ABSTRACT: Sensitivity of tributaries to water-level fluctuations along the St-Lawrence corridor, Québec, Canada.

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During the course of the last century, variations in the St-Lawrence water levels, caused by different uses of the river and the Great Lakes, have already had major impacts on riparian habitats and on a number of tributaries. Anticipated changes caused by climate change will only accentuate these impacts. At present, climate change scenarios forecast a decrease of the St-Lawrence water discharge by 20% over the next fifty years, which would correspond to a drop in water level between 0.5 and 1 metre at Montréal. Because the St- Lawrence corridor is in a region of lowlands, such fluctuations in water levels are expected to cause major adjustments in the morphology and longitudinal profiles of the tributaries through the erosion and incision of the river bed. These changes are important because ultimately, the sediment loads delivered to the St-Lawrence river are linked to the erosion processes occurring along its tributaries. However, given the important physical diversity of the tributaries, their sensitivity will differ, making it difficult to predict their individual response to environmental changes, especially given the paucity of data on the present state of the tributaries. Thus, our goal was to obtain data on the variability in morphosedimentology and dynamics of the tributaries in order to infer their sensitivity to fluctuations in baselevels. Bathymetric, hydraulic, and sedimentological surveys were conducted in 2004 and 2005 on five tributaries of the St-Lawrence: the Yamachiche, the St-Maurice, and the Batiscan rivers on the north shore, and the Richelieu and St-François rivers on the south shore. These tributaries cover a wide range of sizes and sedimentological characteristics, from cohesive clays to silts and gravel. Their hydrological regimes are also varied and are in some cases regulated, limiting the sediment capacity of certain tributaries. For example, the Yamachiche and St-François rivers have been under intense agricultural pressure, have fine sediments and have migrated significantly over the last fifty years, highlighting their instability and suggesting that changes in water levels may accentuate these erosion processes. On the other hand, the St-Maurice is highly regulated, has greater bank stability, due to its coarser sediment and intact riparian vegetation, and has remained fairly unmoved. This suggests that this river will respond more slowly to fluctuations in water levels. Investigating the current physical conditions of the tributaries will allow a better understanding of the long-term impacts of sustained periods of low water levels due to environmental change, and will help us develop a sensitivity index of the response of these rivers.

Bibliography of Canadian Geomorphology