Dams & Diversions

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Overall, Canada is blessed with freshwater resources. But their availability varies considerably from season to season and year to year, and from one region to another. Faced with floods, droughts, and other problems of water supply, engineers have stretched available resources by means of structures: dams, which hold back flows for release when they are more useful or less destructive, and diversions, which redirect the resource to where it is more useful. In fact, dams and diversions often go together: water is stored in a reservoir formed behind a dam, and then withdrawn by ditch, canal, or pipeline from its natural course for transfer elsewhere.


Generally, rivers are dammed to create reservoirs for power production, downstream flood control, recreation, or irrigation. Canada ranks as one of the world's top ten dam builders. Although the Canadian Dam Association's register of dams (2003) reports 933 large dams*, there are many thousands of small dams.

Number of large dams in Canada

Source: Canadian Dam Association's register of dams (2003)

In Canada, large dams are used primarily for hydroelectric power generation (596 dams), but are also used for the following purposes:

  • multi-purposes (86 dams)
  • tailings (82 dams)
  • water supply (57 dams)
  • irrigation (51 dams)
  • flood control (19 dams)
  • recreation (7 dams)
  • other purposes (35 dams)

When a dam is constructed it can have an effect on the water quality of a river system. The land behind it is flooded which may mean the loss of valuable wildlife habitat, farmland, forests, or town sites. Accumulation of sediments in the reservoir can have a detrimental effect on water quality by creating increased concentrations of harmful metal and organic compounds in the reservoir. If vegetation is not removed behind the dam before flooding, other problems can occur. For example, the eutrophication process may occur at a faster rate and adversely affect the water quality.

* A large dam is defined as being higher than 15 metres or, under certain conditions, higher than 10 meters.


In the past, diverting flow from one basin to another has been primarily based on economic development through energy generation, irrigation, and industrial output. In Canada, major diversion projects have been developed by power utilities to increase flows for hydroelectric production, especially in northern projects such as the Churchill-Nelson region in Manitoba and in the James Bay region in Quebec. Projects have also been constructed for irrigation purposes and industrial development such as aluminum production.

Interbasin diversions can have undesirable social and environmental consequences. For example, the amount of water being removed in relation to the amount of water available, existing water demand and uses, quality of water being transferred, including the potential for the introduction of undesirable non-native species and pathogens, can all have significant impacts. The implication of introducing non-native species is particularly significant when major drainage basins are involved.

Social structures may also be affected. Sometimes communities are flooded out or people are forced to change their livelihood or otherwise modify their traditional way of life.

While, in the past, major diversions and transfers have been used to fulfill water resource and economic development objectives, it is widely recognized that we have moved away from the era of large scale diversions and transfers in Canada and the United States. Environmental and social considerations are making these transfers of water a less desirable option. Present approaches now favour reducing the demand on water uses.

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