Two-thirds of the Banks from Montréal to the Berthier–Sorel Archipelago are Eroding

The erosion of the shoreline between Montreal and Lake Saint-Pierre is a major concern for stakeholders studying the St. Lawrence River ecosystem. In this section of the river, banks are retreating by one to three metres in many places.

Environment Canada specialists conducted an analysis of the erosive phenomenon using data from 1964 on the island habitats of freshwater reaches between Montréal and the Berthier–Sorel archipelago, including the Îles de la Paix. It is estimated that approximately 250 km of the 400 km of shoreline in this sector are eroding (Dauphin and Lehoux, 2004).

Erosion is threatening biological components of interest over a total distance of approximately 15.5 km. Wetlands, spawning areas, rare birds or plant habitats, breeding grounds for aquatic birds and habitats are among the sites where erosion-reducing protection strategies are planned.

Recent studies suggest that 50–65% of the suspended particulate matter exported to Quebec City (6.0 to 7.5 million tonnes) comes from the erosion of banks and shores, mainly in the Montréal–Lake Saint-Pierre section. Sea clay erosion is closely correlated with water level seasonality and other meteorological variables of the St. Lawrence.

During the summer of 1999, a team of research scientists from Environment Canada undertook a study of the processes of erosion in the Verchères region. The results of this work showed that the erosion of marine clays is closely linked to the seasonal cycle of water levels and other meteorological conditions affecting the St. Lawrence River.

Study area
Map: Study location
May 15, 2000November 8, 2000
Photo: High and low water levels observed at Île au Boeuf (Verchères Islands)

High and low water levels observed at Île aux Boeufs, Verchères

At the end of the winter, the freeze-thaw cycle sets off the fracturing and weakening of riverbanks. Then, in summer, low water levels and high temperatures contribute to the drying out of large portions of the banks, increasing their vulnerability to erosion. Episodes of rain, wind and waves, as well as shipping traffic cause the erosion and resuspension of large quantities of fine particles, which are discharged toward the estuary by fluvial currents.

Photo: Shoreline erosion
Water level 1997–2007
Illustration: Water level 1997-2007
Photo: Erosion at l'île aux Boeufs, 5 mars 2001
Îles aux Boeufs, Verchères

Specialists from Environment Canada observe greater erosion in the area when the water level is high enough for the waves to reach the banks (the upper part of the shoreline). This is because water levels affect erosion factors such as:

  • Current strength and clay bank exposure
  • Wave strength, shape and propagation
  • The frequency of passing ships
  • Shoreline profile

Erosion can adversely affect many environmental components of an ecosystem, thus threatening its integrity, biodiversity and sustainability.

The consequences of erosion are considerable, including the silting up of watercourses, spawning beds and drainage ditches, and the increase in contaminants transported by suspended particulate matter. In addition to the decreased productivity of animal and plant communities, erosion can reduce the recreational and aesthetic value of certain sites and even, in some cases, result in economic losses.

Prevention strategies are recommended to reduce the phenomenon of erosion. Reducing the speed of commercial ships and pleasure boats, stabilizing banks through bioengineering works, managing species at risk and regulating water levels are all measures that could slow the erosion of the banks of the St. Lawrence River.

Literature

Dauphin, D and D. Lehoux. 2004. Bilan de la sévérité de l’érosion dans le Saint-Laurent dulcicole (Montréal – Archipel de Berthier-Sorel, incluant les îles de la Paix) et stratégies de protection recommandées pour les rives à plus grande valeur biologique. Environment Canada – Quebec Region, Environmental Conservation, Canadian Wildlife Service. (French only)

Gaskin, S. J., J. Pieterse, A. Al Shafie, and S. Lepage. 2003. Erosion of undisturbed clay samples from the banks of the St. Lawrence River. Canadian Journal of Civil Engineering 30: 585–595.

Lepage, S. and L.-F. Richard. 2001. Sediment resuspension and erosion in the Varennes–Contrecoeur stretch of the St. Lawrence River. In Proceedings of the Canadian Coastal Conference. May 16–19, 2001, Université Laval, Quebec City, Quebec. Edited by Bernard F. Long. Canadian Coastal Science and Engineering Association, Ottawa, Ontario, pp. 525–532.

Related Links 

St. Lawrence Vision 2000 – The St. Lawrence Shoreline between Cornwall and Montmagny

Le Devoir – Le Saint-Laurent s'asphyxie (in French only)