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Understanding how subsurface water storage—created and structured by the geology and geomorphology of the critical zone—governs hydrologic connectivity between landscapes and streams is essential for explaining spatial and temporal variation in stream water chemistry. Most headwater studies have focused on high‐resolution stream water chemistry at the catchment outlet, rarely examining the spatial variability among tributaries and the main channel, or how these patterns relate to the underlying geology and geomorphology. Linking upstream spatial and temporal variability with chemical dynamics at the outlet over time is even less common. We conducted weekly synoptic sampling along Lookout Creek, located within the HJ Andrews Experimental Forest Long Term Ecological Research programme. Lookout Creek is in the volcanic terrain of the western Cascades, Oregon. The catchment spans multiple geologic units (e.g., lava flows) and geomorphic features (e.g., earthflows). We measured stream chemistry along the main stem and five tributaries to assess how varying degrees of hydrologic connectivity influence solute concentrations and transport across this geologic and geomorphologic template. To identify the timing and magnitude of hydrologic connectivity between tributaries, the main stem, and the catchment outlet, we analysed spatiotemporal patterns in stream chemistry using concentration‐discharge relationships, principal component analysis, and a metric of subcatchment synchrony. We found that in previously glaciated catchments with active earthflows, solute concentrations and base‐cation‐to‐silica ratios were higher, and more solutes had a chemostatic or mobilising behaviour, indicating high subsurface storage. This variability in subsurface storage, and its influence on hydrologic connectivity, ultimately determined the degree of chemical synchrony with the catchment outlet. Our findings suggest that, under future climate scenarios with shifts in precipitation phase and timing, headwater systems with substantial subsurface storage are likely to be more chemically resilient. 

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 Changing climate conditions are expected to cause increases in the frequency and severity of drought conditions in many areas around the world, including the Pacific Northwest region of North America. While drought impacts manifest across the landscape, headwater streams are particularly susceptible to droughts due to limited deep‐water habitats and low water volumes that allow for substantial increases in water temperature. While low volumes of water and increased stream temperature will likely affect all aquatic species to some degree, the response of different taxa to these impacts is expected to vary with differences in physiological needs and habitat preferences among species. Using a before–after control‐impact (BACI) experimental design, this study investigates how reduced streamflow and increased stream temperature affect the two dominant apex predators in headwater streams of the Pacific Northwest, coastal cutthroat trout (Oncorhynchus clarkii clarkii) and coastal giant salamander (Dicamptodon tenebrosus). In a second‐order stream in the H.J. Andrews Experimental Forest in OR, USA, experimental flow diversions created decoupled drought conditions of reduced streamflow and elevated temperatures. Low‐flow conditions were created by diverting water around a 100‐m stream reach and this diverted water was passively warmed before re‐entering a downstream channel to create an increased temperature reach. We compared fish and salamander abundances and stream habitat in an upstream unmanipulated reference reach to the two experimental reaches. Relative increases in temperature ranged between 0.41 and 0.63°C, reflecting realistic stream warming in this region during drought events. Trout responded positively to increased temperatures, showing an increase in abundance, biomass, condition factor, and growth, whereas salamanders responded negatively in all metrics except condition. The low‐flow reach diverted approximately 50% of the flow, resulting in a relative pool area reduction of about 20%. Relative to the reference reach, salamanders displayed a net positive abundance response while trout declined in the low‐flow reach. The contrasting responses of these populations to decoupled drought conditions suggest that interactions of flow and temperature changes together will influence drought responses of the vertebrate communities of headwater streams. 

The protection of headwater streams faces increasing challenges, exemplified by limited global recognition of headwater contributions to watershed resiliency and a recent US Supreme Court decision limiting federal safeguards. Despite accounting for ~77% of global river networks, the lack of adequate headwaters protections is caused, in part, by limited information on their extent and functions—in particular, their flow regimes, which form the foundation for decision-making regarding their protection. Yet, headwater streamflow is challenging to comprehensively measure and model; it is highly variable and sensitive to changes in land use, management and climate. Modelling headwater streamflow to quantify its cumulative contributions to downstream river networks requires an integrative understanding across local hillslope and channel (that is, watershed) processes. Here we begin to address this challenge by proposing a consistent definition for headwater systems and streams, evaluating how headwater streamflow is characterized and advocating for closing gaps in headwater streamflow data collection, modelling and synthesis. 

Tracer-aided hydrological models (TAHMs) are one of the most powerful tools to identify new (event) and old (pre-event) water fractions contributing to stormflow because they account both for streamflow and tracer mixing dynamics in model calibration. Nevertheless, their representativeness of hydrograph dynamics is often limited due to the unavailability of high-resolution conservative tracer data (e.g., water stable isotopes or chloride). Hence, there is a need to identify alternative tracers yielding similar flow partitioning results than “ideal” ones while requiring fewer financial resources for high-frequency monitoring (e.g., sub-hourly). Here, wecompare flow partitioning results of a TAHM calibrated using high-frequency electrical conductivity (EC) and water stable isotope (18O) data collected during 37 rainfall-runoff events monitored during variable hydrometeorological conditions in the Zhurucay Ecohydrological Observatory, a tropical alpine catchment located in southern Ecuador. When the model was calibrated using the sampling resolution of stables isotopes (6-hours to 1-hour), no statistically significant differences of pre-event water fractions (PEWFs) using both tracers for model calibration were found. PEWF differences between both tracers for 89% of the events were < 20% regardless of the events’ antecedent moisture and rainfall conditions. Model transfer functions were also similar suggesting that catchment internal processes inferred using both tracers are comparable. Events presenting larger differences (n = 4; up to 27% PEWF difference) had no samples collected during peak flow. Calibration of the model using EC data collected at sub-hourly intervals (every 5-minutes) showed a significant increase in model performance as compared to the frequency of collection of isotopic data. Similarity in flow partitioning results can be attributed to a quasi-conservative nature of EC due to the presence of organic-rich riparian soils (peat-type) overlying compact bedrock across the catchment. Findings also highlight the importance of capturing rapidly occurring catchment mixing processes though high-temporal frequency monitoring of tracer data. Our study encourages the value of assessing the use of alternative tracers, such as EC, to identify fast occurring rainfall runoff processes, while lowering the costs needed to implement and sustain tracer data collection for long time periods. 

Abstract Humans affect ecosystems in many ways, and scientific field studies are no exception. If data collection disrupts environments or biota too much, it can lead to inaccurate conclusions in the study of interest or in subsequent studies. We evaluated whether stream electrofishing surveys could measurably disturb the benthic biofilms in two forested headwaters in western Oregon, USA. While the consequences of electrofishing to macroinvertebrates and fish have been assessed, to date no studies have quantified its influence on benthic biofilms. We observed declines in the standing stocks of accrued benthic chlorophyll a directly after electrofishing in both streams. After electrofishing, the standing biofilm stocks declined by an average of ~15% in Oak Creek, a small third-order stream in the Oregon Coast Range Mountains, and by an average of ~34% in a third-order section of Lookout Creek, which is located in the western Cascade Mountains of Oregon, USA. In returning to Oak Creek 2 weeks after electrofishing, the standing stocks had fully recovered to their prefishing levels. While the benthic biofilm standing stocks did decline in association with electrofishing, the effects were small when compared with those of disturbances from common flow events and when scaling to the whole stream system. In Oak Creek, the proportional biofilm standing stock decline from electrofishing activity was about 26% of what was observed following a moderate flow event (40% of bank-full discharge), and about 15% of the decline in biofilm standing stocks following a complete bank-full discharge event (140% of bank-full discharge).