Observations of Suspended Sediment Flux over a Tidal Cycle in the Region of the Turbidity Maximum of the Upper St. Lawrence River Estuary
A series of four 36-hour anchor stations was occupied along the axis of the upper St. Lawrence Estuary from the freshwater zone near Quebéc, across the turbidity maximum to near sea water salinities at the mouth of the Saguenay Fjord. The anchor stations reveal that in the region of the turbidity maximum near surface suspended sediment concentrations are maximum during and just following the ebb phase of the tidal cycle while near the bottom sediment concentrations are greatest shortly after the occurrence of maximum current speeds. The intense near-surface maximum in turbidity coincides with the reduced vertical stratification during the ebb which is considered to increase the upward vertical diffusion of suspended sediment from lower depths. It appears that the turbidity maximum is not maintained by residual advection according to the classical two-layer estuarine circulation but by upstream Reynolds transport arising from higher suspended sediment concentrations on the flood than on the ebb in the lower portion of the water column. Thus contaminants may be recycled through the turbidity maximum by means of this tidal pumping. A bulk vertical settling velocity of 0.3 mm/s was estimated at one station near the turbidity maximum from a simple vertical transport model which is in close correspondence with the settling velocity of the average particle size observed in several studies in the area.
KeywordsSuspended Sediment Sediment Concentration Tidal Cycle Suspended Sediment Concentration Sediment Flux
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