Abstract The redistribution of atmospherically derived radionuclides by two Japanese rivers has been carried out. This work concerns their potential health impact related to the redistribution and the geochemical cycle of radionuclides in river watersheds, including connected water bodies. The chapter focuses on cesium-137, a radionuclide originating mainly from nuclear weapon tests fallout and partly from the Chernobyl accident. Lead-210 and beryllium-7 of natural origins are also considered in some analyses. A load of fluvial discharge of these radionuclides in particulate form exhibited a log-log relationship with the river water flow rate. A budget analysis for 137Cs in the Kuji River watershed showed that approx. 0.05% of the accumulated 137Cs in the watershed was discharged annually during the period 1987–1988. In the Kuji River watershed, the particulate form was dominant (90% or more) in the annual load of fluvial discharge of 137Cs, 210Pb, and 7Be. This finding, along with results from previous studies by others in other watersheds, suggests that this dominance is coupled with the concentration of suspended solids, which is derived from the geohydrological features of a watershed. A budget analysis for 137Cs in the Hi-i river watershed of Shimane in Japan, where the contribution of the Chernobyl fallout is minor, revealed a temporal decrease in the rate of fluvial discharge of 137Cs from ∼1% in the 1950s and 1960s, when the fallout of 137Cs was greatest, to ∼0.15% in the 1990s, when the fallout has become much less. This decrease may be due to progressive infiltration and fixation of 137Cs in the soil layers within the watershed.