Controls on sediment provenance in the Baghmati river catchment, Central Himalaya, India

Rapidly uplifting Himalaya contributes a huge sediment load that governs the morphological characteristics of the rivers draining it and the flood hazards associated with them. Sediment budgeting of the Himalayan rivers has been a challenge in geomorphology due to complex lithotectonic terrains, varied tectonic activity and rainfall distribution, and extremely diverse topography. Such a situation calls for studies of individual catchments where sediment contributions from different lithotectonic and topographic units can be estimated and controls on sediment yield can be determined to inform understanding of local-scale geomorphic processes and hazard management. This study of Baghmati river determines the sediment contributions of the Nepal Himalaya from the Lesser Himalaya (LH) (with crystalline rocks, high relief and low tectonic activity), and the Siwaliks (with soft rocks, low relief and intense tectonic activity). The sediment contributions from the two lithotectonic terrains through time were determined using: (i) geochemical tracers of optically dated alluvial sediments; (ii) an empirical model based on physical processes, and (iii) a decade long record of hydrological data downstream of the mountain front. The key results are: the low-relief (Siwaliks) terrain provides more sediment compared to the LH because of lithology and tectonics and implies that in tectonically active terrains, relief acts as a secondary control on sediment delivery. The contribution of the Siwaliks to the sediments in Himalayan rivers can be significant and should not be ignored.