Temperature Variability Along an Entire Stream Longitudinal Profile and a Stream Temperature Factor Analysis at the Agricultural Reach Scale in a Mediterranean-Type Climate Watershed in Western Oregon, USA

The ability to maintain or improve cool stream temperature conditions year-round is essential to support native Pacific Northwest salmonid fish populations. This is particularly concerning if climate change increases air temperatures and decreases summer streamflow in Western Oregon, as predicted. Research on the factors affecting stream temperature have yielded inconsistent results due to complex heat exchange processes and diverse watershed conditions across land use, geology, topography and geographical regions. Site-specific reach and watershed scale data are most helpful in understanding cooling processes and identifying management scenarios to improve conditions. Study objectives were to 1) determine year-round stream and air temperature variability along the longitudinal profile and 2) identify factors most strongly correlated to summer stream temperature in two agricultural reaches along Oak Creek, a fourth-order stream in western Oregon. Average streamflow approximates 0.05 m3s-1 in summer, base-flow conditions, and 0.3 m3s-1 in winter. Oak Creeks adjacent land use includes forests in upper elevations, rural residential and agriculture in mid and lower elevations, and urban in the lower valley floor. From perennial headwaters to the mouth, year-round temperature data were collected over four years at 24 in-stream, five riparian air, and two weather station locations. Preliminary results indicate daily mean stream temperature warming trends (+5 to 7C) in the down-valley direction during summer and cooling trends (-1 to 2C) during winter. Riparian air temperatures followed similar trends but to a lesser amount (+/- 1 to 2C). Agriculture-reach data measurements for one summer month included canopy cover, solar radiation, wetted width and depth, sediment sizes, in-stream cover, and gradient every 10 m, stream temperature every 50 m, streamflow and air temperatures above and below the two reaches, and other information to characterize the sites, such as buffer widths and groundwater temperatures. Preliminary results indicate stream and air temperatures are substantially correlated in each reach, and the lower reach has higher stream temperature sensitivity to air temperature (i.e., thermal sensitivity) despite more canopy cover, wider buffers, deeper pools and more streamflow.