The deposition chronology of algal pigments, organic C, N, P, trace metals, and organic contaminants (PCB congeners, DDD, DDE, Mirex, hexachlorobenzene) is described for two sites in fluvial lakes (St. Francis and St. Louis) of the St. Lawrence River. Pigment concentrations rapidly decrease in the top 10 cm due to degradation, but show a broad secondary peak consistent with higher primary productivity between 1960 and 1975. In Lake St. Francis, deposition of organic C, N, and P peaked between 1950 and 1975. In both lakes, trace metal concentrations reached maximum values between 1960 and 1970 and decreased considerably between 1970 and 1980. At both sites, surficial trace metal concentrations are approaching (within a factor of 3) those observed in preindustrial sediments except Cd, which is still 5–6 times higher. Concentrations of Cr, Cu, Ni, and Zn are inversely related (p < 0.05) to river discharge whereas those of Pb are positively related to discharge. Concentrations of organic contaminants have declined by a factor of 5–10 between the mid-1960's and the early 1980's, except Mirex, which shows no obvious trend with time. Historic trends in pollution by organic chemicals appear to have been influenced not only by changing anthropogenic inputs, but also by river discharge.

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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.

Water quality degradation is a serious concern for the St. Lawrence River. While some environmental data are available for the St. Lawrence ecosystem, long‐term monitoring data are generally lacking. To infer past environmental changes, we undertook a paleolimnological assessment of diatom assemblages preserved in four 210Pb‐ and 137Cs‐dated sediment cores from two fluvial lakes in the river, and used diatom transfer functions to infer past shoreline habitat characteristics. At sites in Lake Saint‐François, a fluvial lake downstream from Cornwall, water quality decreased this century in response to human impacts (e.g. macrophyte density and nutrient levels increased). These trends were apparent from an increase in epiphytic diatom taxa, followed by an increase in eutrophic planktonic taxa. Water quality, however, appears to have improved somewhat in response to rehabilitation measures during the last two decades. From a sediment core near Montréal (Lake Saint‐Louis), we also noted a large proportion of eutrophic and epiphytic taxa, but less evidence was recorded of a recent improvement in water quality. The diatom‐based inference model for habitat characteristics appeared to reconstruct environmental conditions in the St. Lawrence River during the last century. The most notable shift has been an increase in diatom taxa commonly associated with macrophyte substrates. Trends in some of the planktonic diatoms were similar to those recorded in paleolimnological investigations from Lake Ontario, but cores from the river also may be reflecting local environments. This study shows that diatom‐based paleolimnological studies are possible in large river systems, if coring sites (e.g. fluvial lakes) are carefully selected.

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.