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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 δ¹⁸O 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 δ¹⁸O, 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 δ¹⁸O 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. 

Abstract As Arctic warming accelerates, understanding hydroclimate shifts is key to projecting glacier melt and sea‐level rise. We assess the climatic signature of the Little Ice Age (LIA; ∼1250 to 1900) by quantifying changes in equilibrium‐line altitude (ΔELA) for 215 Alaskan glaciers from the LIA maximum to present (2016–2024), using remote sensing and geographic information system methods. ELAs have risen by 170 ± 8 m, equivalent to 1.6 ± 0.3°C summer warming (assuming constant precipitation) or 248 ± 89 mm w.e. annual precipitation increase (assuming 2.3°C warming). The latter is ∼4X the precipitation change observed since 1950. Glacier morphology and topographic setting explain 32% of ΔELA variance, likely reflecting differing sensitivities to climatic shifts and elevation‐dependent warming. Spatially interpolated ΔELA residuals are most strongly correlated to winter precipitation (r = −0.67). Results suggest the LIA was characterized by (a) colder, drier conditions and (b) a weak, westward‐displaced Aleutian Low that has since strengthened and shifted eastward. 

Understanding how diverse headwater streams contribute water downstream is critical for accurate modelling of seasonal flow dynamics in larger systems. This study investigated how headwater catchments, with diverse subsurface storage, influence downstream flows within Lookout Creek—a 62 km², 5th‐order catchment in the rain‐snow transition zone in western Oregon, USA. We analysed one year of hydrometric and water stable isotope data collected at 10 stream locations, complemented by a decade of precipitation isotopic data. As expected, isotopic data revealed that most of the streamflow was sourced from large fall and winter storms. Generally, stream isotope ratios decrease with elevation. However, some streams had higher isotopic values than expected, reflecting the influence of isotopically heavy storms and relatively low storage. Other streams that tended to have low flow variability in response to precipitation inputs had lower isotopic values, indicating higher elevation water sources than their topographic watershed boundaries. Both hydrometric data and water isotope‐based end‐member mixing models suggest storage differences among headwater catchments influenced the seasonal water contributions from tributaries. Most notably, the contributions of Cold and Longer Creeks, which occupy less than 10% of the Lookout Creek drainage area, sustain up to 50% of the streamflow in the summer. These catchments have high storage and high groundwater contributions, as evidenced by flat flow duration curves. Finally, our data suggest that geologic variability and geomorphic complexity (presence of earthflows and landslides) can be indicators of storage that dramatically influence water movement through the critical zone, the variation in streamflow, and the response of streams to precipitation events. Heterogeneity in headwater catchment storage is key to understanding flow dynamics in mountainous regions and the response of streams to changes in climate and other disturbances. 

Abstract Field measurements of hydrologic tracers indicate varying magnitudes of geochemical separation between subsurface pore waters. The potential for conventional soil physics alone to explain isotopic differences between preferential flow and tightly-bound water remains unclear. Here, we explore physical drivers of isotopic separations using 650 different model configurations of soil, climate, and mobile/immobile soil-water domain characteristics, without confounding fractionation or plant uptake effects. We find simulations with coarser soils and less precipitation led to reduced separation between pore spaces and drainage. Amplified separations are found with larger immobile domains and, to a lesser extent, higher mobile-immobile transfer rates. Nonetheless, isotopic separations remained small (<4‰ for δ 2 H) across simulations, indicating that contrasting transport dynamics generate limited geochemical differences. Therefore, conventional soil physics alone are unlikely to explain large ecohydrological separations observed elsewhere, and further efforts aimed at reducing methodological artifacts, refining understanding of fractionation processes, and investigating new physiochemical mechanisms are needed.