The Oginosawa River catchment lies 15 km south-west of the Fukushima Dai-ichi nuclear plant and covers 7.7 km2. Parts of the catchment were decontaminated between fall 2012 and March 2014 in preparation for the return of the evacuated population. The General-purpose Terrestrial Fluid-flow Simulator (GETFLOWS) code was used to study sediment and 137Cs redistribution within the catchment, including the effect of decontamination on redistribution. Fine resolution grid cells were used to model local features of the catchment, such as paddy fields adjacent to the Oginosawa River. The simulation was verified using monitoring data for river water discharge rates (r = 0.92), suspended sediment concentrations, and particulate 137Cs concentrations (r = 0.40). Cesium-137 input to watercourses came predominantly from land adjacent to river channels and forest gullies, e.g. the paddy fields in the Ogi and Kainosaka districts, as the ground in these areas saturates during heavy rain and is easily eroded. A discrepancy between the simulation and monitoring results on the sediment discharge rate following decontamination may be explained by fast erosion occurring after decontamination. Forested areas far from the channels only made a minor contribution to 137Cs input to watercourses, total erosion of between 0.001 and 0.1 mm from May 2011 to December 2015, as ground saturation is infrequent in these areas. The 2.3–6.9% y−1 decrease in the amount of 137Cs in forest topsoil over the study period can be explained by radioactive decay (approximately 2.3% y−1), along with a migration downwards into subsoil and a small amount of export. The amount of 137Cs available for release from land adjacent to rivers is expected to be lower in future than compared to this study period, as the simulations indicate a high depletion of inventory from these areas by the end of 2015. However continued monitoring of 137Cs concentrations in river water over future years is advised, as recultivation of paddy fields by returnees may again lead to fast erosion rates and release of the remaining inventory.