Fluvial dynamics of an anabranching river system in Himalayan foreland basin, Baghmati river, north Bihar plains, India

Abstract

Anabranching river systems are now regarded as a separate class in river classifications owing to their distinctive morphological/hydrological characteristics and fluvial processes. A better understanding of anabranching rivers still needs detailed data from different environmental and geographical settings. This paper presents a detailed account of an anabranching river system from the Himalayan foreland basin. The Baghmati river system from north Bihar Plains, eastern India provides a typical example of an anabranching river system located in the interfan area between the Kosi and the Gandak megafans. The river system is braided in upstream reaches and meandering in downstream reaches, but the midstream anabranching reach is characterized by low width–depth ratio (11–16), gentle gradient (0.00018–0.00015), variable peak discharge, frequent flooding and high sediment load. The anabranching in the midstream reaches is a response to its inability to transport high sediment load due to gentle channel slope and dominance of aggradation process. The development of anabranches is related to rapid and frequent avulsions of the river channels with eight major avulsions observed in the 30-km-wide floodplain in the last 230 years. The decadal scale avulsion history of the Baghmati river system makes it ‘hyperavulsive’ and the major causative factors for such channel instability are sedimentological readjustments and active tectonics in the basin area.