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Where does the water come from? Examining water stable isotopes across river basins
Citation:
McGill, L., A. Steel, AND J. Renee Brooks. Where does the water come from? Examining water stable isotopes across river basins. 2018 Salish Sea Ecosystem Conference, Seattle, Washington, April 04 - 06, 2018.
Impact/Purpose:
Much of the nation's water supply comes from surface waters. Understanding the spatial and temporal dynamics of water sources for river networks will be key to managing water supply and demand needs for future climate fluctuations. In this study, we use stable water isotopes to characterize water sources within 5 basins within Washington, 3 of which feed the Puget Sound. Our results will help managers understand how river flows may change with changing snowpack in the Cascade Mountains.
Description:
Global warming is expected to dramatically alter the timing and quantity of water within the nation’s river systems; however, these impacts will be heterogeneous both within river basins and across regions. A detailed understanding of the spatial and temporal dynamics of water sources across river networks is therefore central to managing the impacts of climate change. Stable isotopes of water (2H:1H & 18O:16O) provide an emerging method for elucidating the contributions of varying water sources to rivers. Because the stable isotope composition of precipitation varies geographically, variation in the stable isotope composition of river water indicates the volume-weighted integration of source water within the watershed. During September 2017, we collected stable isotope samples across five river networks in the Pacific Northwest and SE Alaska that span a range of hydraulic and climatic conditions: the Snoqualmie, Green, Wenatchee, and Skagit Rivers as well as Cowee Creek. For each basin we regressed isotope values against computed spatial covariates such as elevation, mean annual precipitation, and geology to determine which covariates explained the greatest proportion of isotopic variation. We compared best-fit models for each basin to examine how major correlates of isotopic variation differed among river networks. Results indicate that rivers on the west side of the Cascades displayed a far stronger relationship to elevation than the Wenatchee. We then built spatial stream network models to display and communicate spatial patterns of isotopic variation within the Snoqualmie and Wenatchee basins. We also employed a mixing-model approach to determine where sources of water in the mainstems of each river originated (e.g. snow zone, rain-on-snow zone, or rain) during summer low flow conditions. Our results can be used by managers and watershed groups to estimate how river flows may shift in a changing climate.