It is essential to evaluate secondary migration caused by riverine input and resuspension from seabed sediments to estimate the future distribution of radioactive cesium (137Cs) in the coastal area off Fukushima Prefecture. In particular, the inflow from rivers cannot be ignored because most of the 137Cs inflow from rivers is deposited on the coast without elute into seawater. Two mooring systems were installed near the Ukedo River's mouth (Fukushima Prefecture) from February 2017 to February 2018. The first contained a sediment trap system, collecting sinking particles during the period. The second comprised a turbidity sensor and a current sensor. The contribution of resuspension and inflow from the river to the mass flux was quantitatively evaluated using multiple regression equations. The results showed that resuspension caused 79%–83% of secondary 137Cs migration in nearshore areas, whereas the influence of riverine 137Cs input on the sediment was only 7% per year.