The Spatial and Temporal Dynamics of Water Sources Across the Snoqualmie River Basin: A Stable Isotope Approach
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In mountainous river basins of the Pacific Northwest, climate models predict winter warming resulting in increased precipitation falling as rain and decreased snowpack. A detailed understanding of the spatial and temporal dynamics of water sources across river networks is central to managing the impacts of climate change on river flow regimes. Because the stable isotope composition of precipitation varies geographically, variation in surface-water isotope signatures indicates the volume-weighted integration of upstream source water. We measured the stable isotope ratios of surface-water samples collected in the Snoqualmie River basin in western Washington over the 2018 water year. We used a class of geostatistical models that account for the spatial dependencies of dendritic river networks (e.g., branching and longitudinal connectivity), called Spatial Stream Network (SSN) models, to generate surface water isoscapes for the basin across summer low flow, fall wet up, winter wet period, and spring snowmelt and quantify the relative role of in-stream versus landscape processes contributing to variability in isotope ratios. We found that overall our models explain 90 – 95 % of the variation in δ 18O values across seasons. Mean watershed elevation explained between 81 – 92% of the total variation and network autocovariance structure explained the remainder. Although small tributaries isotope ratios show little seasonal change, the mainstem Snoqualmie and the outlets of major tributaries show large shifts in isotope ratios across the year, likely reflecting changing source waters and shifting contributions of snowmelt. Isotope ratios were most depleted in the spring and most enriched in the dry, Mediterranean summer. We estimated that the mean elevation of source water shifted 630 m during this time. Furthermore, we see spatial variability in water sources across tributaries. During spring the proportion of water at major tributary outlets derived from the snow zone (elevations above 900 m) ranged from 18 – 83 %. Shifts in precipitation quantity and quality (e.g. rain vs. snow) will be combined with our water source estimates to assess the short and long term climate sensitivity of basin flow. Future research will link this information with water temperature data to help restoration efforts for endangered salmon.