Inferring snowpack contributions and the mean elevation of source water to streamflow in the Willamette River, Oregon using water stable isotopes
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Much of the water that people in Western Oregon rely on originates as snow in the Cascade Range, which is predicted to decrease in coming years with climate change. The Cascade snowpack has varied dramatically in the past decade when compared to median peak snowpack values from 1981-2023, ranging from 150% in 2022-23 to 17% in 2014-15. Over the past 13 years, we have monitored the stable isotopes of water (SIW) from streams in the Willamette River basin (18,500 km2) in western Oregon, including two sites on the mainstem and 63 streams draining small catchments across a representative range of elevations on both the windward (Cascade Range) and leeward (Coast Range) sides. The SIW values from small catchment streams sampled during base flow conditions reflect recent mean annual SIW values in precipitation. Within the Willamette Basin, baseflow SIW in the small catchments vary strongly with elevation and provide a marker for determining the mean elevation from which water in the Willamette River is derived. In the winter, when snow accumulates in the mountains, precipitation from mid- and low-elevations (primarily rain falling below 1200 m) contributes the largest proportion of streamflow to the Willamette River. During summer when rainfall is scarce and demand for water is the greatest, streamflow in the Willamette River is mainly derived from higher-elevation areas above 1200 m, representing < 12% of the basin area, where snow dominates the precipitation regime.
We observed that the antecedent year snowpack was positively correlated with the proportion of summer streamflow derived from the snow zone (>1200 m). Additionally, over the last 13 years, we observed a decline in streamflow originating from high-elevation during summer low-flow regardless of antecedent snowpack size. Since high-elevation water dominates summer low-flow, this reduction in high-elevation water was reflected in decreased Willamette average monthly flow during summer low flow over the monitoring period, even in this highly regulated river system marked by 13 dams and associated reservoirs. Improved understanding of the origin of, and trends in, summer season streamflow in the Willamette River will aid in reconciling human demands with biological instream requirements under future hydroclimatic regimes marked by increased rainfall and decreased snowpack.