The Ogaki Dam Reservoir is one of the principal irrigation dam reservoirs in the Fukushima Prefecture and its upstream river basin was heavily contaminated by radioactivity from the Fukushima Daiichi Nuclear Power Plant accident. For the purpose of environmental assessment, it is important to determine the present condition of the water in the reservoir and to understand the behavior of sediment-sorbed radioactive cesium under differentmodes of operation of the dam, as these factors affect further contamination of arable farmlands downstream of the reservoir through sediment migration. This paper addresses this issue with numerical simulations of fluvial processes in the reservoir using the two-dimensional Nays2Dcode. Wedistinguish three grades of sediment (clay, silt, and sand), as cesium adherence depends on sediment grain size and surface area. Boundary conditions for the simulations were informed bymonitoring data of the upstreamcatchment and by the results froma separate watershed simulation for sediment transport into the reservoir. The performance of the simulationmethod was checked by comparing the results for a typhoon flood in September 2013 against fieldmonitoring data. We present results for sediment deposition on the reservoir bed and the discharge via the dam under typical yearly flood conditions, forwhich the bulk of annual sedimentmigration fromthe reservoir occurs. The simulations showthat almost all the sand and silt that enter into the reservoir deposit onto the reservoir bed. However, the locationswhere they tend to deposit differ, with sand tending to deposit close to the entrance of the reservoir, whereas silt deposits throughout the reservoir. Both sand and silt settlewithin a fewhours of entering the reservoir. In contrast, clay remains suspended in the reservoirwater for a period as long as several days, thus increasing the amount that is discharged downstreamfromthe reservoir. Under the current operatingmode of the dam, about three- quarters of clay that enters the reservoir during the flood is discharged downstreamduring and in the days following the flood. By raising the height of the damexit, theamount of clay exiting the reservoir can be reduced by a factor of three. The results indicate that the damcan be operated to buffer radioactive cesiumand limit the contamination spreading into lowland areas of theUkedoRiver basin. These results should be a factor in considerations for the future operation of theOgakiDam, and will be of interest for other operators of damreservoirs in areas contaminated by radioactive fallout.