Chemical weathering affects the material properties of rock and soil — thereby influencing the pace and mechanisms of erosion including landslides — and produces dissolved elements in groundwater and surface waters. These solutes determine water quality and regulate the long-term global carbon cycle by taking up carbon dioxide. High weathering rates in the Himalaya have led to heated debates over the potential role of mountain building in long-term evolution of global climate. The south-north gradient in the Melamchi Valley offers an exciting opportunity to measure how chemical weathering responds to the dramatic change in erosion and topography associated with uplift of the High Himalaya. While much work has been done on this general topic, the deep weathering that occurs in steep, rapidly eroding mountains is still not well characterized.

We are quantifying weathering across the Melamchi Valley by collecting samples of exposed soils and underlying weathered rock, and by drilling a series of boreholes to probe weathering at depths of 50-100 meters. We are analyzing the bulk composition of these samples as well as their mineralogy and the structure of fracture systems.

In tandem, and in close coordination with our work on water flowpaths and transit times (see below), we are using the chemical and isotopic compositions of spring and stream waters to better understand water flow paths and mineral-fluid reactions which determine the water chemistries — providing a complementary dataset on weathering processes and rates.

Mountains have been described as the water towers for the world, because they are the source for a large proportion of the global fresh water supply. This water is delivered to downstream environments and human communities by streams and rivers. Streamflow is sustained year-round through storage in glaciers, snow, and mountain groundwater aquifers, but many questions remain about the role of groundwater storage and transfer within mountains. Understanding how long it takes water to transit from precipitation, through groundwater, and into streamflow is critical for understanding how mountains will buffer changes in water supply in a changing climate.

We are using two main approaches to determine water transit times and flow paths in the Melamchi Valley: (i) time-series of stable isotopes in water (¹⁸O and ²H) from three small catchments to understand young (2-3 month old) water contributions to streamflow and (ii) multiple transient tracers (³H/³He, SF₆ and CFCs) in groundwater springs throughout the valley that allow us to better understand older (1-50 year old) water contributions.

We aim to gain general understanding through this work about the main controls on water transit times, as well as locally-relevant information: streams and springs are used by the residents of the Melamchi Valley for all their water requirements, and the Melamchi River supplies water to the Kathmandu Valley as part of the Melamchi Water Supply Project.

Landslides impact lives and infrastructure while also shaping landscapes over time. In steep mountainous terrain, landslides are the main erosional process that detaches rock from valley sides and transports it into river beds, and eventually out of mountain ranges as sediment.

Understanding landslide occurrence during environmental stressors such as earthquakes and storms requires information about the topography as well as the critical zone structure, and the associated coupling between chemical weathering and the geomechanical properties of hillslopes. We are developing a multi-scale approach to connect macro-scale observations (i.e., multi-temporal landslide inventories across the Melamchi Khola based on mapping from satellite images) to the critical zone architecture assessed using scalable geophysical approaches and geomechanical field observations (at the meso-scale) and the strength of the material for given rock structure and state (quantified through detailed geomechanical and geochemical testing of our outcrop and borehole samples). By linking the across these scales, we aim to gain fundamental understanding of when, where, and why landslides occur over the large area of the Melamchi Khola.

Building on this knowledge, we are modeling the influence of landslides on erosion and sediment transport rates within the Melamchi valley. Using topographic data of the area, landslide inventories and geomechanical properties obtained in the field, we vary the triggering conditions and rheology of landslides to assess their consequences on sediment transport. These results can then be analyzed in terms of risk to the inhabitants and infrastructures of the region, as well as the role of landslide erosion in shaping this landscape.