Why Floods Occur
The main cause of riverine flooding is excessive runoff following heavy RAINS. The rate of runoff (and hence the peak discharge) can be increased by urbanization (the conversion of open land to impervious surfaces by roads, buildings and the like) and by removal or change of vegetative cover (water runs off cropland faster than forested land). Runoff is also augmented by snowmelt in early spring. For much of Canada, spring is the peak flood season.
Floods can also be caused by ice jams, when upstream water is blocked by accumulations of ice, often at a constriction point in a river such as a bridge or a narrow channel. Heavy convectional storms also cause floods in the summer months, especially in small watersheds.
In snowmelt and spring runoff floods, the river generally rises slowly (a long flood-to-peak interval), allowing time for the construction of temporary defenses or evacuation. In convectional storm floods the flood-to-peak interval can be very short. Similarly when an ice jam breaks up, the resulting rush of water can be very rapid.
High-water problems also occur on lakes, but usually the rate of rise and fall of the lake level is much slower than on rivers. Damage to shoreline installations and cottages is not uncommon on the Great Lakes and occurs occasionally on most other lakes such as Lake Winnipeg.
Coastal flooding may be caused when seasonally high tides are augmented by storm activity, or when a TSUNAMI, generated by EARTHQUAKES, is driven ashore. This phenomenon occurs occasionally on the West Coast. Coastal flooding related to HURRICANES sometimes occurs in Atlantic Canada.
One of Canada's most difficult flood problems is on the RED RIVER in Manitoba. Snowmelt waters from the US flow north through a wide, flat plain (the bed of former glacial LAKE AGASSIZ), and severe flooding can create havoc in many small communities as well as in the city of Winnipeg. Large floods are known to have occurred in 1776 and 1826. There were also serious floods in 1950, 1966 and 1979 and 1997.
The 1950 flood led to the construction of a large flood-diversion channel, the Red River Floodway. The high cost led some to disparage the project as "Duffs Ditch" (after the then premier of Manitoba, Duff ROBLIN), but its value was amply demonstrated in 1997. The catastrophic flood in the spring of that year was caused by snowfall in the neighbouring United States that was 300% above normal and 200% above normal in southern Manitoba. This unusually high accumulation of snow fell on top of ground that was still saturated from the previous autumn.
Some 1950 km2 were flooded, including 2500 homes, and about 30 000 people were evacuated. A massive flood-fighting operation, assisted by the Canadian Armed Forces and many volunteers, was mounted. These efforts, and the Red River Floodway, saved Winnipeg from being flooded. Had this effort not succeeded, floodwaters in Winnipeg would have been 1 m above the 1950 level.
The success of the flood-fighting efforts was not without controversy. The distribution of flood damage compensation payments caused public concern, and the saving of Winnipeg may well have been at the cost of higher flood levels elsewhere. The magnitude of the flood was such that not all communities could be protected with the existing dikes.
The Saguenay region of Québec experienced severe flooding in July 1996. An intense summer storm, combined with inadequate storage capacity of some dams, led to floods throughout the region. The damage was high because houses had been constructed and other developments allowed to proceed on lands that were assumed to be protected against such events. Premier Lucien BOUCHARD described the floods as "an act of God," but there were many who disputed this assertion.
There is also a significant flood risk in the lower FRASER RIVER valley in British Columbia. In 1948, 16 000 people were evacuated from their homes, and damage was extensive. Since then there has been considerable expansion of development and population in the Richmond area. Although the area is protected by dikes, the risk of them being overtopped cannot be ruled out.
In 1954 heavy rains associated with the passage of HURRICANE HAZEL caused flooding on the Don and Humber rivers in Toronto and resulted in 81 deaths and severe damage. Since then much of the damageable property has been removed from the flood plains and restrictions on development have largely prevented the growth of damage potential.
In July 1986, 900 Edmonton residents fled their homes as the rain-swollen waters of the North Saskatchewan River rose 7.6 m above their normal level, causing the worst flooding since 1915.
Many rivers in Canada (MACKENZIE, COPPERMINE, CHURCHILL and Moosonee) run northward to Hudson Bay and the Arctic Ocean. Meltwaters from the southern headwaters of these river basins in combination with still frozen lower reaches often cause severe spring flooding. Remote First Nations communities such as AKLAVIK, NWT, in the Mackenzie delta, and Fort Albany, Ontario, are used to coping with expected spring floods.
Nevertheless, catastrophic floods do happen. In 1986 the community of Winisk, Ontario, was destroyed and 2 people killed when an ice dam broke causing water and car-size blocks of ice to rush through the townsite. The community was subsequently rebuilt at a new site further upstream on higher land and renamed Peawanuck.
Damaging floods have also occurred at Red Deer, Alberta (Red Deer River); Swift Current and Moose Jaw, Saskatchewan (Swiftcurrent Creek and Moose Jaw River); Fredericton (Saint John River); along the Châteauguay, Richelieu, Yamaska, St-François and Chaudière rivers in Qué; and at many other localities in Canada.
A traditional approach to flooding has been to attempt to control high flows by the construction of dams, dikes and diversion channels. While there are no accurate and comprehensive statistics on national flood damage, some evidence suggests that such engineering works have not prevented a rise in nationwide damage. In many regions, urban and suburban developments for housing, industry and commerce have been located on floodplains, behind dikes or downstream from flood control dams. When extreme, low-probability floods occur, they can overtop the protective works and cause more damage than in the past because more property lies in the path of floodwaters.
Because of this risk, the federal government invited the provinces to join in a co-ordinated program to address flood losses in a more comprehensive way. In 1975 a flood damage reduction program was announced. Its main element was the preparation of flood-risk maps to identify hazardous areas. The maps, along with floodplain land-use regulations by the provinces and municipalities, were designed to discourage further development. Some 300 locations throughout Canada were mapped and the maps made available to floodplain managers, urban planners, property developers and the public.
Provincial and municipal policies, regulations, bylaws and public information activities are based on such maps. Construction of new homes and businesses in these flood zones is discouraged (but not entirely halted) in favour of reserving the land for parks and other open spaces, recreation facilities and wildlife habitat. The program has successfully alerted provincial and municipal governments to the need for a more comprehensive approach to reducing flood damage, including flood forecasting and warning systems, emergency measures, structural measures for flood control and land-use planning.
In 1996-97, the federally sponsored program was phased out during a period of fiscal restraint, but the concept of adjusting the location, pattern and type of human settlement to the flood hazard has replaced the concept of simply trying to control floods.
What the Future Holds
Flooding is a natural phenomenon and will continue to occur. Until the mid-1990s it seemed that improvements in the management of the flood hazard were beginning to contain flood damages, and that losses would be considerably reduced, although not eliminated. This confident view has been called into question by the recent major floods in the Saguenay region and on the Red River. There is a possibility that these events (and other major floods in the United States) are linked to CLIMATE CHANGE, and that Canada and other countries may experience even larger and more frequent floods in the future.
Author IAN BURTON
Links to Other Sites
The Nature of Water
This website offers an overview of Canada's water resource management programs. Topics covered include: water quantity, use and quality; governance of water in Canada; and building and sharing knowledge for better water management. From Environment Canada.
Know the Risks
This site offers useful tips and guidelines for preparing and responding to earthquakes and other extreme natural events. Click on the "hazards poster" on the right to download a map of Canadian locations prone to various natural events. Includes brief notes about each type of event. From Public Safety Canada.
Flooding Events in Canada
See historical accounts of major flood events across Canada. Also includes maps and related data tables. From Environment Canada.
Top Ten Manitoba Floods Since 1800
Facts about Manitoba's major flood events. From the Government of Manitoba.
The Red River Flood of 1950
Detailed accounts of Red River flooding in the Winnipeg region. From the Manitoba Historical Society.
Red River Floodway
The website for the Red River Floodway, an extensive infrastructure system designed to redirect flood waters around the City of Winnipeg. See a detailed history of the project, dramatic annimations showing the coures of floodwater with and without the floodway, and much more.