Sediment budgets typically require an estimate of the proportional yield from erosion sources to sediments in transport and storage. This becomes increasingly difficult as catchments become larger, and erosion, storage, and deposition processes become more complex. We demonstrate how fallout radionuclides can be used to estimate the proportional contributions to sediment load, from a tripartite classification of erosion sources in a large catchment (the mid-Murrumbidgee, 13,500 km2). The three major potential sources of sediment within this catchment are cultivated lands (∼22% of the surface area), uncultivated pastoral lands (∼78%), and the numerous channels and gullies found in this region. Concentrations of the fallout radionuclides 210Pbex and 137Cs in representative samples from each of these three sources are significantly different. Employing these values in a simple mixing model shows that the largest contribution of material is currently derived from subsoil channel/gully sources. Alterations to the suspended sediment 210Pbex signature by in situ labeling and decay are also considered. Applying the model with different concentrations of 210Pbex (decayed as a function of residence time in channels) suggests that the mean residence time of fine-grained material within this system is 10±5 years. However, differences in 137Cs concentrations observed between flood and low-flow sediments, and the presence of the short-lived, cosmogenic 7Be, suggest that residence time of some of this fine-grained material may be of the order of only weeks to months.