Willamette River Flow Sensitivity to Cascade Snowpack Variation: Insights from Water Stable Isotopes
Citation:
Brooks, J. Renee, H. Johnson, S. Cline, C. Segura, AND W. Rugh. Willamette River Flow Sensitivity to Cascade Snowpack Variation: Insights from Water Stable Isotopes. Water Resources Seminar Series -Portland State University and Oregon State University, Portland/Corvallis, Oregon, May 22 - 23, 2018.
Impact/Purpose:
Much of the water that people in Western Oregon rely on comes from snowpack in the Cascade Range, and this snowpack is expected to decrease in coming years. Water managers need tools to understand how the lose of high elevation water sources will influence water supply for the diverse demands, particularly during the summer low flow period. We devised a stable isotope tool for understand which parts of the watershed are contributing to flow over time. Our data indicate that the proportion of water from high elevation sources decreased with decreasing snowpack, with a memory effect of at least one year. Reconciling the demands between human use and biological instream requirements during summer will be challenging under climatic conditions in which winter snowpack is reduced compared to historical amounts.
Description:
Much of the water that people in Western Oregon rely on comes from snowpack in the Cascade Range, and this snowpack is expected to decrease in coming years. In fact, the past 7 years have shown dramatic variation in snowpack, from a high of 174% of normal in 2010-11 to a low of 11% for 2014-15, one of the lowest on record. During this timeframe, we have monitored the stable isotopes of water within the Willamette River twice monthly, and mapped the spatial variation of water isotopes in small streams across the basin in order to partition water into its various sources. Within the Willamette Basin, stable isotopes of water in small headwater streams vary strongly with elevation and provide a marker for determining the mean elevation from which water in the Willamette River is derived. In winter, when snow accumulates in the mountains, low elevation precipitation (primarily rain) contributes the largest proportion of water to the Willamette River. During summer, when rainfall is scarce and demand for water is the greatest, water in the Willamette River is mainly derived from high elevation snowmelt. Our data indicate that the proportion of water from high elevation sources decreased with decreasing snowpack, with a memory effect of at least one year. We combine this information with river flow data to estimate the volume reduction related to snowpack reduction during the dry summer. Observed reductions in the contribution of high elevation water to the Willamette River after just 2 years of diminished snowpack indicate that the hydrologic system responds relatively rapidly to changing snowpack volume. Reconciling the demands between human use and biological instream requirements during summer will be challenging under climatic conditions in which winter snowpack is reduced compared to historical amounts.