The Magdalen Islands, Gulf of Saint Lawrence

Dubois, Jean-Marie M.; Grenier, Anna; ,

The Magdalen Islands, Gulf of Saint Lawrence

by Jean-Marie M. Dubois, Univ de Sherbrooke, Sherbrooke, Canada,
Anna Grenier, Univ de Sherbrooke, Sherbrooke, Canada,

Document Type: Proceeding Paper

Part of: Coastlines of Canada

Abstract:

The Magdalen Islands constitute the largest barrier-island system in the Gulf of Saint Lawrence. They represent 435 km of coasts at the 1:50 000 scale, two-thirds of which are sandy and one-third rocky. The main islands form bedrock knolls united by double tombolos containing shallow lagoons. At the extremities of the archipelago, two huge spits curve towards the East. The archipelago lies on a sandy covered marine platform forming an important shoal in the Gulf. The rocky coasts are mainly formed by soft and weathered Paleozoic sedimentary rocks which are largely erodible. They represent active cliffs reaching 80 m in height. The sandy coasts, formed mainly from the storm induced sand-bars, are heavily reworked by the wind and sometimes the sand dunes can reach over 10 m in height. These bars are in return eroded during storms, mainly on the West side, and the active cliffs can reach a height of 7 m. Following the influence of the dominant winds, the tombolos of the West coast migrate towards the interior of the lagoons while the tombolos and spits of the East prograde with the accumulation of sediments provided by the longshore currents. Storm action is important and has permitted the opening and closing of dozens of tidal inlets since 1755. Erosion which is predominant can be related to both storm action or to glacio-isostatic subsidence occurring in the islands and potentially estimated at close to 1 mm per year for the last 6 000 years. A detailed analysis of the coastal evolution based on aerial photographs was carried out between 1963 and 1983 and the balance is a loss of land reaching 254 410m².

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