Abstract The surficial sediments of the Miramichi estuary, New Brunswick, contain a much higher quantity of organic matter than would be expected to occur in an estuary of this type, which is shallow and well-mixed. The organic carbon distribution in general corresponds with the distribution of fine-grained sediment, although higher concentrations occur in sediments of the drowned river channel than in sediments of the bay portion of the estuary. The high quantity of organics in all the sediments, and differences in concentrations between river channel and bay sediments, are the direct result of the discharge of pulp-mill effluent into the upper reaches of the estuary. The organic carbon isotopic composition of sediments suggest that land-derived material is the predominant source of organics throughout the estuary. Marine organic material is restricted to the sediments near the estuarine mouth. Dispersal of the organic matter is achieved largely by transport in the upper freshwater effluent layer, and by settling through the water column when transport energies are reduced. Settling of organic-matter is highest in areas where deposition of fine-grained inorganics prevails.