Decadal scale geomorphic changes and tributary confluences within the Ganga River valley in Varanasi region, Ganga Plain, India

摘要

Allahabad to Buxar, the Ganga River shows distorted menders, huge meander cut-offs, ox-bow lakes, scroll bars and abandoned linear channel belts, all suggesting to a fault system-related tectonic control on the river. The confluence dynamics of northern tributaries Gomati and Varuna Rivers which have shifted progressively from north to south, and confluences of southern tributary Jirgo which shifted from south to north, further corroborate the view and also suggest that the presence of a NE-SW trending dextral strike slip fault having a gravity component.The Ganga River came close to Varanasi around 40 ka ago following a tectonic event that caused extensive liquefaction of the sediments due to a seismic pulse and upheaval in the area. Between 40 to 7ka, the Ganga River was freely oscillating within its valley when another tectonic event took place and the Ramnagar cliff was raised due to upheaval and channel incision along NE-SW trending fault. After 7ka, the Ramnagar cliff was degraded before it was inhabited by man at around 3800 years BP, which have been contemporaneous to antiquity of Varanasi.Based on available remote sensing data, the morphometry has shown that in 1972 active channel covered 12% area in Ganga River Valley and aerial extent remained 8% in 2010. The decrease of sand bar areas (14%-11%) in the studied stretch reflects the decrease in monsoon strength over the last four decades. However, the decline in the aerial extent of relict valley features, viz. meander scars, meander cut-offs and alluvial islands, is quite characteristic and may be related to sediment recycling by migrating channels present within the active channel belt and floodplain areas of the Ganga River.