Seasonal and Elevational Variation of Surface Water δ¹⁸O and δ²H in the Willamette River Basin

Presented at The Oregon Water Conference, May 24-25, 2011, Corvallis, OR.Climate change is expected to dramatically alter the timing and quantity of water within the nation’s river systems. These changes are driven by variation in the form, location and amount of precipitation that will affect the temporal and spatial distribution of river source water over time. To manage the impact of climate change, we will need to understand how water sources for rivers are shifting over time. Yet methods for knowing where river water comes from within the drainage basin at various times of the year are not well developed. Because stable isotopes of precipitation vary geographically, variation in the stable isotopes of river water can indicate source water dynamics. We monitored the stable isotopes (18O and 2H) of river and stream water within the southern Willamette River basin in Western Oregon over two years. We sampled sites along the Willamette River, and up six major river tributaries to the Willamette, and eight small catchments along each tributary that spanned the elevation range in the tributary. All sites were sampled four times a year, with a selected set of sites being sampled eight times a year. Seventy-five percent of the isotopic variation in stream water from the small catchments could be explained by the mean elevation of the catchment. A decrease in catchment water isotope values with increasing elevation is caused by Raleigh distillation of precipitation where heavy isotopes fall first, and rain is progressively lighter isotopically as storms move eastward from the Coast Range, across the Willamette Valley and up the Cascade Mountains. Coast Range catchments did not have a clear elevation pattern in the water isotopes. \ud \ud Water within the lower Willamette River showed distinct isotopic seasonal patterns. Isotopic values were at their lowest during summer low flow and at their highest during Feb/March when snow was accumulating in the mountains. This seasonal variation likely comes from a change in source elevation for water in the river. During winter when rain occurs in the valley and snow is accumulating in the mountains, the river isotopic signal reflects the valley bottom rain sources. During the dry Mediterranean summer, valley soils are dry and the water comes from snow melt and high elevation spring water. Using our relationship between catchment elevation and water isotope values, we estimated that the mean elevation of the source water shifted upward approximately 350 m during the summer low flow period. Reliance on high-elevation snowmelt water during summer low flow highlight the vulnerability of this system to influences of climate change, where snowpacks in the Cascade Mountains are predicted to decrease in the coming year