ABSTRACT: Evolution recente du delta de la Yamachiche (Quebec): processes naturels et impacts anthropiques.

CGRG Bibliography of Canadian Geomorphology
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Abstract

Lake St. Pierre, being an enlargement of the St. Lawrence River, could experience a significant base level drop in the next fifty years as a consequence of global climate warming. Seven tributaries flow into this shallow lake, and base level changes could lead to increased sedimentation rates. We have studied the delta of the Yamachiche River, a tributary of Lake St. Pierre. The objective of the study is to document the active geomorphic processes and the response of the river to past environmental changes. We have reconstructed the evolution of the Yamachiche delta from aerial photos, historical maps, sedimentological and stratigraphic analysis of six sections and fourteen boreholes, and by sediment dating using optically stimulated luminescence, 21 OPb and 14C. The subsurface deposits are divided into two facies: sandy low-water channel deposits at the base, and silty delta plain deposits at the top. Sedimentary sequences show changes along a longitudinal gradient driven by the level of the lake and the river dynamics on annual and decadal scales, and a high lateral variability driven by migration of the channel and by high-energy waves from Lake St. Pierre, which erode the downstream eastern part of the delta plain. Rapid progradation of the delta has occurred in the last 150 years, as demonstrated by the poor development of the delta on the 1859 map. Optically stimulated luminescence dating provides ages of 140 and 280 years at the base of the depositional sequence. These results are consistent with the chronological sequence deduced from the other dating methods. This framework gives mean accumulation rates between 0.5 and 1.5 cm/year. These high rates of sedimentation are likely linked to human activities in the watershed, which affected sedimentary sources since colonization of the St. Lawrence Lowlands 200 years ago. The young age of the delta could also be explained by the late stabilization of Lake St. Pierre at its present level, between 1000 and 150 years ago.

Bibliography of Canadian Geomorphology