Source-to-sink patterns of grain size along the Yamuna River in the Indian Himalaya

Abstract

Material supply, water transport, and river engineering structures such as dams and barrages potentially disrupt and alter the downstream grain size pattern in a coarse-grained stream. Here, we document such complex interactions for a 70 km-long stretch of the Yamuna River and its tributaries in the western Indian Himalaya, where we quantify the downstream pattern of grain size. We particularly illustrate the grain size pattern changes in response to the hillslope-derived material supply to the stream path, and evaluate how such patterns evolve as the stream crosses a tectonically active mountain belt and finally enters a foreland basin. We also look into whether anthropogenic constructions like dams and barrages disrupt the source-to-sink sediment cascade in this stream. Grain size was measured on digital images taken at 18 locations on the gravel bars > 1000 m² with an uncrewed aerial vehicle (UAV), and the data was supplemented with previously published datasets. The results show that the material is at its coarsest and characterized by a large intra-bar variability of grain size (poorer sorting) where the river crosses the deeply dissected mountainous terrain. This suggests a supply control on the grain size distribution where material supply by hillslope processes in a steep terrain imprints the size and the sorting of the material in the channel. Such a supply control is also inferred along the Yamuna River reach across the smooth landscapes of the Dehradun valley, which hosts the terrace sequences deposited by the Yamuna River and its tributaries during the Pleistocene, and which is currently dissected. Erosional recycling of these terrace sediments may explain why, across the Dehradun valley, the grains in the Yamuna River are nearly uniform in size. As the Yamuna River enters the Ganga foreland basin that experiences active subsidence, the material becomes finer-grained and better sorted (low intra-bar variability of grain size). The stream finally transitions from a pebbly to a sandy stream c. 40 km downstream of the orogenic front. We interpret this downstream fining trend in the Ganga foreland basin as a result of the selective deposition of the coarser-grained fraction of the bedload material, driven by the formation of accommodation space in the subsiding foreland basin. We finally found that the impact of engineering structures, including barrages and dams, on grain size is limited. Overall, this investigation demonstrates that the particle size distributions in this coarse-grained stream represent a complex system of supply and sedimentation controls, with no discernible impact from anthropogenic structures on grain size.