River flow regimes strongly shape floodplain ecosystems. However, human activities are increasingly altering these freshwater systems with uncertain consequences for floodplaindependent wildlife. The subtropical Karnali River floodplains at the foot of the Himalayas are a prime example of this. We previously showed that since the shift of the main discharge channel (Geruwa River) away from the western boundary of Bardia National Park in 2009, this reduction in the river flow regime was followed by lower dry-season water availability, reduced hydromorphological dynamics, and the expansion and changes in floodplain vegetation. These changes in the river system can strongly influence the pioneer grasslands and wetlands that serve as the primary forage for the vulnerable greater one-horned rhino (Rhinoceros unicornis), a floodplain habitat specialist, and support prey base of the endangered Bengal tiger (Panthera tigris tigris). However, quantitative relationships between altered flow regimes and the habitat quantity and quality of terrestrial wildlife remain understudied.

The dynamics of the bifurcating Karnali river in the western plains of Nepal and India is governed by the geomorphological processes in an alluvial fan. The dynamic branches showcase a notable degree of braiding, dominant channel switching and unequal discharge partitioning. Since recent switching of the dominant channel of Karnali system occurred after an intense monsoon in 2009, the eastern Geruwa branch of the system, which used to be dominant channel passing through the Bardiya National Park, is now receiving a lower share of discharge. This situation exacerbates in the low flow periods when there is very small flow in the Geruwa branch. This decreasing discharge has been associated with depleting diversity of wildlife habitat in Bardiya National Park (Bijlmakers et al., 2023). For sustainable habitat management in the Bardiya National Park, there is a necessity to study the dynamic Karnali river and its two branches, the eastern Geruwa branch and the western Kauriala branch. Activities such as sediment mining, construction of irrigation and hydropower and inter-basin water transfer projects will potentially influence the system dynamics. Our objective is to understand the switching behaviour of the Karnali system to the natural dynamics such as bend sorting (Baar et al., 2020; Parker & Andrews, 1985) of sediments at the location where water from the main Karnali enters the Geruwa branch, and offer understanding of system response to human interventions especially with regards to the distribution of discharge between the Geruwa and Kauriala branches. We combine the technique of field observations and numerical modelling to study the system.

Rivers flowing from the highlands to the lowlands, characterize the Terai Arc Landscape (TAL) in Nepal and India, but also the neighbouring region in Bhutan. The sudden change in slope results in deposition of nutrient-rich sediment, which forms large alluvial or fluvial fans that enhance biodiversity and economic activities in the region. Our objective is to identify similarities and differences in hydro-morphodynamics between the TAL rivers, with a focus on the Karnali and Koshi systems in Nepal. It is expected that such insight in river dynamics will aid the sustainability and management of the TAL rivers. To this end, we investigate and compare discharge characteristics, historical flow paths, and morphodynamic characteristics such as channel shifting, braiding, and flow partitioning to identify and understand the fan scale dynamics of the two systems. Hereto we used optical remote sensing images and river discharge time series from Chisapani (Karnali) and Chatara (Koshi) gauging stations. In addition, we conducted a field campaign in November 2022.

Himalayan rivers in Nepal flow through the mountains with high gradient to emerge in lowlands as large rivers carrying enormous amount of discharge and sediments. They release significant quantity of sediment forming alluvial fan as a result of sudden decrease in gradient when they enter the lowland and gain braided form. This braided form has made the river morphologically more dynamic in nature. Division of channels into numerous anabranches leads to formation of temporary or permanent islands in between them. These islands in long run are either eroded gradually by the river channel or develop into vegetated islands. The development of vegetation may be long term with growth of trees or they may develop into grasslands that may be seasonal which is usually inundated during floods. The river channels and islands along with the surrounding floodplain with vegetation act as perfect recipe for the development of complex wetland ecosystem.

Koshi River in Nepal is among such rivers emerging from the mountains to flat plains of Terai thereby flowing into multiple channels within a large width of about 5 km, which is then controlled by Koshi Barrage at 41 km from the gorge. This dynamic river system feeds the Koshi Tappu Wildlife Reserve, a Ramsar site in the reach. The change in river course and vegetation of this large area which otherwise would be challenging to study can be done rather easily by the use of satellite imageries and cloud computing. Google Earth Engine (GEE) has been used in this study for analysing the morphological changes of the river as well as vegetation changes within the study area using the multiple satellite images taken at different times. NDWI has been calculated and used to identify the occurrence of water in the river channels, thus the morphological changes. While NDVI is used for intensity of vegetation. The temporal and spatial analysis of the morphodynamics and corresponding changes in vegetation is performed from 1987 to 2020 within the selected area.

The preliminary assessment of the results shows that the vegetation dynamics of the area has been affected by the continuous erosion and deposition caused by the morphological changes apparently due to the barrage. Over time, river has been channelizing and branching several times causing the existing islands to erode along with their vegetation as well as forming new islands with vegetation cover. This shifting of the river and resulting vegetation dynamics appear to have affected the habitat of the wild water buffaloes (Arna) as well as, other endangered species native to the area. Additional analysis on the effect of river morphology and vegetation dynamics to the flood pattern and other ecological components will be carried out to support the initial findings and draw generalized conclusions.