The impacts of climate-induced changes in discharge and base level in three tributaries of the Saint-Lawrence River, Québec, Canada, are modelled for the period 2010-2099 using a one-dimensional morphodynamic model. Changes in channel stability and bed-material delivery to the Saint-Lawrence River over this period are simulated for all combinations of seven tributary hydrological regimes (present-day and those predicted using three global climate models and two greenhouse gas emission scenarios) and three scenarios of how the base level provided by the Saint-Lawrence River will alter (no change, gradual fall, step fall). Even with no change in base level the projected discharge scenarios lead to an increase in average bed material delivery for most combinations of river and global climate model, although the magnitude of simulated change depends on the choice of global climate model and the trend over time seems related to whether the river is currently aggrading, degrading or in equilibrium. The choice of greenhouse gas emission scenario makes much less difference than the choice of global climate model. As expected, a fall in base level leads to degradation in the rivers currently aggrading or in equilibrium, and amplifi es the effects of climate change on sediment delivery to the Saint-Lawrence River. These differences highlight the importance of investigating several rivers using several climate models in order to determine trends in climate change impacts.

Abstract:More frequent extreme flood events are likely to occur in many areas in the twenty-first century due to climate change. The impacts of these changes on sediment transport are examined at the event scale using a 1D morphodynamic model (SEDROUT4-M) for three tributaries of the Saint-Lawrence River (Québec, Canada) using daily discharge series generated with a hydrological model (HSAMI) from three global climate models (GCMs). For all tributaries, larger flood events occur in all future scenarios, leading to increases in bed-material transport rates, number of transport events and number of days in the year where sediment transport occurs. The effective and half-load discharges increase under all GCM simulations. Differences in flood timing within the tributaries, with a shift of peak annual discharge from the spring towards the winter, compared to the hydrograph of the Saint-Lawrence River, generate higher sediment transport rates because of increased water surface slope and stream power. Previous research had shown that channel erosion is expected under all GCMs' discharge scenarios. This study shows that, despite lower bed elevations, flood risk is likely to increase as a result of higher flood magnitude, even with falling base level in the Saint-Lawrence River.

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This study examines the effects of changes in St. Lawrence water levels and in hydrological regimes of two tributaries of the St. Lawrence, the Richelieu and Saint-François Rivers, on the sedimentary dynamics of their confluence in an historical context and in response to projected climate changes. Analysis of historical data indicates that alteration to St. Lawrence mean water levels and spring flood characteristics (duration, timing, and magnitude) have already impacted flow and sediment dynamics at tributary confluences. The major progradation of more than 1.5 km of the sedimentary front of the Saint-François delta into the shallow water of the fluvial Lake Saint-Pierre between 1859 and 2001 is associated with the combined effect of water level reduction in the lake, as a consequence of dredging of the navigation channel, and discharge regulation. High tributary discharges during periods of low water levels in the St. Lawrence have also contributed to accelerated delta front progression. Increases in temperature and changes in the precipitation regime projected for the current century were used into the HSAMI model to obtain hydrological simulations for the tributaries. Results indicate that the greatest hydrological changes will be observed during the winter and spring seasons, a time that is critical for many ecosystems. The mean spring discharge is projected to decrease by 17% for the Richelieu and 40% for the Saint-François by the end of the century. During this season, the frequency of discharge higher than the sediment transport threshold will decrease by 63% for the Saint-François and by 17% for Richelieu when compared to the reference period. However, during winter, the frequency of these high discharges will be four times higher for both rivers. These changes are expected to occur simultaneously with a decrease of the St. Lawrence water levels, a situation that will have significant impacts on sedimentary processes. A hydro-sedimentological modeling approach based on SEDROUT was used to investigate the morphological response of the river beds to the projected changes on the hydrology of St. Lawrence tributaries and to three base level scenarios. Results show that the combined hydrology and base level effects will lead to an increase in sediment supply compared to the current state, and to the extension of accumulation zones. While the effects Guest editors: M. Power, J. will vary among the tributaries according to their particular characteristics, the projected increase in sediment supply will modify the extent of freshwater wetlands at the mouth of the St. Lawrence tributaries with feedback effects on local flow and sediment distributions.