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Abstract In snowmelt‐driven mountain watersheds, the hydrologic connectivity between meteoric waters and stream flow generation varies strongly with the season, reflecting variable connection to soil and groundwater storage within the watershed. This variable connectivity regulates how streamflow generation mechanisms transform the seasonal and elevational variation in oxygen and hydrogen isotopic composition (δ18O and δD) of meteoric precipitation. Thus, water isotopes in stream flow can signal immediate connectivity or more prolonged mixing, especially in high‐relief mountainous catchments. We characterized δ18O and δD values in stream water along an elevational gradient in a mountain headwater catchment in southwestern Montana. Stream water isotopic compositions related most strongly to elevation between February and March, exhibiting higher δ18O and δD values with decreasing elevation. These elevational isotopic lapse rates likely reflect increased connection between stream flow and proximal snow‐derived water sources heavily subject to elevational isotopic effects. These patterns disappeared during summer sampling, when consistently lower δ18O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ18O and δD values expected in warmer seasonal rainfall. The consistently low isotopic values and absence of a trend with elevation during summer suggest lower connectivity between summer precipitation and stream flow generation as a consequence of drier soils and greater transpiration. As further evidence of intermittent seasonal connectivity between the stream and adjacent groundwaters, we observed a late‐winter flush of nitrate into the stream at higher elevations, consistent with increased connection to accumulating mineralized nitrogen in riparian wetlands. This pattern was distinct from mid‐summer patterns of nitrate loading at lower elevations that suggested heightened human recreational activity along the stream corridor. These observations provide insights linking stream flow generation and seasonal water storage in high elevation mountainous watersheds. Greater understanding of the connections between surface water, soil water and groundwater in these environments will help predict how the quality and quantity of mountain runoff will respond to changing climate and allow better informed water management decisions.
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This content will become publicly available on August 1, 2026
Insights Into Heterogeneous Streamflow Generation Processes and Water Contribution in Forested Headwaters
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.
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- Award ID(s):
- 2025755
- PAR ID:
- 10644589
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Hydrological Processes
- Volume:
- 39
- Issue:
- 8
- ISSN:
- 0885-6087
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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