Fluvial fans in the Himalayan Terai: A gradual shift of the Karnali River from double to single branch

Abstract. Fluvial fans in the Himalayan Terai are essential for water resources and provide crucial habitats for endangered species, including tigers. Switching of the dominant channel in these fans influences such habitats by changing the distribution of water and sediment. This study addresses such a transition in the Karnali River, one of the least human-altered large rivers in Nepal and India. For over two centuries, the Karnali maintained a double-branch system, but in recent years it has gradually consolidated into a single branch. Our primary objective is to describe this shift and to assess its trigger. By analyzing flow duration curves, fluvial fan topography, and channel properties, we suggest the cause is an extreme monsoon season in 2009, when two major peak discharges seem to have initiated the subsequent gradual deposition of coarse sediment at the upstream end of the eastern branch (Geruwa), effectively gradually plugging its flow. To better understand the balance between natural and anthropogenic influences, we compare the Karnali with the more heavily altered Koshi River. While embankments and infrastructure developments have significantly shaped the Koshi’s morphology, the Karnali’s shift appears to be driven primarily by natural sediment dynamics. Human interventions (such as embankments and existing hydropower dams) appear to have played little to no role in the transition. With rapid hydropower expansion and ongoing modifications to the river system, we anticipate that Karnali’s single-channel configuration will persist, with profound implications for water distribution and habitat conservation in Bardiya National Park.