Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 11/1/2006
Publication Date: 12/15/2006
Citation: Johnson, H.M., Anderson, C., Kendall, C., Lajtha, K., Mcdonnell, J.J., Frentress, J., Griffith, S.M., Grove, R. Insights into the Hydrology of the Willamette River Basin Using d18O of Water during Summer Baseflow Conditions. American Geophysical Union Meeting Abstracts. Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract B13A-1061
Technical Abstract: The influence of different water sources on the Willamette River, Oregon were characterized using d18O of water during summer baseflow conditions in August 2006. We collected samples from the entire length of the Willamette River, a distance of more than 275 km. Mid-river grab-samples were collected from the Willamette River at sites selected to provide both uniform geographic coverage plus indications of inputs from different water sources that may be chemically distinct. Near-shore grab-samples were collected from all major tributaries near their confluence with the Willamette River, including the mouths of minor tributaries to the Willamette River that had largely urban or agricultural land use, and major sub-tributaries, reservoirs, or other sites of local hydrologic interest. These samples were collected as part of a larger effort to begin to understand sources and cycling of nitrogen in the Willamette River Basin. Values of d18O ranged from -12.1 per mil to -11.3 per mil in the Willamette River. For approximately 130 kilometers downstream from the confluence of the Coast Fork Willamette River and Middle Fork Willamette River, the isotopic variation of d18O in the river was minimal, with a median value of -11.9 per mil. At this point, d18O values of Willamette River water began increasing, and reached a maximum value of -11.3 in downtown Portland. The change in character approximately corresponds to the inflow of the Santiam River and also to the location of a major geologic structure in the Willamette Valley. d18O in major and minor tributaries seem to reflect the elevation and geographic location of the headwaters of those tributaries. The headwaters of the McKenzie River and Clackamas River are in the High Cascades and d18O values in those rivers were among the lowest values we observed (-12.2 per mil and -12.1 per mil, respectively). Rivers with headwaters in the Western Cascades had d18O values between -9.3 and -10.0 per mil. The Santiam River has significant headwaters in both the High and Western Cascades and had an intermediate d18O value of -11.5. Rivers with headwaters in the valley floor or low foothills of the Western Cascades had larger d180 values than other Cascades rivers, and typically were greater than -9.3. Rivers draining the Coast Range were isotopically distinct from those draining the Cascades. Coast Range rivers tended to have the largest d18O values of among all samples collected. The lowest d18O values in Coast Range rivers was around -9 per mil and the greatest was -5.5 per mil. d18O data collected to date appear to reflect the hydrology, geography and land use in the WR basin. Further analyses and future studies are expected that will provide greater detail on the influence of tributaries, point sources, and various land-use types on local and regional hydrology in the basin. With an understanding of the relative contribution of various water sources in conjunction with other isotopic and chemical tracers, we seek to identify potential chemical sources and reactive pathways that contribute to nutrient or aquatic food web processes in the Basin.