Geologic and geomorphic setting control of water sources and flow paths in mountainous headwater catchments

Communities downstream from mountainous regions rely on snowmelt for water supply. As climate change reduces the reliability of the snowpack in these regions it is important to understand how and when rain and melt water are stored and released as runoff. In mountainous regions the prediction of water movement is especially complex because water storage capacity and overall water input magnitude and form vary over short distances given variable geology, geomorphology, and topography. We used water stable isotopes (WSI) and water chemistry (ions and cations) to investigate seasonal water sources and contributions in a 64 km² headwater mountain catchment in Oregon (USA). In one study, we collected > 1,000 synoptic samples between 2021 and 2022 across different seasons in 12 headwater streams (600–1,200 m in elevation and drainage areas 0.1–5 km²) and analyzed them for WSI. Results demonstrate that despite season there are localized variations in WSI within less than 1-km² between catchments underlain by similar geology but characterized by different geomorphic history of mass wasting events (landslides and earthflows). We also observed weak relationships between elevation and WSI in some streams suggesting that their sources of baseflow are not directly controlled by seasonal precipitation but by differences in storage over spatially variability geomorphic history.  In a second study, we investigated relative streamflow contributions from five tributaries (0.7–17 km2) over the whole year based on weekly WSI and water chemistry in grab and precipitation samples. We found strong differences across streams; the most interesting was a spring-fed stream, whose water contribution varies widely throughout the year, resembling a snowmelt system (with high relative water input in the summer). The depleted WSI signal and relatively high cations concentrations of this stream reveals higher elevation snowfall is moving to the stream through relatively long flow paths. This stream is underlain by porous lava flows demonstrating a strong geologic control on runoff generation. The contribution of this stream to the whole watershed is over 27 times larger in the summer compared to any other season. This finding challenges the idea of streamflow scaling with drainage area because the effective drainage area of this stream varies between 0.8 and 47 km² while the topographic derived drainage area is 0.7 km².