Sediment dynamics in the fluvial lakes of the St. Lawrence River: accumulation rates and characterization of the mixed sediment layer

Abstract

Permanent sedimentation (210Pb and 137Cs), sediment mixed layer thicknesses, and mixing coefficients (7Be) were measured in the St. Lawrence River in order to evaluate the importance of sediment retention in the particulate matter budget and to characterize the system's resilience to changing contaminant loads. Net sediment accumulation (1 to > 18 kg·m-2·year-1) is observed at most sites deeper than 4.5 m located outside the main channels. Annual sediment retention in the lakes ranges from 1.5% (Lake St. Pierre) to 17% (Lake St. Francis) of their total load of suspended solids. 7Be profiles indicate that the average mixed layer thickness, mixed layer mass, and mixing coefficient are 3.3 ± 0.2 cm, 17.8 ± 1.7 kg·m-2, and 14.9 ± 2.8 cm2·year-1, respectively. The average depth of the long-term (approximately 5 years) mixed layer determined from the 137Cs : anthropogenic Pb ratio is 5.1 ± 0.4 cm, corresponding to 30.6 ± 4.6 kg·m-2. Because the mixing coefficient in superficial sediments is relatively high, and because annual particulate matter loading to the river is comparable with its mixed sediment inventory, the system is expected to have a rather short memory of past conditions and to recover rapidly (2-5 years) following a decrease in contaminant loading.