Sediment dynamics and the transport of suspended matter in the upstream area of Lake St. Francis

Abstract

A long-term project was initiated in autumn 1994 to monitor the suspended matter (SM) in the upstream area of Lake St. Francis. Over a 32-month period, 190 SM samples were collected at six study sites while conductivity and current velocity measurements were made to study resuspension and transport of SM. Weather data from a nearby station and daily discharge rates for the St. Lawrence River were also utilised. Overall, the study shows that the SM load in the central portion of Lake St. Francis is not evenly distributed. On the northern side of the lake, the SM load is mainly a function of the SM load carried by the St. Lawrence River waters coming from the Great Lakes. On the southern side, an important contribution to the SM load comes from sediment resuspension and from the local tributaries. Calculations show that wave action is likely to resuspend surficial sediments in depths shallower than 2 m, a surface area estimated to be 32-35 km2 between Cornwall Island and Thompson Basin. Also, important fluctuations of the south shore tributaries' winter discharge are thought to contribute to sediment resuspension and redistribution of contaminants such as mercury and polychlorinated biphenyls.