Stream Temperatures are Rising. A number of landscape factors and stream characteristics influence stream temperatures such as the amount of solar radiation streams receive; evaporation rates; bed conduction; friction of the water with the bed and the banks; air temperature; valley and channel morphology such as channel slope and orientation, and features such as pools, riffles and rock steps; amount of groundwater and/or hyporheic flows; the presence of human modifications such as impoundments; and other factors (Webb et al. 2008). In a study of the landscape, stream, and channel factors that control summer stream temperatures in unregulated Pacific Northwest streams, Mayer (2012) finds that regionally, baseflow index (groundwater influence in a stream) and stream channel slope best explained summer stream temperatures and thermal sensitivities. Research in the Pacific Northwest by Isaak et al. (2011) on unregulated streams found statistically significant increasing stream temperature trends with rates of warming highest during the summer (raw
trend, +0.17°C/decade; reconstructed trend, +0.22°C/decade). Air temperature increases were found to be the dominant factor explaining long-term stream temperature trends (82–94% of trends) and inter-annual variability (48–86% of variability), except during the summer when discharge accounted for approximately half (52%) of the inter-annual variation in stream temperatures.
Aresmendi et al. (2012) found that not all streams in the Pacific Northwest exhibit warming trends depending on the time scale, suggesting possible local variation and other factors at play. This study recommends more long-term data on minimally impacted streams is needed throughout the region. Following up, five long-term gage stations (>30-years) from western Oregon were analyzed by Aresmendi et al. (2013b) and key findings include: warming of sites in both winter and summer, with the greatest trends in the winter; during the summer, although both increased, daily minimum temperatures increased more than daily maximum values; and duration and frequency of cold events are declining, whereas warm events are more frequent and longer in duration than cold events. Research from long-term temperature monitoring sites in Europe also show strong evidence of warming stream temperatures correlated to rising air temperatures and climate cycles (e.g., Pacific Decadal Oscillation) with deviations associated with landscape factors and stream characteristics as described above (Webb and Nobilis 2007; Hari et al. 2006).