This article provides an outline of conditions of particulate material and sediments in the river, estuarine, and gulf sections of the St. Lawrence (Québec, Canada). The concentration of solids in the river above lac St. Pierre varies between 4 and 10 mg/L, whereas downstream towards Québec the concentrations rise to 30–50 mg/L. With the exception of the Sorel delta, this section of the river does not constitute a major sink of sedimentary materials, but the surface sediments of the river are enriched with phosphorus, polychlorinated biphenyls, copper, and lead. The portion immediately upstream from Île d'Orléans is the interface of riverine water and salt water from the Gulf of St. Lawrence and at this point is a region of high concentrations of suspended solids (up to 300 mg/L). The process of sedimentation in the estuary is complicated by the coagulation of fine particles after contact with salt water. As a result, there is a zone of high sedimentation rates of fine participates enriched in phosphorus, organic matter, and polychlorinated biphenyls in the area of Île d'Orléans, making constant dredging necessary. Downstream from Île d'Orléans is a decrease from 40–80 mg/L suspended solids to 1–2 mg/L in the Gulf of St. Lawrence.