Recent authors have suggested that a significant proportion of the worldwide terrigenous sediment budget is trapped landward of the river-ocean boundary in high-load, tectonically active basins. To test this idea, modern flood-plain sediment accumulation rates were determined along a 110 km reach of the Brahmaputra (locally Jamuna) River using 137Cs geochronology of sediment cores and geographic information system (GIS) extrapolation to adjacent areas. 137Cs accumulation rates decrease exponentially away from the channel, from >4 cm/yr on the natural levees to <1 cm/yr within a few tens of kilometers basinward. Important controls on sedimentation in this area include proximity to distributary channels, local topography, and interannual variability of the flood pulse. Model results indicate that an average of 23 m.t./yr of sediment are sequestered in this section of the flood plain. Extrapolated basinwide, as much as 39%–71% of the river sediment budget may be trapped landward of the Ganges-Brahmaputra mouth.