Bed sediments in a three-mile reach of the Ottawa River are largely composed of sand-sized mineral grains and wood chips, with finer, organic-rich sediments segregated towards channel margins. The amount of leachable mercury in the bed sediment correlates strongly with leachable iron content, and less well with mean (phi ) grain size and organic content. These results are best explained by surface adsorption of mercury on grain coatings of hydrated iron oxide and complexing with organic particles. Mercury concentration therefore increases with decreasing grain size and/or increasing content of wood chips. Bedload transport occurs mainly by dune migration; the rate was about 4 x 10 5 kg/day in early June, 1974, yielding a mercury discharge of about 10 g/day in bed sediment. The main sources of pollution were controlled in 1971, and mercury levels are now declining about 66% per year in the coarser mid-channel sediment, and about 40% annually in the finer channel-margin sediments. The differential rate is thought to be due to the greater influence of mechanical transport in areas with coarser sediment, whereas chemical desorption is the principal means of mercury loss from finer deposits.