National Marine Conservation Areas of Canada
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Canada’s National Marine Conservation Areas System Plan
Canada's Great Lakes Aquatic Environment
Map showing Park Canada's 5 Great Lakes marine regions used in the context of its national marine conservation areas program
© Parks Canada, 1996
Though in a strict sense the Great Lakes are not marine, they have been described as "freshwater seas" because of their size — the term 'lake' seems patently absurd when applied to these vast expanses, as anyone who has looked upon them can attest. At 245,000 km² they are also the world's largest freshwater lake system, with some 18% of the planet's supply and display many of the same attributes as true marine environments.
The length of the Canadian Great Lakes shoreline is about 9500 km, including islands. The lakes form a step-like sequence, with Lake Superior being at the highest elevation (183 m above sea level) and Lake Ontario at the lowest (74 m asl). The most dramatic drop (100 m) occurs at Niagara Falls. The whole system drains into the Atlantic Ocean via the St. Lawrence River.
Following the repeated advances and retreats of the glaciers over a million-year period, the present-day shorelines of the Great Lakes have emerged. These are primarily rocky shores or low cliffs and tend to be straight except where more resistant rock have been carved into bays, headlands and islands. Outcrops of the Niagara Escaprment and the Canadian Shield serve as natural dams and effectively separate the lakes from one another.
The Great Lakes do not have true tides, but lake levels do vary as a result of seasonal factors, the weather and the influence of man. Seasonal variations on the order of 0.5 m occur, though fluctuations of up to 2 m have been recorded, notably seiche effects resulting from storms. Thermal stratification is a common feature of the lakes. Ice forms over most of this environment, particularly along the shores, and lasts four to five months.
© Parks Canada