The seasonal waxing and waning of rivers is one of nature's most crucial cycles, influencing everything from the success of salmon runs to having enough water during parched summers to irrigate crops.
But by this measure, many of Canada's major rivers are in trouble, contends a new report that says many of the best known rivers have suffered major alterations in their natural flows due to hydro dams, irrigation schemes and withdrawals by industry, and could be further compromised by the effects of global warming.
The report, by WWF-Canada, one of the country's major environmental organizations, says the rivers that have been most altered from their natural state include the St. Lawrence and the South Saskatchewan, whose "ecosystems are in serious trouble" as a result. But it warned that if safeguards aren't put in place soon, some of North America's last free-flowing rivers, including the Skeena in B.C., the Athabasca in Alberta, and the Mackenzie in the Northwest Territories "could soon be in trouble as well."
The report, titled Canada's Rivers at Risk, is believed to be the first evaluation of rivers in Canada based on how much human activity has changed their natural flows. Previous studies of rivers by government or environmentalists have focused on such traditional threats as pollution from industries or sewage treatment plants.
"The concept of environmental flows is really emerging globally as a fundamental indicator of how to look at river health," said Tony Maas, director of freshwater programs at WWF-Canada.
The report assessed 10 major rivers based on more than 300 scientific papers, and is being released Thursday.
Most rivers have a distinct annual flow pattern on which wildlife depend, with amounts peaking in spring when melting snow augments flows. Low levels occur during winter freeze-ups or summer droughts. But dams and other human interferences smooth out these natural fluctuations.
It highlighted the fact that responsibility for most large rivers in Canada is shared by several governments, making efforts to regulate their flows more difficult.
Among the most endangered rivers the study found were:
It's one of the few truly wild rivers left on the continent, and is in decent shape. The river flows through the rugged Coast Mountains, and its watershed boasts grizzlies, dense forests and Canada's second largest wild salmon fishery. While development hasn't yet harmed flows, there are worries that coal bed methane extraction, pipelines carrying crude from the Athabasca oil sands, mines and hydro projects could undermine its status as nearly untouched. A two-year blockade by members of the Tahltan First Nation led to a moratorium on coal bed methane projects in its headwaters. An independent scientific panel has raised concerns that the water management regime isn't able to protect salmon habitat from the cumulative impact of development.
It's the most threatened large river in Canada. In some areas, more water has been allocated for such purposes as irrigation than is available, leading it to run almost dry, according to the WWF. The report says flows have declined to "crisis levels," pointing to the dramatic decision by the Alberta government in 2006 to institute a moratorium on new withdrawals.
Maintaining a healthy ecosystem would require that about 85 per cent of its water stay in the river, but currently 70 per cent of its flow has been allocated for human uses.
While current conditions are worrisome, the future could be even worse if global warming leads to a further reduction in the glaciers that provide its source water.
The river is the country's longest, and is in good shape, although there are some worrisome signs. Summer flows on the Slave River, one of its tributaries, have fallen by 35 per cent since 1950 due to climate change and the Bennett Dam in British Columbia.
WWF says the Mackenzie is "one of the world's few large wild rivers" and underpins an ecosystem of continental and global importance.
Most of the immediate threats to the Mackenzie arise from development pressure in upstream tributaries, such as the Athabasca, where oil sands development has large water needs. Protecting the Mackenzie will be difficult because it will require the co-operation of Ottawa, British Columbia, Alberta, Saskatchewan, Yukon and the Northwest Territories.
The river is one of the most altered in Canada, with its natural flows changed by hydroelectric development in Ontario and Quebec. The dam building by the two provinces has "dramatically altered" natural flows and compromised distribution of fish and shoreline vegetation, according to the WWF.
The river provides drinking water for more than a million people, but it also contains some of Canada's rarest fish, including lake sturgeon, and its floodplains and wetlands are home to more than 300 bird species.
The WWF says no comprehensive plan exists for integrating the flow of water needed to support wildlife with the requirements of hydroelectric operations, suggesting the river "is likely to continue its decline." Dams are so extensive that only one set of large rapids on the lower section of the river remains, the Deschenes, between the city of Ottawa and Aylmer in Quebec.
The river, which drains the five Great Lakes, carries the largest amount of water of any in Canada. But it could be one of the big losers from climate change, based on models suggesting global warming will reduce levels at Montreal by up to 1.2 metres by mid-century.
The reason for the decline is falling water levels upstream on the Great Lakes due to reduced rainfall and warmer temperatures causing more evaporation.
Water flows have been highly influenced by human activity. The Moses-Saunders dam near Cornwall has reduced natural variability on the upper St. Lawrence by 70 per cent. Further disturbance has occurred because of the locks and canals needs for the St. Lawrence Seaway.