Drainage basin, area of land that contributes the water it receives as precipitation (except for losses through evaporation, transpiration from plants, incorporation into the SOIL or GROUNDWATER, etc) to a RIVER or network of rivers.
Drainage basin, area of land that contributes the water it receives as precipitation (except for losses through evaporation, transpiration from plants, incorporation into the SOIL or GROUNDWATER, etc) to a RIVER or network of rivers. Drainage basins are defined by topographical features, called drainage divides, which determine the direction of flow of water. Canada has 6 major drainage basins, each of which are described below. Note that upward revisions in river flow volumes, especially for the northern and western basins, are warranted with the new data we now have available, but good current regional estimates are not yet available.
This basin of 3.58 million km2 receives streamflow from the northern two-thirds of Alberta, northern BC, northern Saskatchewan, the YT, the NWT and Nunavut, including the arctic islands. The estimated average annual discharge rate is 15 500 m3/s. The largest river of the region is the MACKENZIE. Its watershed, 1.787 million km2, comprises about 50% of the arctic drainage area, about 20% of Canada's area and 60% of the drainage water of the arctic region. The most important tributaries, the PEACE, ATHABASCA and LIARD rivers, all rise on the eastern slopes of the Rocky Mountains. The Mackenzie has an average discharge of 9910 m3/s into the Beaufort Sea. The annual streamflow is equivalent to a mean depth on the watershed of 170 mm. The longest flow route is 4241 km, from the head of the Finlay River in northern BC via the Peace River, Lake Athabasca, Great Slave Lake and the Mackenzie River to its mouth. This length exceeds by over 1000 km that of any other Canadian river system. The Mackenzie's peak flow occurs in June; otherwise it is quite uniform from May to September, when 70% of the annual discharge occurs. This uniformity is attributable to the influence of the relatively flat Barren Lands east of the river and to the many lakes, the largest of which, GREAT BEAR and GREAT SLAVE, each cover about 30 000 km2.
ICE is significant to the HYDROLOGY of the region and much of the terrain is underlain by PERMAFROST. Lakes and rivers are open from about June 15 to November 1 in the more northerly part of the Mackenzie Basin, less in the arctic islands. The Mackenzie River valley has been the subject of intensive technical, social and cultural studies during the 1970s because of concern over the potential impact of proposed petroleum PIPELINES. The delicate ecological balance existing between native people and their natural environment has given rise to many questions about the effects of TRANSPORTATION corridors in the valley.
The 1.08 million km2 Cordilleran region, west of the Continental Divide and extending from the international border at the boundary of Alberta and BC into the YT near the Mackenzie Delta, sheds water into the Pacific Ocean at an average rate of 21 200 m3/s. This rate is equivalent to an annual depth of 620 mm over the area, the largest hypothetical average depth of any hydrologic region of Canada. The high average discharge is attributable to the large amounts of precipitation received at higher elevations of the Rockies and coastal mountain ranges, caused by moist air masses moving off the Pacific Ocean. Stream water supply is, however, highly variable. In the dry interior valleys the equivalent annual depth of runoff is 100 mm or less; in the coastal zone it is 3200 mm.
The 1368 km FRASER RIVER flows through spectacular canyons and rich agricultural land to provide drainage to more than 230 000 km2 of interior BC. The mean discharge rate is about 3620 m3/s. Highest flows are experienced in June; lowest in March. The risk of flooding in the Fraser's lower reaches has long been a matter of concern.
The COLUMBIA RIVER system in southeastern BC has an average flow at the international boundary of 2800 m3/s. A portion of this flow originates from 50 000 km2 of drainage area in the US. Three large RESERVOIRS in the Columbia system provide significant FLOOD-control benefits to the US and will ultimately allow installation of 4000 MW of hydroelectric-generating capacity in Canada (see BC HYDRO). The Okanagan River, which flows to join the Columbia south of the international boundary, is of great benefit to fruit growers in the semiarid valleys.
Coastal rivers, including the Fraser, swarm with SALMON during the season when they return inland to their spawning grounds. The other large rivers, the SKEENA, NASS and STIKINE, discharge a combined average of 3800 m3/s into the Pacific. The Yukon, with a drainage area of 800 000 km2, has an average discharge of 2300 m3/s where it crosses into Alaska. Only a small fraction of the potential hydroelectric-generating capacity has been installed on any of these streams. In addition to the benefits to fisheries and agriculture and for HYDROELECTRICITY, the waters of the Pacific drainage provide a valuable resource to the LUMBER, PULP AND PAPER and mineral processing industries and to municipalities.
Western Hudson Bay and Mississippi Drainage
This region of 2.64 million km2 includes all of Manitoba, most of Saskatchewan, the southern third of Alberta, most of the District of Keewatin, a part of Ontario west of Lake Superior and about 150 000 km2 immediately south of the international boundary. A 27 500 km2 zone of southern Saskatchewan and southeastern Alberta drains into the Mississippi Basin and thence to the Gulf of Mexico. The major rivers discharging into western HUDSON BAY are the HAYES, NELSON, CHURCHILL, KAZAN, and THELON. The estimated mean rate of discharge for the region is about 9400 m3/s.
The most important drainage basin is the 1 million km2 drained by the Saskatchewan-Nelson river system. The BOW, OLDMAN and North and South SASKATCHEWAN RIVERS rise on the eastern slopes of the Rockies and flow east for 1900 km through the central plains. Extensive tracts in the plains region drain into interior depressions (sloughs). Many of these sloughs have no outlet, and many others have outlets only in the wetter years to the principal streams which enter a chain of lakes in south-central Manitoba. Here the flow is joined with that of the RED RIVER flowing in from the south and the WINNIPEG RIVER from the east. The latter carries drainage from southeastern Manitoba, the Lake of the Woods region of Ontario, and Minnesota. The flow discharges from Lake Winnipeg (24 000 km2) into Hudson Bay via the Nelson River.
The mean flow of the Nelson at its outlet is 2370 m3/s, equivalent to a mean depth of about 125 mm annually over the watershed. The principal source areas of streamflow for the Nelson are the eastern slopes of the Rockies and the lake area of Manitoba, northwestern Ontario and the adjacent states. The latter region contributes over 50% of the total annual discharge. The several large lakes and hundreds of smaller ones, totalling over 50 000 km2 in area, regulate the Nelson's flow, so that the mean flow rate in any month is seldom 30% greater or less than the annual average. Highest discharge occurs in July; lowest flows in January-March. The Nelson and Churchill rivers are sites of large hydroelectric-power generating facilities with significant potential for further development.
The North and South Saskatchewan river system is important to agriculture since it flows through a region of fertile soils limited in their productivity by uncertain rainfall. The water supply from the Bow, Oldman, North and South Saskatchewan rivers and their tributaries is vital to cities, towns and industries of Alberta and Saskatchewan. The high variability of local tributary flow has necessitated construction of many dams to provide carry-over storage from years of abundant flow to years of low discharge.
South and East Hudson Bay Drainage
This region includes most of northern Ontario and western and northern Québec, an area of about 1.4 million km2. There are at least 15 individual watersheds of 30 000 km2 or more in area. The largest of these are the SEVERN, draining into Hudson Bay, the ALBANY (134 000 km2), the MOOSE (108 000 km2); LA GRANDE RIVIÈRE (98 000 km2) flowing into JAMES BAY; and the KOKSOAK (133 000 km2) draining into UNGAVA BAY. The region, part of the Canadian Shield, consists of massive ancient crystalline rocks. As a result of GLACIATION there are large areas of lakes, ponds and swamps. In some parts as much as 15% of the drainage area is covered by fresh water. The many lakes result in fairly uniform rates of river flow throughout the year. The waters are generally of good quality and very little influenced by human activities.
Although the rivers and lakes are ice-covered annually for 6 months, water continues to flow under the ice. The lowest flows occur in March for the more southerly streams, April for those draining into Ungava Bay. The time of highest discharge is late May or June, following the breakup of the ice cover in mid-May. About 45% of the total annual flow occurs May-July, except for streams draining into Ungava Bay, where about 60% of the annual flow occurs June-August. The average annual rate of flow of all streams draining into Hudson, James and Ungava bays is estimated at 20 000 m3/s, which is equivalent to a mean annual water depth of 450 mm over the entire region and represents about 60% of precipitation received. There is considerable hydroelectric potential in the region. The JAMES BAY PROJECT is one of the largest in the world. The average flow in La Grande Rivière is to be increased from 1700-3300 m3/s and the eventual generating capacity will be 10 000 MW.
Great Lakes-St Lawrence Drainage
This basin, with an area of 1 million km2 above Montréal, is dominated by the GREAT LAKES, covering 245 000 km2. About one half the basin is Canadian, Lake Michigan is wholly within the US; the international boundary passes through Lakes Superior, Huron, Erie, Ontario and St Clair. Most of the land drainage from the Canadian portion is from the Canadian Shield; the ST LAWRENCE LOWLANDS make a small contribution. The lowlands include peninsular Ontario, bounded by Georgian Bay, Lakes Huron, St Clair, Erie and Ontario and the fringes of the St Lawrence River. The Great Lakes-St Lawrence system provides an important navigable waterway of nearly 4000 km from Ile d'Anticosti to the head of Lake Superior (see ST LAWRENCE SEAWAY).
Most streams in the lowlands are less than 300 km long. They are important for industry, agriculture and urban water supply, and provide recreational opportunities for the region's urban population. The intensive land use in the southern parts of the basin, coupled with variability of the glacial deposits, results in high variability of seasonal flows, localized flooding, periodic low summer flows and impaired WATER quality.
The average flow in the NIAGARA RIVER at Queenston, between Lake Erie and Lake Ontario, is 5760 m3/s. By the time the St Lawrence River reaches Cornwall, downstream of the Great Lakes, the average flow is 6430 m3/s. After the river is joined by the OTTAWA RIVER at Montréal and by other tributaries, its average flow is 9800 m3/s, equivalent to an annual average depth of 300 mm over the entire basin. The St Lawrence Basin is the most southerly in Canada. This factor, coupled with the lakes' relatively large areas of open water, results in significant evaporation losses. Streamflow is reduced to about 25-30% of received precipitation. Nevertheless, the large lakes maintain a steady rate of flow throughout the year, and from one year to the next.
The region includes that part of Canada draining to the St Lawrence River (from Montréal eastward), the Gulf of ST LAWRENCE and the Atlantic Ocean. It includes all of Nfld, NS, NB, PEI and part of Québec, an area of 1 million km2.
The drainage from the north shore of the St Lawrence and Labrador is largely from the Canadian Shield, where 10-15% of the area is freshwater lakes. The balance of the area is part of the Appalachian Region, consisting of a mixture of crystalline and sedimentary rocks, with thin surficial materials. The rivers are short and drainage is well developed as a result of long periods of erosion. About 9% of Newfoundland and 5% of NS are lakes. There are few lakes in NB and PEI. The largest watersheds discharging into the St Lawrence River are the Manicouagan (45 000 km2) and the Saguenay (88 000 km2). The Churchill River in Labrador drains 79 800 km2 and discharges directly to the Atlantic Ocean. The SAINT JOHN RIVER, in NB, draining 55 400 km2, discharges into the Bay of FUNDY. Most of the large rivers of the basin have extensive hydroelectric installations.
The total discharge of the watersheds in the basin averages 21 000 m3/s, ie, an equivalent annual depth of 650 mm of water, or about 50% of the annual precipitation. The annual rate of runoff per-unit area is lowest (17 L/s/km2) for the St Maurice River in the most westerly part of the region; the highest (45 L/s/km2) is in southwestern Newfoundland.
The minimum daily rates of runoff from the larger watersheds range from 3 L/s/km2 for the Churchill River to 1 L/s/km2 for the Aux Outardes River. Many smaller streams dry up completely for a short period during summer or as a result of ice formation in winter. Local problems of water-quality impairment are encountered in regions of intensive agricultural, forestry and industrial activity.