ELSEVIER Sedimentary Geology 121 (1998) 239–258 Floodplain processes in the Bengal Basin and the storage of Ganges–Brahmaputra river sediment: an accretion study using 137 Cs and 210 Pb geochronology Steven L. Goodbred, Jr. Ł , Steven A. Kuehl Virginia Institute of Marine Science, P.O. Box1346, Gloucester Point, VA 23062, USA Received 4 February 1998; accepted 3 June 1998 Abstract Floodplain deposits are recognized as potentially large sinks for fluvial sediments, yet spatial and temporal patterns of accumulation are not known for many river systems. In the tectonically active Bengal Basin, the Ganges and Brahmaputra rivers coalesce and have formed a large floodplain=delta complex, where widespread overbank flooding may deposit a significant portion of the river’s estimated sediment discharge. To investigate the magnitude and distribution of this process, accumulation rates were determined by 137 Cs and 210 Pb radioisotope geochronology at 60 sites located in three regions of the Bengal Basin differing in age, physiography, and river influence. These geochronological methods have been applied to floodplain environments in only a few recent studies, and the present research offers a revised approach for heterogeneous systems. Patterns of sediment deposition in the Bengal Basin reveal three dominant controls on floodplain accretion: channel processes, overbank flooding, and surface runoff. Accretion is most rapid in the river braidbelt and adjacent floodplain, decreasing rapidly with distance from the main channel. Accumulation rates increase again in low-lying distal basins, where several meters of annual precipitation and associated runoff rework surrounding floodplain surfaces and transport remobilized sediments to local catchments. The extent and magnitude of this process indicate its comparable importance with overbank deposition for floodplain accumulation. A sediment budget for the study area reveals that at least 15% of the estimated 1 billion tonnes of fluvial sediment discharge is stored annually and not reaching the oceans as previously assumed. Extrapolated to the remaining ¾60,000 km 2 of floodplain not covered in this project, the sequestered load could rise to >30% of annual sediment discharge.  1998 Elsevier Science B.V. All rights reserved. Keywords: Ganges–Brahmaputra river delta; Bangladesh; accretion study; floodplain processes; radioisotope geochronol- ogy; radiocesium 1. Introduction 210 Pb radioisotope geochronology has been widely applied in aquatic and wetland environments where Ł Corresponding author. Tel.: C1-804-6847279; Fax: C1-804-6847250; E-mail: goodbred@vims.edu relatively constant grain size and ‘steady-state’ nu- clide fluxes generally satisfy inherent assumptions (Appleby and Oldfield, 1978). Similarly, fallout 137 Cs has been broadly used to investigate erosion and de- position in terrestrial, wetland, and aquatic systems (DeLaune et al., 1978; Ritchie and McHenry, 1990), often in conjunction with 210 Pb in the latter two set- 0037-0738/98/$ – see front matter c  1998 Elsevier Science B.V. All rights reserved. PII:S0037-0738(98)00082-7