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To predict the behavior of particulate and dissolved radiocesium (137Cs) in river water, somesoil and land use categories in the watershed should be considered because these may affectthe dissolved 137Cs concentration. This recommendation is based on measurements madeafter the Chernobyl accident (Smith et al., 2004). However, it is difficult to predict thedissolved 137Cs concentration in Japanese rivers based only on studies after theChernobyl accident because 137Cs dynamics differ between the areas surroundingthe Chernobyl and Fukushima Daiichi Nuclear power plants because of climateand soil differences (Evrard et al., 2015; Konoplev et al., 2016). In this study, toinvestigate the factors that affect dissolved 137Cs concentrations in Japanese rivers, wemeasured dissolved 137Cs concentrations in 67 rivers under baseflow conditions 13–280km from the Fukushima Daiichi nuclear power plant in August and September2017. The watershed area of each sampling point was 3.2–3,600 km2 and the averaged 137Csinventory for each watershed in July 2011 was 4.7–1,700 kBq/m2(calculated from the 137Csinventory distribution map by Kato et al., under review). Land use in the watersheds variedand included forest watershed (up to 99% forest area) and urban watershed (up to 73%building area). To research the influence of water quality on the dissolved 137Csconcentration, we coincidentally measured the main coexisting solutes, pH value,and electric conductivity in the river water. Additionally, to research the effect ofthe 137Cs source, the land use ratio, soil components, and watershed topographicwetness index were calculated from geographic information data published on theInternet. The dissolved 137Cs radioactivity concentrations in river water were 0.10–120 Bq/m3andstrongly depended on the averaged 137Cs inventory in the watershed (r = 0.70, p < 0.01).Therefore, the dissolved 137Cs concentration was normalized by dividing it by theaveraged 137Cs inventory (converted into the range 4.5×10−6–2.2×10−3 m−1) and thecorrelations among these values and each water quality factor and 137Cs source wereanalyzed. Significantly high positive and a negative correlation existed between the normalizeddissolved 137Cs concentration and building area ratio (r = 0.78) and forest area ratio (r =-0.60), respectively. The high concentration of coexisting ions in the water was a cause ofsuch a high correlation because it would impede 137Cs absorption in soil particles because thebuilding area ratio has a high positive correlation with electrical conductivity (r = 0.55). [ABSTRACT FROM AUTHOR]