Development of a hyper-avulsive river course during the Holocene on the Himalayan frontal plains

Abstract

The rivers flowing over the Quaternary foreland plains of the Himalayas embody a history of hyper-avulsiveness. The present study is an attempt to identify a major flow trajectory of the Torsa River and the dynamics of the major flow path during the Holocene. This study is focused on identifying the major distributive system of the Torsa River, active during the Holocene and preceding known historical palaeochannels, using geomorphic and sedimentary archives of the floodplain on the foreland part of the eastern Himalayas. Satellite image processing, sediment analysis from trenches and cores, geochemical and paleocurrent analysis along with the determination of the age of deposits using the Infrared Stimulated Luminescence (IRSL) dating technique were used to identify paleochannels and validate the findings. Moreover, the sequence of channel succession and major avulsion events was also identified from the facies reconstruction and stratigraphy of fluvial depositions. The oscillatory nature of the river is well preserved within the floodplain stratigraphy in the form of lenses of channel deposits intercalated with proximal floodplain depositions. A total of three avulsion events were found between 1.6 ± 0.1 ka and 0.9 ± 0.1 ka where the latest significant channel switching occurred between 1.14 ± 0.1 ka and 0.9 ± 0.1 ka within the distributive system identified in this study and an eastern branch of the fluvial system. The newly identified palaeo-flow trajectory of the system can be utilised for water resource management and the avulsion regime may be useful for understanding future flow oscillations within the river basin.