This study investigates the hydro-morphological processes driving the channel shift of the Karnali River in Western Nepal after the 2009 monsoon season. Exceptionally high discharge during this year caused significant morphological changes to the bifurcation system, leading to a drastic discharge reduction in the branch that flows into and provides habitat for the Bardiya National Park. The park is home to various endangered species, such as the Royal Bengal Tiger, and the change in flow distribution is impacting ecosystem dynamics and increasing human-wildlife conflict. An extensive field campaign, combined with satellite imagery analysis of planform evolution,  provided the foundation for a conceptual model. Two hypotheses are tested: (1) bend sorting and flood-induced aggradation in the Geruwa branch, and (2) erosion-driven opening of the Kauriala branch. These hypotheses are tested using a schematized version of the bifurcation system in a depth-averaged 2D hydro-morphological numerical model (Delft3D).  Model results suggest a self-reinforcing feedback loop initiated by increased sediment load during peak flows in the Geruwa branch, leading to aggradation and reduced flow capacity. This process promoted further sedimentation and the eventual partial closure of the branch. The model highlights the critical role of mixed-sized sediment interactions, particularly the processes that influence the local sediment supply of finer grain sizes, such as hiding and exposure. Additionally, the processes that influence the direction of sediment particles, namely secondary flow and transverse bed slope gravity pull, significantly impact the discharge distribution. The size, magnitude, and direction of particles towards a particular branch can lead to exceeding its transport capacity and subsequent aggradation, resulting in the dominance of the other branch. Furthermore, the model reveals limited sensitivity to slope changes in the Kauriala branch. Although limited by the lack of calibration and validation data, the results from this schematized model offer valuable insights into the drivers behind the real-world system and may inform future studies and potential interventions.

Growing concerns about elevated nitrate levels in natural springs on the southern slope of the Irazu Volcano, Cartago province in Costa Rica, were the driving force for a multi-disciplinary study on the problem. The region is characterized by its high agricultural output and steep slopes. More than sixty springs located in the area are managed by a large number of local water authorities, ASADAS. The study focused on determining the main sources of nitrate pollution. Anthropological activities such as agricultural practices and domestic actions are found to contribute the most. A multivariate polynomial regression model was used with a large set of parameters. From the results it can be seen that human activities within the by law determined 200 meter radius protection zone around the springs, are most influential on high nitrate concentrations in the springs. Furthermore a stakeholder analysis, fieldwork, financial analysis of alternatives and farmer interviews were performed to produce a comprehensive list of recommendations for the various stakeholders. The recommendations are compiled to ensure a future with clean drinking water for all the citizens in the region.

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. @en