Absence of well-developed floodplains along the lowland rivers and controls of the hydro-geomorphic conditions in the Western Ghat, India

Abstract

Lowland rivers across the world are characterized by accumulation of sediment due to a decrease in the channel slope. Despite the antiquity of the topography, the lowland part of the west-flowing rivers of Western Ghat, India is characterized by poorly developed floodplains except for the southern part of the Ghat. This poses a question of the sediment transportation and deposition capacity in these lowland rivers, which in turn governs geomorphic characteristics. In this study, we adapted a novel method to integrate hydrological model-derived discharge estimates, channel slope estimates from digital elevation models, bankfull channel width estimates from high-resolution Google Earth satellite images and field observations. We modelled the downstream distribution of total stream power ( ), specific stream power ( ) and shear stress ( ) for four west-flowing river systems from different lithological and climatic settings. The 75th percentile discharge from monthly time series data was used in modelling to assess channel processes. The peak values of are distributed all along the longitudinal profile, whereas the peak values of and are consistently higher in the escarpment. Scaling relationships between hydrogeomorphic parameters were also established to understand the inherent controls on the distribution of these driving parameters. We inferred that is sufficient to transport large grain sizes even in the low-gradient lowland stretches. Hence sediments are mostly absent in the channel, and floodplains are not well developed. Despite having a low gradient, the lowland coastal rivers can efficiently evacuate most of the eroded sediments to the offshore region. Finally, the lithological control is manifested by the variability of channel width regulating the sediment transport in the coastal rivers of the Western Ghat.