Spatial and temporal dynamics of water sources in a mountain river basin inferred through δ2H and δ18O of water
In mountainous river basins of the Pacific Northwest, climate models predict that winter warming will result in increased precipitation falling as rain and decreased snowpack. A detailed understanding of the spatial and temporal dynamics of water sources across river networks will help illuminate climate change impacts on river flow regimes. Because the stable isotopic composition (2H/1H, 18O/16O) 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 ordinary least squares regression and geostatistical Spatial Stream Network models (SSNMs) to relate surface water isotope ratios to mean watershed elevation (MWE) across seasons. Geologic and discharge data was integrated with water isotopes in order to generate a conceptual model of streamflow generation for the Snoqualmie River. We found that surface water stable isotope ratios were most depleted in the spring and most enriched in the dry, Mediterranean summer, but related strongly to MWE throughout the year. Depleted isotope ratios in spring reflect the input of snowmelt into high elevation tributaries. Enriched summer isotope ratios suggest that groundwater is sourced from low elevation areas and recharged by winter precipitation. Overall, our results suggest that baseflow in the Snoqualmie may be resilient to predicted warming and subsequent changes to snowpack in the Pacific Northwest.