Sediment traps were used to measure particle-settling fluxes in serial moorings offshore of Great Whale River (Hudson Bay), both under the late winter sea-ice cover and during and after breakup. Before breakup, the settling fluxes ranged between 0.25 and 2 g cm−2 100 a−1, increasing from April to May in response to the progressively larger under-ice algal biomass. Fluxes also increased with depth. During and after breakup, including the early summer period of peak runoff, sedimentation rates increased to values of up to 33 g cm−2100 a−1. These fluxes agree with the mean sedimentation rate determined from 210Pb activities in the underlying sediments.