Subsidence control on river morphology and grain size in the Ganga Plain

The Ganga Plain represents a large proportion of the current foreland 14 basin to the Himalaya. The Himalayan-sourced waters irrigate the Plain via major river 15 networks that support approximately 10% of the global population. However, some of 16 these rivers are also the source of devastating floods. The tendency for some of these 17 rivers to flood is directly linked to their large scale morphology. In general, the rivers 18 that drain the east Ganga Plain have channels that are perched at a higher elevation 19 relative to their floodplain, leading to more frequent channel avulsion and flooding. In 20 contrast, those further west have channels that are incised into the floodplain and are 21 historically less prone to flooding. Understanding the controls on these contrasting river 22 forms is fundamental to determining the sensitivity of these systems to projected climate 23 change and the growing water resource demands across the Plain. Here, we present a new 24 basin scale approach to quantifying floodplain and channel topography that identifies 25 areas where channels are super-elevated or entrenched relative to their adjacent 26 floodplain. We explore the probable controls on these observations through an analysis 27 of basin subsidence rates, sediment grain size data and sediment supply from the main 28 river systems that traverse the Plain (Yamuna, Ganga, Karnali, Gandak and Kosi 29 rivers). Subsidence rates are approximated by combining basement profiles derived 30 from seismic data with known convergence velocities; results suggest a more slowly 31 subsiding basin in the west than the east. Grain size fining rates are also used as a proxy 32 for relative subsidence rates along the strike of the basin; the results also indicate higher 33 fining rates (and hence subsidence rates for given sediment supply) in the east. By 34 integrating these observations, we propose that higher subsidence rates are responsible 35 for a deeper basin in the east with perched, low gradient river systems that are relatively 36 insensitive to climatically driven changes in base-level. In contrast, the lower subsidence 37 rates in the west are associated with a higher elevation basin topography, and entrenched 38 river systems recording climatically induced lowering of river base-levels during the 39 Holocene. 40 41 42