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The Fukushima nuclear accident in March 2011 generated a 3000 km$^{2}$ plume of soils heavily contaminated with $^{137}$Cs. Decontamination was completed early in 2019. Typhoon Hagibis was the first extreme event that occurred in the region after decontamination. Its impact on sediment sources and sediment $^{137}$Cs contamination was investigated through the application of a sediment fingerprinting procedure using spectrocolorimetry and geochemical properties. Sediment deposits ($n=24$) were collected in the Mano and Niida River catchments after the 2019 typhoons, and their signature was compared to that of potential sources (e.g., cropland, forests, and subsurface; $n=57$). Results demonstrate the dominance of cropland as the main source of sediment (mean: 54%) followed by forests (41%) with much lower contributions of subsurface material (5%). Overall, $^{137}$Cs concentrations in sediment were on average 84%–93% lower than the levels recorded after the accident in 2011, which demonstrates the effectiveness of cropland decontamination.