The association of many environmentally sensitive chemicals and their transformation products with mineral and organic substrates is of considerable importance for environmental monitoring, prediction and management purposes in rivers and their basins . Our understanding of these relationships is poor . This paper reviews processes of particular concern, including the physical behaviour of fine-grained (< 63 µm) sediment in freshwater ; the role of flocculation as a transport vector ; the processes that control freshwater flocculation including microbiological factors ; the uncertainty in conventional sediment transport models for predicting pathways of sediment-associated chemistry ; the relationship between suspended sediment and toxicity in the water column ; and the partitioning of chemicals between the sediment, organic and water phase, including the significance of these in predicting chemical transport on suspended matter . 177