Sediment that is transported fluvially is subject to the processes of sorting, abrasion and breakage. Each of these processes may affect the properties used to determine the origin of the sediment; so they should be evaluated in sediment tracer studies. As part of a study to determine the dominant sources of sediment in Tarago Reservoir, southeastern Victoria, Australia, the effects of is aggregation and grain abrasion on the distribution of 137Cs with respect to particle size fractions were studied in soils derived from the three different rock types of the Tarago catchment. The main action of abrasion over short distances is aggregate breakage. In each soil, the material produced by abrasion had approximately the same 137Cs concentration as the corresponding size fractions of unabraded soil. The <40 μm size fraction, which is the dominant sediment size in the Tarago Reservoir, includes material generated by partial disaggregation and grain abrasion as well as unabraded soil. Consequently, determining the sources of sediment in the Tarago Reservoir using 137Cs is unlikely to be significantly affected by grain abrasion or disaggregation. These results improve the confidence with which 137Cs can be used to trace the origin of sediment at other sites.