Floods: general information

Introduction

Over the past century, natural disasters have taken millions of lives and caused billions of dollars in damage. Flooding ranks as one of the most damaging forms of natural disaster in the world, together with droughts, earthquakes, and cyclones. Coastal storms, ice jams, snowmelt runoff, rainstorms and tsunamis are all causes of floods that Canada has in common with other countries. Together, we face the paradox of flooding: that flooding is at the same time essential to a healthy environment and a hazard to developed areas and to human activities in the floodplain.

Viewed in terms of the global ecosystem, flooding is part of the hydrologic cycle in which water is forever recycled between the earth and the atmosphere. Spilling over the floodplain, water nourishes the wetlands of Canada's major deltas. For instance, the recurrent inundation of the Peace-Athabasca Delta in northern Alberta has fostered an environment in which plant and animal life has achieved a balance dependent on flooding.

The Floodplain

A floodplain is the relatively flat land beside a lake or river, naturally liable to flooding if any overflow occurs. It is really part of the water's living space, which it uses periodically to stretch and spread out. Floods are most destructive in that part of the floodplain known as the floodway, where the water flows fastest.


The floodplain

The floodplain, a level expanse carved out by the river, is a natural extension of the river channel. In spite of potential peril, people are drawn to the area because of the many benefits the floodplain provides. Unfortunately, this use of the floodplain generally does not make accommodation for the river's eventual return. As the development of floodplains continues, conflicts with many ecological processes increase. The marshes and swamps of the floodplains provide required habitat for numerous species of fish and wildlife. As well, these areas act as filters for the river system and allow the recharging of groundwater supplies. When floodplains are developed, marshes and swamps are drained and low-lying areas filled in. 

Floodplains are subjected to long-term cycles of high or low water, as well as to rapid seasonal fluctuations of water levels. The ecology of floodplains, as well as the rivers and lakes, has adapted to annual and longer term cycles. Wetlands and shallow surface water, in particular, rely on the fluctuation of water levels to maintain their ecological balance and productivity. If there are long-term changes to the water levels, then the existing vegetation and habitat will follow a long-term succession. The direction of change can be from an aquatic habitat toward a terrestrial one, or vice versa.

Floods are Natural

Flooding is a common phenomenon in Canada, resulting from an increase in streamflow beyond the point where the normal stream channel can contain the water. When water overspills riverbanks, it spreads out along the adjoining floodplain. Flood-waters may occupy the floodplain for a matter of hours, as in the case of flash floods, or for up to two months, as sometimes occurs during the spring snowmelt runoff period.

Given the proper conditions, every river has reaches that have the potential to flood. Across Canada, flooding is a common event because these conditions are met regularly. Although flooding is generally perceived to be associated with the spring runoff, floods have been known to occur at any time of the year. Other causes of flooding include storms and ice jams. While many floods are due to one cause, the most severe floods tend to result from the compounding influences of two or more causes, such as a heavy rainfall together with the spring snowmelt runoff.

Photo - Canada Geese crossing road flooded by the Thames River, December 30, 1990 (Reproduced with permission from The London Free Press / Morris Lamont. Further reproduction without written permission from The London Free Press is prohibited.)
Canada Geese crossing road flooded by the Thames River, December 30, 1990 (Reproduced with permission from The London Free Press / Morris Lamont. Further reproduction without written permission from The London Free Press is prohibited.)

Floods are not mysterious freak happenings, but entirely natural events. They rarely strike without some advance warning. Although people generally think of floods in terms of damage to property and loss of life, floods can benefit the natural environment and sustain many ecosystems.

Economic Development Pressures

Since earliest time people have lived along the edge of rivers and lakes. Apart from the obvious source of drinking water, the river supported a food source, provided a means of transportation, and eventually supplied power to mills. People have continued to occupy the floodplain but for different reasons. The floodplain and riverbank provide aesthetically pleasing sites for housing, the level land is cheaper to build on, and services are easier to install.

As the development of the floodplains increased, concurrently the damage caused by flooding also increased. Therefore protective measures such as dykes, dams and diversions have been widely employed.

Photo - Hatzic Dyke Break, British Columbia
Hatzic Dyke Break, British Columbia

Economic development in concert with an increasing population has brought pressure to alter the flow regime of surface water systems and the landscape of the floodplains. Human efforts to constrict the active zone of floodplains include the construction of dams, dykes and diversion channels, channel dredging and realignment, and drainage of wetlands. These measures, although perhaps beneficial in the interests of economic development, have resulted in the decline of fish and wildlife habitats, and the disruption of entire ecosystems. Wetlands have been eliminated, shoreline erosion has increased, and the sediment filtration capabilities of the floodplains lost.

Although structures may successfully protect areas, they also can engender a false sense of security. Often development continues in the floodplain after the structural approach has apparently solved the flood problem. However, structures are not infallible. They require maintenance, particularly dykes, which can be eroded over time or become saturated and fail during extended periods of high flow. No matter to what level of flooding structures are designed, there may come a time when the flood levels will exceed the design flood. Whatever the reason, once the structural system fails, the damage from the flooding will be greater than if the structures had never been put in place.

Acknowledgements

A substantial amount of the information in the Floods section of the Freshwater Web site is taken from the following publication:

Canada Water Book on Flooding. 1993. Jeanne Andrews (ed.). Environment Canada. Ottawa, Ontario.

We gratefully acknowledge the many individuals and organizations -- including federal and provincial departments and agencies, and private citizens who witnessed flood events -- who contributed ideas and information for the Canada Water Book on Flooding.

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