Sediment fluxes in rivers have proved difficult to measure or model at timescales relevant to land and water resource management decisions. Determining transport rates and residence times is complicated by the intermittent movement and temporary storage of sediments in natural streams. This paper presents a new tool for estimating the residence times of river-borne sediments, based on the observed disequilibria between the naturally occurring radionuclides 228Ra and 232Th in the thorium decay series. Disequilibrium between concentrations of 228Ra (half-life 5.75 years) and its long-lived parent 232Th has been observed in marine environments and in terrestrial freshwaters but has not been previously reported in fluvial sediments or soils. Here we report 228Ra excesses in sandy stream and silty lake sediments from southeastern Australia. The excess is developed in hillslope soils in the catchment headwaters and begins to decay back to secular equilibrium once the sediment is finally removed from contact with the soil water. The short half-life of 228Ra enables this process to be used to investigate fluvial sediment transfers, and alluvial and lacustrine sediment storages, over the past 30 years. When combined with indicators of sediment source, such as 226Ra/232Th and 230Th/232Th ratios, observation of this excess should provide a useful new tool for examining the residence and transit times of bed load and suspended load sediments in streams and lakes.¿ 1997 American Geophysical Union