Evolution of Kosi river fan, India: structural implications and geomorphic significance

The Kosi river fan located in the northern part of India is one of the largest alluvial cones built by any river in the world. This 180 km long and 1501cm wide alluvial cone shows evidence of lateral shift exceeding 140 km during the past 250 years. The Kosi alluvial cone and its adjoining area has been studied in detail by remote sensing techniques. The data have been integrated with the available geological and geophysical information to decipher the causes responsible for the lateral shift of such a high magnitude fan. The satellite image better defines the old palaeo-channels of the Kosi river and its confluence with the Mahananda river north of Lava prior to 1731. The dominant structural control, in shifting the river towards the west from one fault zone to another, is also discernible. Integrated study reveals the occurrence of dominant NNW-SSE and NNE-SSW trending faults/lineaments colinear with the pre-existing basement weak zones and their reactivation to date. The recent Himalayan relief, the thrusting of Siwaliks over the Gangetic alluvium, the development of inclined Holocene terrace in the piedmont zone and ocurrence of recent earthquakes amply demonstrates the continued neotectonic movements in the recent past. The structure contour map at the basement and Neogene-Quaternary level suggests a regional tilt from east to west along the Malda-Kishanganj fault, which got accentuated in the recent past due to reactivation along the faults/lineaments resulting in westward migration of the river. Thus neotectonic movements result in rapid uplift in the catchment area and a gradual regional westward tilt responsible for building up this gigantic alluvial cone.