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Distributions of 137Cs in dissolved and particulate phases of the downstream reaches of seven rivers and adjacent nearshore and offshore waters as far as ∼60 km south of the Fukushima Dai-ichi nuclear power plant (FDNPP) were studied during the high-river-flow period (June–September 2019) and during the period of October 2019 after typhoon Hagibis. Dissolved 137Cs activities in nearshore water were higher than those in rivers and offshore waters, and this distribution was more intensified after the typhoon, indicating the desorption of 137Cs from riverine suspended particles in addition to the ongoing release of contaminated water from the FDNPP and re-entry of radiocesium via submarine groundwater discharge. This scenario is also supported by the reduction of distribution coefficient (K d) from a geometric mean value of 5.5 × 105 L/kg in rivers to 9.8 × 104 L/kg in nearshore water. The occupation of desorbed 137Cs to the dissolved activity of this nuclide in nearshore water was estimated to be 0.7%–20% (median: 9.7%) during the high-river-flow period, increasing to 1.4%–66% (32.3%) after the typhoon, suggesting that the desorption during the flood period such as typhoons further contributes to the increase in dissolved 137Cs levels in nearshore water.