Willamette River Basin surface water isoscape (δ18O and δ2H): temporal changes of source water within the river

Abstract

Determining how water sources for rivers vary over time can greatly enhance our understanding and management of land use and climate change impacts on rivers. Because the stable isotope composition of precipitation can vary geographically, variation in the stable isotope composition of river water may be able to identify source water dynamics. We monitored the stable isotope values (δ18O and δ2H) of river and stream water within the southern Willamette River Basin in western Oregon over two years. Within this basin, eighty‐four percent of the isotopic variation in small tributary streams was explained by the mean elevation of the catchments, whereas seasonal variation was minimal. However, water within the Willamette River had distinct isotopic seasonal patterns that likely occurred because of changes in the mean elevation of source water for the river. River isotopic values were lowest during summer low flow and highest during February/March when snow accumulated in the mountains. We estimated that the mean elevation of the source water for the Willamette River shifted over 700 m, seasonally. During winter when rain occurred in the valley and snow accumulated in the mountains, the river reflected a mixture of low mountains and valley bottom precipitation. During the dry Mediterranean summer, 60–80% of the river water came from the snow zone above 1200 m, which is only 12% of the land area and accounts for 15.6% of the annual precipitation within the Willamette Basin. This high elevation area contains the High Cascades geological region with highly permeable bedrock that sustains late‐summer baseflow compared to the Western Cascades with low permeable bedrock. Reliance on high‐elevation water during summer low flow highlights the vulnerability of this system to influences of a warming climate, where snowpacks in the Cascade Mountains are predicted to decrease in the future.