[1] River channel confluences play a major role in the dynamics of all fluvial systems, and yet our understanding of bed load routing at these sites is very sparse. The dynamics of confluences are a function of the momentum ratio between the combining flows and the three-dimensional geometry of the junction. Recent experiments have shown that discordance in bed height between the confluent rivers increases turbulence intensity and enhances upwelling of flow within the confluence. However, the significance of these flow characteristics on sediment transport is still unknown. To examine the relations between flow and sediment transport, we have measured near-bed flow turbulence, bed load transport rates, and changes in bed morphology for eight different flow conditions at a sand bed discordant confluence. Detailed analysis of the near-bed flow patterns reveals that within the shear layer, low mean flow velocities are combined with the highest values of Reynolds shear stresses and that turbulence generation is associated with intense upward movements of flow. High sediment transport rates are found at the edges of the shear layer region where horizontal-vertical cross stresses (rUw 0 ) are high. These patterns match changes in bed morphology where erosion occurs along the shear layer. The relation between the shear layer and sediment transport confirms the role of bed discordance on the dynamics of the confluence. Migration of the shear layer into the confluence, as a result of a change in momentum ratio, modifies local near-bed flow characteristics, sediment transport rates, and the spatial distribution of deposition and erosion zones.
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.
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