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Have Wetlands Increased or Decreased Along the St. Lawrence?

Environment Canada’s research findings indicate that the surface area of wetlands (marshes and swamps) along the St. Lawrence generally increased by nearly 3% between 1990-1991 and 2000-2002. Wetlands have increased mainly in the estuary upstream from Trois-Rivières. However, wetlands have suffered net losses in the fluvial section, principally near Montreal and Lake Saint-Pierre.

Did you know? The surface area of wetlands in the St. Lawrence Valley increased from 28,228 ha in 1990 to 28,992 ha in the early 2000s.  The wetlands of the St. Lawrence are valuable because they perform essential ecological functions. They are genuine natural wastewater treatment plants. Characterized by remarkable biodiversity, they supply wildlife with food and provide buffers against shoreline erosion.

The above figure represents changes observed in the wetlands of the St. Lawrence, near Montreal, between the 1970s and the 1990s. More than 40 hectares of wetlands, the equivalent of about 30 soccer fields, underwent changes during this period.

Montreal: Significant Artificialization of Wetlands

Shoreline urbanization in Montreal, in Boucherville, on Charron Island and along the channels between the Boucherville Islands caused the loss of more than 25 ha of wetlands between 1970 and 1990. In addition, wildland or forest replaced nearly 15 ha of wetlands. From the 1990s to the early 2000s, more than 10 ha of marshes and swamps vanished, especially around the Great Tailhandier Sand Bar, mainly giving way to open water. 

Did you know? 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.

The loss of wetlands in the St. Lawrence Valley is not a recent phenomenon. When colonization began, for example, there were five times as many of these ecosystems in the greater Montreal area. They began to vanish more quickly near urban centres from the 1940s to the 1970s. According to Martin Jean, Wetland Project Leader, “The dynamic of the wetlands in the St. Lawrence Valley is complex. They disappear in some places and normally appear elsewhere. Their surface area has been relatively stable for about thirty years. However, wetlands are undergoing significant internal changes.”

Scientists observe that 37 ha of low marshes have been transformed into bogs in the St. Lawrence Valley near Montreal. These natural transformations have occurred especially quickly. En 2000-2002, the reed canary grass  (Phalaris arundinacea) and the common water reed (Phragmites australis), two invasive plant species, colonized nearly all of these new bogs.

Lake Saint-Pierre: Farmland Encroaching on Wetlands

Lake Saint-Pierre is home to the largest concentration of marshes and swamps throughout the St. Lawrence Valley, that is, 16,098 ha in 2000‑2002, not including water plant communities. This region, which is home to a migratory bird sanctuary (Nicolet) and an ecological reserve (Marcel-Léger), became a Ramsar site in 1998 under the Convention on Wetlands of International Importance.

Losses of marshes and swamps totalled 82 ha between 1990-1991 and 2000-2002. They mainly gave way to agriculture along the South Shore and open water elsewhere. As in the Montreal region, the rapid transition from low marshes to bogs is among the major internal changes that have affected Lake Saint-Pierre. The colonization of these wetlands by invasive species is mainly seen around Lavallière Bay and Saint-François Bay.

The Fluvial Estuary: Wetlands Have Gained Ground

The situation is different in the fluvial estuary, since wetlands have gained ground, increasing by nearly 450 ha, from 5589 ha in 1991 to 6036 ha in 2000-2002. These gains have occurred mainly along the river’s south shore, at the expense of open water. This section of the St. Lawrence is characterized by the existence of tides whose levels do not, however, seem to explain these changes in area. One hypothesis to explain gains in wetlands would be the rapid development of vegetation in open areas following a deposit of sediment.

Finally, scientists have observed the transformation of low marshes into treed swamps near the port of Gentilly during this same period. No invasive plant species seem to have colonized the fluvial estuary in the early 2000s.

What are the Consequences of these Changes?

Changes in the area or structure of wetlands are likely to affect the integrity of their ecological functions. Thus, potential impacts on hydrodynamics, water quality, erosion, sedimentation and biodiversity can be expected in aquatic ecosystems as a result of such changes in wetlands.

Are Water Levels Also to Blame?

Sources of direct pressure on wetlands are well known: urbanization, agriculture, port activities, infilling, dredging, erosion, drying and so on. Scientists generally observe a trend toward the drying of wetlands, turning low marshes into bogs. Drying can happen naturally. However, the decrease in the water levels of the St. Lawrence, recorded during the 1990s, may have caused drying to occur more quickly, especially in Lake Saint-Pierre.

Water-level fluctuations foster greater wetland growth, diversity and productivity in a hydrographic system like the Great Lakes–St. Lawrence basin than no fluctuations. ― Martin Jean, Project Leader, Environment Canada

Interannual variations in water levels ensure that no one community invades or dominates an environment to the detriment of others. Water levels have an effect on the diversity and areal extent of wetlands. According to scientists, this diversity can be preserved in the long term by maintaining a reference water level around which interannual fluctuations oscillate.

What is the Government of Canada Doing?

The Federal Policy on Wetland Conservation, approved in 1991, is one of the tools that Canada has developed to meet its commitment to guidelines for rational use of wetlands on federal Crown lands. Among other things, this policy recommends that wetlands on federal Crown lands should be monitored to detect signs of deterioration. Various legal provisions are in effect in Canada to protect wetlands, such as:

• the CanadaWildlife Act;
• the Canadian Environmental Protection Act;
• the Fisheries Act and the Policy for the Management of Fish Habitat;
• the Historic Canals Regulations;
• the Migratory Bird Conservation Act;
• the Canada National Parks Act;
• the Navigable Waters Protection Act.

It must be noted, however, that wetlands are still under significant pressure. Vigilance is in order, and only a long-term commitment to continue monitoring the status of wetlands can serve as a guide to action.

To Find Out More…

Jean, Martin and Guy Létourneau. 2008. Changements dans les milieux humides du fleuve Saint-Laurent de 1970-2002 [changes in the wetlands of the St. Lawrence River, 1970-2002]. Environment Canada, Science and Technology, Water Quality Monitoring – Quebec Region. Science and Technical Report.

Savage, C. and M. Jean. 2008. Invasive Plant Species of the St. Lawrence Wetlands. Environment Canada – Quebec Region and Department of Sustainable Development, Environment and Parks of Quebec. Fact sheet, “State of the St. Lawrence River” collection.