Spatio-temporal variability in stream power distribution in the Upper Kosi River basin, Central Himalaya: Controls and geomorphic implications

Abstract

River processes in a drainage basin are strongly governed by the spatio-temporal variability in stream power. Recently, various approaches have been developed to derive the spatial variability of stream power in a river basin; however, the temporal variability in stream power due to hydrological changes remains poorly understood. An assessment of the temporal variability of stream power is critical for river studies in monsoonal regimes because significant geomorphic work is performed during this period, particularly in the Himalayan rivers. This paper presents a novel methodology to derive the stream power distribution at a basin-scale using a physically-based-hydrological model, the Soil and Water Assessment Tool (SWAT). Hydrological changes in the upper Kosi (also called Koshi) basin in the Central Himalaya were analysed and integrated with slope variability and channel width to obtain the basin-scale distribution of total stream power and specific stream power. Most of the tributaries of the Kosi River are characterized by a multi-peak distribution of stream power. Stream power distribution in the basin shows no systematic downstream increase or decrease of either total stream power or specific stream power. Total and specific stream power peaks during the monsoon period are 100–1500 times higher compared to the non-monsoon period and exceed the threshold for erosion and sediment transport processes. The SWAT model also helped to assess controls on the stream power distribution in the basin. The downstream distribution of stream power between confluence points is influenced by slope variability, while major increases at confluence points are due to discharge variability. Geological characteristics control the spatial distribution of stream power while the annual rainfall distribution influences the seasonal variability of stream power.