Floods are primarily caused by naturally occurring variability in the height of rivers, lakes and oceans. In Canada, Manitoba’s Red River region is one of the country’s most flood-prone areas; historic floods have also occurred in the Prairie provinces and Québec.

Why Floods Occur

The main cause of river flooding is excessive runoff following heavy rains. The rate of runoff can be increased by urbanization (i.e., the conversion of open land to water resistant surfaces such as roads and buildings) 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 storms also cause floods in the summer months, especially in small watersheds.

In snowmelt and spring runoff floods, the river generally rises slowly, allowing time for the construction of temporary defenses or evacuation. By comparison, in storm floods water can rise quite quickly. 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 as well as others (e.g., Lake Winnipeg).

Coastal flooding may be caused when seasonally high tides are augmented by storm activity, or when a tsunami, generated by an earthquake, is driven ashore. This phenomenon occurs occasionally on the West Coast. Coastal flooding related to hurricanes sometimes occurs in Atlantic Canada.

Flood-Prone Areas

One of Canada's most flood-prone areas is on the Red River in Manitoba. Snowmelt waters from the United States 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, 1979 and 1997.

Historic Floods

Disastrous floods occur in Canada each year. Using the number of people affected and the damage incurred as measures, below are the details of some of the worst floods in Canadian history.

Manitoba

The Red River in southern Manitoba floods regularly. Major floods of the 19th century occurred in the years 1826, 1852 and 1861, while the worst of the 20th century took place in 1950, 1979 and 1997. Focusing on those of the 20th century, in May 1950 a flood inundated many valley towns and one-sixth of Winnipeg; more than 100,000 people had to be evacuated. The construction of floodways to carry off spring overflow alleviated the problem around the city; rural areas, however, remained vulnerable.

About 30 years later these floodways were put to the test when, in April and May of 1979, the Red River flooded to within centimetres of the levels seen in 1950. This time, however, Winnipeg went largely unaffected, while over 7,000 people were evacuated from valley communities.

The flood that occurred from mid-April to early May 1997 was named “The Flood of the Century,” though in terms of the amount of water the Red River discharged, it was in fact the largest flood since 1826, almost two centuries earlier. The flooding of over 1,800 km2 of land resulted in the evacuation of 30,000 Manitobans. Some 8,600 troops were mobilized to assist civil authorities.

Prairies

Heavy rainfall in the spring of 2005 caused several rivers to flood in southern Alberta, including the High, Bow, Elbow, Oldman and Red Deer rivers. Over 7,000 people were evacuated from these regions, and with an estimated $400 million in damages, it was one of the most expensive disasters in Alberta’s history.

In 2010, the same region of Alberta, along with the southern portions of Saskatchewan and Manitoba, experienced one of the wettest springs and summers on record. Saskatoon, for example, received 645 mm of rain between April and September, breaking a record of 420 mm dating back to 1923. Between Saskatchewan and Alberta, just over 2,000 people were evacuated over the course of the season, according to Public Safety Canada. The excess precipitation made the growing season incredibly challenging for Prairie farmers, with Statistics Canada reporting 15 per cent less wheat harvested as compared to 2009.

In the spring of 2013, flooding affected the southern quarter of Alberta, including Calgary. As many as 100,000 people evacuated the area when the Bow River watershed experienced above average spring runoff and rainfall. In addition, four people died of drowning. In Calgary, 3,000 buildings were flooded and infrastructure destroyed. With damages estimated at $6 billion, it was Canada’s costliest disaster.

Québec

The Saguenay region of Québec experienced severe flooding on 20–21 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 and other developments had been constructed on lands that were assumed to be protected against such events. Some 16,000 people were evacuated temporarily; 10 people lost their lives.

In the spring of 1974, over 300 municipalities were affected by province-wide flooding along the Gatineau, Ottawa, Richelieu, St. Lawrence, Châteauguay, Saint-Maurice and Chaudière rivers. Over 7,000 people had to be evacuated, 3,000 of whom were from Maniwaki.

British Columbia

There is also a significant flood risk in the lower Fraser River valley in British Columbia. In the spring of 1948, 16,000 people were evacuated from their homes, and damage was extensive. Since then there has been an increase in population and development in the Richmond area. Although the area is protected by dikes, the risk of them being overtopped cannot be ruled out.

Ontario

In 1954, heavy rains associated with 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 have been placed on its development.

Other Areas

Damaging floods have also occurred at Red Deer, Alberta (Red Deer River); Swift Current and Moose Jaw, Saskatchewan (Swift Current Creek and Moose Jaw River); Fredericton (Saint John River); along the Châteauguay, Richelieu, Yamaska, St-François and Chaudière rivers in Québec; as well as many other places in Canada.

Flood Control

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 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 coordinated 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.

Flooding is a natural phenomenon and will continue to occur. However, 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.

See also Climate Change.