Stability of the gravel-sand transition of the Ganga Plains recorded in Siwalik stratigraphy; implications for extreme floods

Many rivers of the Indo-Gangetic Plain are prone to abrupt switching of channel courses causing devastating floods over some of the world’s poorest and most densely populated regions. Recent work has identified the gravel-sand transition as an avulsion node for the channels; notably the avulsion of the Kosi River in 2008 occurred in close proximity to its gravel-sand transition. The gravel-sand transition is a geomorphic feature observed within all major mountain-fed, and smaller foothill-fed Himalayan rivers ranging from 10 to 20 km downstream from the mountain front. It is characterised by an abrupt downstream reduction in grain size from gravel to sand and is often associated with a break in channel gradient, which suggests it is a relatively stable feature over the last few thousands of years.

However, new subsurface data from the Kosi mega-fan in eastern Nepal reveals 10-20 Ka gravels located ~50 km downstream from the current gravel-sand transition. The implication is that this key geomorphic boundary can periodically prograde considerably further into the Ganga Plains. A greater long-term (>10⁶ yrs) understanding of the controls on the gravel-sand transition is achieved by studying the stratigraphic record of the Miocene Siwalik Group, which is exhumed as a series of thrusted fault blocks at the Himalayan mountain front. The Siwalik succession is divided into three lithofacies units that coarsens upwards from siltstones and sandstones to coarse conglomerates. The units are termed the Lower, Middle and Upper Siwaliks respectively and reflect the current depositional environments found on the Ganga Plains.
The gravel-sand transition is recorded as the contact between the Middle and Upper Siwaliks. Significant gravel pulses have been identified directly below the Middle to Upper Siwalik contact and suggests that the gravel-sand transition is indeed mobile and can episodically prograde far into the plains. Sedimentological characteristics of the gravel pulses and sediment entrainment calculations suggest that extreme events (e.g. enhanced monsoon, earthquakes and GLOFS) can force gravel far into the Ganga Plains, impacting the position the gravel-sand transition. These episodes of distant gravel progradation must represent extreme floods from which the sedimentological system must take many years to recover. Such events are beyond the historic timescales of human narrative, and hence have not been recognised as a risk to the populations of the plains.