The National Atlas of Canada, 4th Edition, Seasonal
Runoff shows the timing of peak runoff for a large selection
of rivers across Canada.
As with large lakes, many of the largest rivers in the world are
wholly or partly within Canada as shown in Table 3 below.
Table 3. The World's Largest River Drainage Basins
The World's Largest River Drainage Basins
1 |
Amazon |
South
America |
6915 |
6923 |
2 |
Congo |
Africa |
3680 |
1320 |
3 |
Murray |
Australia |
3520 |
N/A |
4 |
Plata-
Parana-
Grande |
South
America |
3100 |
811 |
5 |
Ob' |
Asia |
2990 |
302 |
6 |
Mississippi-
Missouri |
USA/
Canada |
2980 |
510 |
7 |
Nile |
Africa
|
2870 |
100
|
8 |
Yenisei
|
Asia |
2580 |
539 |
9 |
Lena |
Asia |
2490 |
404 |
10 |
Niger |
Africa |
2090 |
302 |
11 |
Amur |
Asia |
1855 |
360 |
12 |
Yangtze |
Asia |
1855 |
1006 |
13 |
Mackenzie |
Canada |
1790 |
330 |
14 |
Ganges
-
Brahmaputra |
India |
1730 |
1386 |
15 |
Volga |
Russia |
1380 |
255 |
16 |
Zambezi |
Africa |
1330 |
18 |
17 |
St
Lawrence |
Canada/
USA |
1030 |
318 |
|
Source: Adapted from: World Water Resources
and Their Uses. Joint SHI/UNESCO Product, prepared by Igor A. Shiklomanov,
[place],1999.
More detailed data about the length, drainage area and discharge
of Canadian rivers is given in the Rivers
section of Facts About Canada.
Measuring Freshwater in Lakes and Rivers
Environment Canada's Water Survey of Canada measures the rate of
flow (discharge) in rivers at more than 2600 locations in Canada.
Useful geographical summaries of flow data are found in the Rivers
section in Facts about Canada as noted above, and the 5th
Edition Streamflow map.
Gauging stations also collect data on the water level (its height
above or below a datum) of their particular lake or river location.
Water level and discharge information is essential for the wise
management of Canada's water resources. Some uses of these data
are the following:
- Allocating water between various users.
- Managing water resources, in particular for minimizing the impacts
of extreme flows. This involves a variety of activities such as
flood protection, floodplain mapping or building diversion canals.
- Designing and constructing specific structures to use or to
function alongside normal water flows. Examples are water supply
facilities, irrigation facilities, bridges and culverts.
- Planning and conducting environmental programs and assessments
related to water quality, fisheries, and wildlife habitat.
- Ensuring that the nation's water resources are developed in
a manner that conserves and protects the environment.
|