CGRG Bibliography of Canadian Geomorphology
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Author : Bondue, V.; Boyer, C.; Lamothe, M.; Roy, A.G.; and Ghaleb, B
Date : 2006.
Title : Recent evolution of the Yamachiche River delta (Québec): Natural processes and anthropogenic impacts.
Publication : Joint Annual Meeting of the Geological Association of Canada and the Mineralogical Association of Canada. University of Quebec in Montreal (UQAM) May 14-17, 2006.
Issue :
Page(s) :
Abstract
Lake Saint-Pierre, 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. In order to understand the response of these rivers to such environmental changes we have studied the delta of the Yamachiche River, a tributary of Lake Saint-Pierre. The objective of the study is to document the active geomorphic processes in the delta formation and to establish a chronology of its progradation. We have reconstructed the evolution of the Yamachiche River delta from aerial photos and historical maps and from the analysis of the sedimentology and stratigraphy of six sections and fourteen boreholes. The chronology was also established by sediment dating using optically stimulated luminescence (OSL), 210Pb and 14C. Subsurface deposits are divided into two facies: sandy proximal deposits at the base, and silty delta plain deposits at the top. Sedimentary sequences show a change from silty dominated to sandy dominated facies along a downstream longitudinal gradient. This is driven by the level of the lake and by the river dynamics on the annual and decadal scales. The stratigraphy also displays a marked lateral variability associated with the migration of the channel and with high-energy waves of Lake Saint-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 deposits. The chronostratigraphic framework obtained by the dating methods leads to an estimated mean deposition rate between 0.5 and 1.5 cm/year. Such high rates of sedimentation are likely linked to human activities in the watershed, which activated sedimentary sources since the beginning of the colonization of the St. Lawrence Lowlands 200 years ago. The young age of the delta could be also explained by the late stabilisation of the Lake Saint-Pierre at its present level, between 1000 and 150 years ago. A significant drop of the lake level would cause an important progradation of the delta and a potentially high lateral and vertical erosion of the stream bed, thus inducing an important detritic accumulation in Lake Saint-Pierre.
Bibliography of Canadian Geomorphology