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Wetlands of the St. Lawrence River: Habitats between Land and Water


St. Lawrence wetlands occupy at least a total area of 33 600 hectares between Cornwall and Trois-Pistoles, excluding aquatic plant communities (Létourneau and Jean 1996). The variety of plant and animal habitats in this area is distributed as follows:

  • 8626 ha of low marshes
  • 18 622 ha of high marshes
  • 6428 ha of swamps

Photo: Wetlands of the St. Lawrence River

Wetland ecosystems perform essential functions. These ecosystems:

  • contain millions of microscopic organisms that act as a natural filter, making wetlands veritable water purification plants
  • retain nutrients and sediments, thus forming a food reserve and creating habitats for a diversity of plants and animals
  • mitigate the impact of waves and also slow and divert currents, thereby protecting banks from erosion
  • are resting, breeding and feeding grounds for more than 150 species of birds
  • are a favourable environment for a number of small fur-bearing mammals (beavers, muskrats, otters and minks)
  • are the preferred spawning grounds of almost all the fish species in Quebec
  • are conducive to the reproduction of many species of amphibians (frogs, toads, salamanders), reptiles (turtles and snakes) and molluscs (clams)
  • represent a refuge conducive to the feeding, breeding and larval development of a wide variety of insects
  • are used to practise a number of activities, including waterfowl hunting, sport fishing, commercial fishing, trapping of small fur-bearing animals like muskrat, pleasure boating, and nature watching.

Photo: Wetlands of the St. Lawrence River

Caroline Savage, Environement Canada

The Many Different Faces of the St. Lawrence Wetlands

The succession of wetland plants along a moisture gradient from the river to dry land is called the hydrosere. It changes according to river section, substrate type and water quality, and determines the composition of plant communities.

Main Wetlands in the Fluvial Section

Map: The Fluvial Section Wetlands

Main Wetlands in the Fluvial Estuary

Map: The Fluvial Estuary Wetlands

Main Wetlands in the Upper Estuary

Main wetlands in the upper estuary

Main Wetlands in the Lower Estuary and Gulf

Map: Main wetlands in the lower estuary and gulf

A Variety of Wetlands to Preserve

Wetlands are very vulnerable to human activities, with agriculture and urban development being primarily responsible for the loss of wetlands. It is estimated that:

  • 80% of the St. Lawrence wetlands present during the earliest days of colonization have disappeared in the Montreal region.
  • 50% of the shoreline between Cornwall and Quebec City has been modified due to urban development and agriculture.
  • 7% (3642 ha) of the wetland area between Cornwall and Matane was lost between 1945 and 1976.

Water-level Fluctuations

Water-level fluctuations foster greater wetland growth, diversity and productivity in a hydrographic system like the Great Lakes–St. Lawrence basin than no fluctuations.

Interannual variations in water levels ensure that no one community invades or dominates an environment to the detriment of others. Variations caused by high tides foster the development of vast intertidal zones in the upstream section of the Upper Estuary. Though water-level fluctuations appear to have an impact on wetlands, these areas play a major role in regulating water levels. They store up and gradually evacuate water, thus regulating flows, moderating changes in the water level and replenishing the water table.

Water levels have an effect on the diversity and areal extent of wetlands. This diversity can be preserved in the long-term by maintaining a reference water level around which interannual fluctuations oscillate. The establishment of a hydrological regime that fosters optimal diversity in St. Lawrence wetlands is critical.

See Also:


Dryade (Le Groupe). 1980. Habitats propices aux oiseaux migrateurs le long des rives: de la rivière Richelieu; de la rivière Outaouais; du fleuve Saint-Laurent; de l'estuaire du Saint-Laurent; de la côte nord du golfe du Saint-Laurent; de la péninsule gaspésienne; des Îles-de-la-Madeleine. Report presented to the Canadian Wildlife Service, Environment Canada, Quebec Region.

Létourneau, G. and M. Jean. 1996. Cartographie des marais, marécages, et herbiers aquatiques le long du Saint-Laurent par télédétection aéroportée. Environmental Conservation Branch, Environment Canada. 101 pages.

St. Lawrence Centre. 1996. Synthesis Report on the State of the St. Lawrence River. Volume 1: The St. Lawrence Ecosystem. "St. Lawrence UPDATE" series. Environment Canada – Quebec Region, Environmental Conservation, and Les Éditions MultiMondes. Montreal.

St. Lawrence Centre. 1991. Wetlands: Habitats on the Edge of Land and Water. Environmental Atlas of the St. Lawrence. Environment Canada. Montreal.

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