ISSN : 0470-0929 105 National Geographer, Vol. XXXI, Nos. 1 & 2, (Jun. & Dec. 1996), Pages 105-128 — Reprint Retrogradation of the Western Ganga- Brahmaputra Delta (India and Bangladesh) : Possible Reasons Sunando Bandyopadhyay Chandernagore College, Chandannagar Manotosh K. Bandyopadhyay Calcutta University, Calcutta Abstract Comparison of multidated maps / images, field evidence and a majority of the recent literature indicate that the Ganga-Brahmaputra delta (GBD), west of the river Haringhata, is retrograding for the last 200 years. The probable reason for this, according to the early workers (1858-1893), are: sediment trapping by the Swatch of No Ground (SNG) submarine canyon, influence of ‘tidal wave’ and abandonment of the western distributaries of the Ganga. To these, the later workers (1927-1994) have added: coastal subsidence, the high regional tidal range and sediments absorption by the lower deltaic marshes and distributaries. Other possible causes include: sediment sink at the reservoirs of the Ganga catchment, sea level rise and deforestation of the coastal mangroves. While all these factors, with exception to the ‘tidal wave’ hypothesis, have certain merits, delta abandonment and sediment interception by the SNG probably are the most important. They together have increased the relative dominance of the erosive marine and atmospheric processes over the accretional fluvial processes at the western GBD. Around the Hugli estuary, a slow progradation of the subaqueous delta is seen although the terrestrial part is getting eroded. INTRODUCTION ituated at the northern apex of the Bay of Bengal, the Ganga-Brahmaputra delta (GBD) is the world’s largest both in terms of land area (c. 150  10 6 km 2 ) and yearly discharge of sediments (~ 1  10 9 t), derived from a catchment area of 1.55  10 6 km 2 . The delta is a composite one, formed by the Ganga, Brahmaputra and many small to medium sized peripheral rivers that emanate from the surrounding uplands. Of these, the Meghna is the largest (Fig. 1A). The current orientation of the Ganga-Padma-Lower Meghna river diagonally divides the GBD into two halves. The southwestern half, along with southern coastline of the delta S