A series of 13 and 25 h anchor stations and individual stations were occupied along the axis of the North Channel of the St. Lawrence Estuary during periods of both high and low river runoff. The data collected have permitted a detailed examination of the variations in salinity, suspended matter concentrations, and the size distributions of the suspended matter and the bottom sediments of the maximum turbidity zone. The high turbidity appears to be maintained by a complex density-driven circulation in combination with the resuspension of bottom sediments near the head of the estuary. The phenomenon may be aided by the breaking of internal waves. A longitudinal structure, which is only slightly influenced by changes in river discharge, is discernible for the zone. The highest concentrations of suspended matter occur in a well mixed region at the head of the estuary and suspended particulate matter levels decrease seaward as a two-layer estuarine circulation becomes better developed.