The Ganges and Brahmaputra rivers in Bangladesh: basin denudation and sedimentation Mohammad Rezwanul Islam,* Syeda Fahliza Begum, Yasushi Yamaguchi and Katsuro Ogawa Department of Earth and Planetary Sciences, Graduate School of Sciences, Nagoya University, Furo Cho, Chikusa Ku, Nagoya 464-8602, Japan Abstract: Every year the Ganges and Brahmaputra rivers in Bangladesh transport 316 and 721 million tonnes of sediment, respectively. These high loads of suspended sediment re¯ect the very high rate of denudation in their drainage basins. The average mechanical denudation rate for the Ganges and Brahmaputra basins together is 365 mm 10 3 yr 1 . However, the rate is higher in the Brahmaputra Basin than that in the Ganges Basin. Several factors, including mean trunk channel gradient, relief ratio, runo, basin lithology and recurring earthquakes are responsible for these high denudation rates. Of the total suspended sediment load (i.e. 1037 million tonnes) transported by these rivers, only 525 million tonnes (c. 51% of the total load) are delivered to the coastal area of Bangladesh and the remaining 512 million tonnes are deposited within the lower basin, osetting the subsidence. Of the deposited load, about 289 million tonnes (about 28% of the total load) are deposited on the ¯oodplains of these rivers. The remaining 223 million tonnes (about 21% of the total load) are deposited within the river channels, resulting in aggradation of the channel bed at an average rate of about 3 . 9 cm yr 1 . Although the Brahmaputra transports a higher sediment load than the Ganges, the channel bed aggradation rate is much higher for the Ganges. This study also documents a wide range of interannual, seasonal and daily variation in suspended sediment transport and water discharge. Interannual variation in sediment deposition within the basin is also suggested. Copyright # 1999 John Wiley & Sons, Ltd. KEY WORDS Ganges; Brahmaputra; suspended sediment load; basin subsidence; denudation; sedimentation INTRODUCTION The two Himalayan rivers, the Ganges and Brahmaputra, are among the most sediment-laden rivers in the world (Milliman and Meade, 1983). Every year these two rivers carry a large sediment load from the Himalayan Mountains to the Bengal Basin and have formed the Ganges±Brahmaputra Delta, which is the largest in the world. Based on a comparison between Rennell's (1781) map and a modern map, Morgan and McIntire (1959) concluded that the delta front had advanced little over the last 200 years. They suggested that most of the sediment deposited within the subsiding delta simply balanced the subsidence. Coleman (1969) suggested that strong tidal currents at the mouths of the river system prevented sediment from settling, so that it was transported through a deep submarine canyon known as the `Swatch of No Ground' to the deeper ocean. On the other hand, studies undertaken by Curray and Moore (1974) and Emmel and Curray (1985) found no evidence of recent sediment accumulation on the Bengal deep sea fan. They found that most of the sediment was being trapped on the lower deltaic plain and on the shelf. CCC 0885±6087/99/172907±17$1750 Received 30 October 1998 Copyright # 1999 John Wiley & Sons, Ltd. Accepted 29 March 1999 HYDROLOGICAL PROCESSES Hydrol. Process. 13, 2907±2923 (1999) *Correspondence to: Dr M. R. Islam, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, 464-8602, Japan. E-mail: islam@eps.nagoya.u.ac.jp