The Influence of Geomorphology on Storage and Surface Water–Groundwater Interactions in Mountainous Headwater Streams

ABSTRACT We investigated the influence of landslide deposits on hydrologic connectivity and subsurface water movement in small headwater catchments in the Western Cascades, Oregon, USA. We examined isotopic variations in surface water across multiple catchments, comparing wet and dry periods to assess how antecedent moisture influences hydrologic connectivity and groundwater interactions. Seasonal shifts in δ 18 O values reveal that hydrologic connectivity increases during wet conditions, resulting in more uniform isotopic signatures across catchments due to enhanced vertical and lateral water movement in the subsurface. In contrast, during dry periods there was greater spatial variability in δ 18 O, reflecting localised groundwater contributions and reduced connectivity. Notably, some catchments with high proportions of earthflow terrain maintain consistent water isotopic ratios across seasons, suggesting persistent groundwater inputs from landslide deposits. Spatial patterns in δ 18 O also point to subsurface inter‐catchment flow paths facilitated by landslide deposits. Streamflow measurements during the dry season further support these findings. Catchments underlain by older, stabilised landslide deposits had highly variable unit discharge and frequent periods of flow cessation, consistent with weaker subsurface connectivity and limited water retention. In contrast, catchments draining active earthflows maintained relatively high unit discharges and perennial flow, indicating stronger subsurface linkages and greater potential for water accumulation that sustains both flow and ongoing slope movement. We estimated storage potential within landslide deposits and then used this to estimate catchment storage potential. Catchment storage was negatively correlated to variability in isotopic ratios, indicating an inverse relationship between catchment storage and variability in water sources in both space and time. Overall, our results demonstrate that geomorphic setting—particularly the presence and structure of landslide deposits—can exert strong control on the spatial distribution of hydrologic connectivity in mountain catchments. These insights improve our understanding of how subsurface properties mediate water movement and streamflow resilience under varying climate conditions.