The historical emission of mercury from a municipal sewage treatment works, which discharges to a lowland river, has resulted in a 17 km contamination plume, the nature of which is strongly dependent upon river bed morphology. Significant variations occur in the spatial distribution of mercury concentrations over very short distances as dictated by sediment type, channel morphology, hydrology and sedimentation rates. The major sedimentalogical process is one of net deposition of paniculate matter rather than downstream transportation, which has buried contaminated sediment beneath a cleaner overlay. The behaviour of methylmercury within the sediment system is not dependent solely upon total mercury concentrations but instead is governed by a complex interplay between sediment redox potential, temperature, sulphur chemistry and the nature of bacterial communities. Detectable levels of methylmercury were confined to the uppermost 40 cm of sediment with peak loadings occurring 4-8 cm from the sediment/water interface. Methylmercury concentrations vary seasonally with minimum loadings in the autumn and highest concentrations in the summer. The distribution of methylmercury within the sediment compartment closely mirrors that of the mercury tolerant bacteria but in-situ and laboratory based experiments have also linked seasonal variations in methylmercury levels to changes in temperature.