| Summary: | The Fukushima Daiichi Nuclear Power Plant accident caused an accumulation of <sup>137</sup>Cs in coastal sediment. The <sup>137</sup>Cs supply from rivers to the ocean can affect the long-term fate of <sup>137</sup>Cs in coastal sediment. Since the Fukushima coastal river basins include large decontaminated and evacuation order areas, considering the decontamination work and resumption of agriculture is important for predicting the <sup>137</sup>Cs supply. We conducted a 30-year prediction of the <sup>137</sup>Cs supply from the Fukushima coastal rivers to the ocean using a distributed radiocesium prediction model, considering the effects of human activities. In river basins with decontaminated and evacuation order areas, human activities reduced the total <sup>137</sup>Cs outflow from agricultural lands, urban lands, and forest areas to the rivers and the <sup>137</sup>Cs supply to the ocean by 5.0% and 6.0%, respectively. These results indicated that human activities slightly impacted the <sup>137</sup>Cs outflow and supply. The <sup>137</sup>Cs supply from rivers impacted by the accident to the coastal sediment was estimated to correspond to 11–36% of the total <sup>137</sup>Cs in the coastal sediment in the early phase of the accident. Therefore, the <sup>137</sup>Cs supply from rivers to the ocean is important for the long-term behavior of <sup>137</sup>Cs in coastal sediment.
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