275-6 The role of landsliding in steep mountain hydrology

Abstract:

The storage and transport of water in high mountain catchments is vital to downstream water supply and critical to the chemical weathering processes that control long-term climate and condition hillslopes for landsliding. The depths and timescales over which recharge passes through mountain catchments are modulated by the structure and hydrological properties of the near surface, which remain challenging to characterize. Landslides themselves heighten these challenges by disrupting hillslopes, yet much remains unknown about how they systematically alter water routing and storage at the catchment scale. We investigate the impact of landslides on infiltration and runoff dynamics using a hydro-geophysical observatory network in a steep mountain catchment in central Nepal as a natural laboratory (the Nepal-FRES observatory in the Melamchi River Valley). Using multiscale datasets, we evaluate the segregation of water pathways through fractured bedrock and surficial deposits with particular attention to variable recharge mechanisms and the role of landsliding in partitioning surface and subsurface flow and weathering of hillslope materials at the catchment scale. Electrical resistivity tomography (ERT) surveys and seismic interferometry (dv/v) are used to infer subsurface water pathways and hydrological structure, while a deep borehole through the Kyul landslide provides direct calibration of slope material properties and weathering states with geophysical measurements. Time-series hydrological monitoring, including δ¹⁸O water isotopes and dissolved solute concentrations from springs, along with stream discharge and precipitation records, provide information on groundwater recharge, subsurface response times, and groundwater reservoir storage capacities.  We find that landslide disturbances govern a dual-flow regime, where rapid stream discharge peaks correspond to shallow flow pathways through landslide colluvium, and slow, prolonged stream response is facilitated by a deeper fractured bedrock. Evidence that the deeper reservoir is fed by recharge at high elevations suggests that these reservoirs are largely segregated, and that precipitation on mid- and lower hillslopes is limited to the shallow reservoir. We posit that this hydrologic behavior is facilitated by sharp permeability contrasts introduced by landslides, which strip hillslopes of regolith and deposit permeable unconsolidated colluvium; these contrasts promote flow above the landslide base, preventing vertical movement into deeper reservoirs. These findings suggest that landslides play an active role in the storage and transport of mountain groundwater, providing a novel perspective on the movement of hydrological fluxes through steep mountain catchments.

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