Sedimentology and Avulsion Patterns of the Anabranching Baghmati River in the Himalayan Foreland Basin, India

Abstract

The Baghmati River, a foothills-fed system in the Himalayan foreland basin of north Bihar, has an anabranching mid-stream reach and floodplains that aggraded rapidly during the late Holocene. The river is characterized by variable discharge, frequent and widespread overbank flooding, and high sediment load. Changes in river course on a decadal time-scale have resulted in temporarily abandoned reaches that are periodically reoccupied. Chute and neck cutoffs, and crevasse splays are also prominent. Borehole logs show that the anabranching reach is underlain by sandy channel bodies up to 25 m thick, separated by mudstone units up to 30 m thick. Extrapolation of floodplain accumulation rates to the mudstones suggests that channels were stably positioned for thousands to tens of thousands of years, allowing thick muds to accumulate. Repeated reoccupation of pre-existing drainage lines may have promoted the creation of thick, narrow channel bodies. Stacked overbank deposits probably form the bulk of the floodplain sediments, but channels that avulse into floodplain lakes (tals) may generate associated avulsion deposits. The Baghmati River sediments are a modern analogue for the deposits of rapidly subsiding extensional and foreland basins in the ancient record.