Jr Brooks

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.

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.

Snowpacks are an important water source for mountainous rivers, worldwide. The timing and volume of streamflow in systems reliant on snowmelt can be affected by changes in snow accumulation and melt time. In the Cascade Range (western USA), seasonal snowpacks are predicted to decrease by over 50% within the next century. During the last decade, Cascade Range snowpacks have varied between 17% and 150% of the median 1981–2023 peak snowpack values. To understand how snowpack variation could affect Willamette River streamflow, we monitored water stable isotopes over 13 years from two sites on the mainstem and 60 streams draining small catchments across the Willamette River Basin. Small catchment water stable isotope values integrated and dampened variation in precipitation isotopes and varied with elevation, providing a marker for determining the mean elevation from which streamflow in the Willamette River was derived. During winter, while snow accumulates in the mountains, most streamflow in the Willamette River originates from rainfall at lower elevations. During summer low‐flow conditions, most streamflow in the river was derived from winter snow that accumulated at elevations above 1200 m, which represents < 12% of the Willamette River Basin area. Peak snow water equivalent from the previous winter was positively correlated with the proportion of Willamette River streamflow derived from > 1200 m during the summer low‐flow period, but both high elevation (> 1200 m) precipitation and temperature trends explained nearly as much variance as snow water equivalent. However, after accounting for climate trends, the estimated amount of high‐elevation streamflow in the Willamette River during summer low‐flow has decreased over the past 13 years. Improved understanding of the origin of, and trends in, summer streamflow in the Willamette River will aid in reconciling human demands with biological instream requirements during periods of low snowpack.

A substantial fraction of nitrogen (N) fertilizer applied in agricultural systems is not incorporated into crops and moves below the rooting zone as nitrate (NO ₃ ⁻ ). Understanding mechanisms for soil N retention below the rooting zone and leaching to groundwater is essential for our ability to track the fate of added N. We used dual stable isotopes of nitrate (δ ¹⁵ N–NO ₃ ⁻ and δ ¹⁸ O–NO ₃ ⁻ ) and water (δ ¹⁸ O–H ₂ O and δ ² H–H ₂ O) to understand the mechanisms driving nitrate leaching at three depths (0.8, 1.5, and 3.0 m) of an irrigated corn field sampled every 2 weeks from 2016 to 2020 in the southern Willamette Valley, Oregon, USA. Distinct periods of high nitrate concentrations with lower δ ¹⁵ N–NO ₃ ⁻ values indicated that a portion of that nitrate was from recent fertilizer applications. We used a mixing model to quantify nitrate fluxes associated with recently added fertilizer N versus older, legacy soil N during these “fertilizer signal periods.” Nitrate leached below 3.0 m in these periods made up a larger proportion of the total N leached at that depth (∼52%) versus the two shallower depths (∼13%–16%), indicating preferential movement of recently applied fertilizer N through the deep soil into groundwater. Further, N associated with recent fertilizer additions leached more easily when compared to remobilized legacy N. A high volume of fall and winter precipitation may push residual fertilizer N to depth, potentially posing a larger threat to groundwater than legacy N. Optimizing fertilizer N additions could minimize fertilizer losses and reduce nitrate leaching to groundwater.

The National Water Model (NWM) provides critical analyses and projections of streamflow that support water management decisions. However, the NWM performs poorly in lower elevation rivers of the western United States (US). The accuracy of the NWM depends on the fidelity of the model inputs and the representation and calibration of model processes and water sources. To evaluate the NWM, we performed a water isotope (δ¹⁸O and δ²H) mass balance using long term mean summer hydrologic fluxes between 2000 and 2019, and gridded precipitation and groundwater isotope ratios. We compared the NWM-flux-estimated (‘model’) river reach isotope ratios to 4503 in-stream water isotope observations in 877 reaches across 5 basins in the western US. A simple regression between observed and mass balance estimated isotope ratios explained 57.9 % (δ¹⁸O) and 67.1 % (δ²H) of variance, though observations were 0.5 ‰ (δ¹⁸O) and 4.8 ‰ (δ²H) higher, on average, than mass balance estimates. The unexplained variance suggest that the NWM does not include all relevant water fluxes to rivers. To infer possible missing water fluxes, we evaluated patterns in observation-model differences using δ¹⁸O_diff (δ¹⁸O_obs − δ¹⁸O_mod) and d_diff (δ²H_diff −8∗δ¹⁸O_diff). We detected evapoconcentration of observations relative to model estimates (negative d_diff and positive δ¹⁸O_diff) at lower elevation, higher stream order, arid sites. The catchment actual evaporation to precipitation ratio, the fraction of streamflow estimated to be derived from agricultural irrigation, and whether a site was reservoir-affected were all significant predictors of d_diff in a linear mixed effects model, with up to 15.1 % of variance explained by fixed effects. This finding is supported by patterns in groundwater levels and groundwater isotope ratios, and suggests the importance of including irrigation return flows to rivers, especially in lower elevation, higher stream order, arid rivers of the Western US.

Stream macroinvertebrate assemblages are shaped by natural and human‐related factors that operate through complex hierarchical pathways. Quantifying these relationships can provide additional insights into stream ecological assessment. We applied a structural equation modeling framework to evaluate hypothesized pathways by which watershed, riparian, and in‐stream factors affect benthic macroinvertebrate condition in the Western Mountains (WMT) and Xeric (XER) ecoregions in the United States. We developed a conceptual model grounded in theory, empirical evidence, and expert opinion to evaluate the following hypotheses: (1) macroinvertebrate assemblages are primarily driven by proximal, in‐stream factors (e.g., water quality and physical habitat); (2) anthropogenic land uses affect macroinvertebrates indirectly by altering in‐stream characteristics; and (3) riparian vegetation cover attenuates land use effects. We tested our model separately on three measures of benthic macroinvertebrate assemblage condition: ratio of observed‐to‐expected taxonomic richness (O/E); a multimetric index (MMI); and richness of Ephemeroptera, Plecoptera, and Trichoptera taxa (EPT). In the WMT, site‐level riparian cover, in‐stream physical habitat (relative bed stability), and water chemistry (total nitrogen) were the top three predictors of macroinvertebrate assemblages, each having over two times the magnitude of effect on macroinvertebrates compared with watershed‐level predictors. In the arid XER, annual precipitation and stream flow characteristics were top predictors of macroinvertebrate assemblages and had similar magnitudes of effect as in‐stream water chemistry. Path analyses revealed that land use activities in the watershed and at the stream site degraded macroinvertebrate assemblages indirectly by altering relative bed stability, water quality, and riparian cover/complexity. Increased riparian cover was associated with greater macroinvertebrate condition by reducing land use impacts on stream flow, streambed substrate, and water quality, but the pathways differed among ecoregions. In the WMT, site‐level riparian cover affected macroinvertebrate assemblages partly through indirect pathways associated with greater streambed stability and reduced total nitrogen concentrations. In contrast, in the XER, watershed‐level riparian cover affected macroinvertebrate assemblages through greater specific stream power. Identifying the relative effects of and pathways by which natural and anthropogenic factors affect macroinvertebrates can serve as a framework for prioritizing management and conservation efforts.

Air quality regulations have led to decreased nitrogen (N) and sulfur deposition across the conterminous United States (CONUS) during the last several decades, particularly in the eastern parts. But it is unclear if declining deposition has altered stream N at large scales. We compared watershed N inputs with N chemistry from over 2,000 CONUS streams where deposition was the largest N input to the watershed. Weighted change analysis showed that deposition declined across most watersheds, especially in the Eastern CONUS. Nationally, declining N deposition was not associated with significant large-scale declines in stream nitrate concentration. Instead, significant increases in stream dissolved organic carbon (DOC) and total organic N (TON) were widespread across regions. Possible mechanisms behind these increases include declines in acidity and/or ionic strength drivers, changes in carbon availability, and/or climate variables. Our results also reveal a declining trend of DOC/TON ratio over the entire study period, primarily influenced by the trend in the Eastern region, suggesting the rate of increase in stream TON exceeded the rate of increase in DOC concentration during this period. Our results illustrate the complexity of nutrient cycling that links long-term atmospheric deposition to water quality. More research is needed to understand how increased dissolved organic N could affect aquatic ecosystems and downstream riverine nutrient export.

Triple oxygen isotope ratios (∆′¹⁷O) offer new opportunities to improve reconstructions of past climate by quantifying evaporation, relative humidity, and diagenesis in geologic archives. However, the utility of ∆′¹⁷O in paleoclimate applications is hampered by a limited understanding of how precipitation ∆′¹⁷O values vary across time and space. To improve applications of ∆′¹⁷O, we present δ¹⁸O, d‐excess, and ∆′¹⁷O data from 26 precipitation sites in the western and central United States and three streams from the Willamette River Basin in western Oregon. In this data set, we find that precipitation ∆′¹⁷O tracks evaporation but appears insensitive to many controls that govern variation in δ¹⁸O, including Rayleigh distillation, elevation, latitude, longitude, and local precipitation amount. Seasonality has a large effect on ∆′¹⁷O variation in the data set and we observe higher seasonally amount‐weighted average precipitation ∆′¹⁷O values in the winter (40 ± 15 per meg [± standard deviation]) than in the summer (18 ± 18 per meg). This seasonal precipitation ∆′¹⁷O variability likely arises from a combination of sub‐cloud evaporation, atmospheric mixing, moisture recycling, sublimation, and/or relative humidity, but the data set is not well suited to quantitatively assess isotopic variability associated with each of these processes. The seasonal ∆′¹⁷O pattern, which is absent in d‐excess and opposite in sign from δ¹⁸O, appears in other data sets globally; it showcases the influence of seasonality on ∆′¹⁷O values of precipitation and highlights the need for further systematic studies to understand variation in ∆′¹⁷O values of precipitation.