ABSTRACT: Nouvelles donnees sur l'erosion du chorre superior a Montmagny, moyen estuarie du Saint-Laurent [ New data on the high marsh erosion at Montmagny, middle St. Lawrence estuary[.

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Abstract

At Montmagny, the high interstadial marsh is being eroded at least since the beginning of the 1980 decade. Between 1984 and 1986, the micro-cliff was cut back at an average rate of 129 cm per year over a distance of about 4 km. Measurements made in the same area between 1996 and 1998 indicate an increase of both the erosion rate and the area eroded. The micro-cliff is now retreating at an average rate of 168 cm per year. Because more than half of the erosion is made during the ice-free season, waves and tide currents are considered the main agents involved in the process. Erosion, however, is increased by geese activity in the intertidal marsh during spring and fall. Removing by shore ice large "peat rafts" from the seaward edge of the high marsh contributes approximately 25 to 35% of the annual erosion. In addition, thawing of the stratified silty sand substrate exposed in the micro-cliff reduces the strength and compaction of the sediment and facilitates subsequently wave erosion. During spring tides, the micro-cliff is exposed to wave erosion for about 6 hours daily.

Bibliography of Canadian Geomorphology