Are Multi-Decadal Stream Temperature Trends for Least-Disturbed Watersheds of Western North America in Accordance with a Recent Warming Climate?

Recent warming of terrestrial climates in most parts of the world has motivated concern about corresponding increases in water temperatures. The link between terrestrial climate and water temperature is complicated in part by uncertainties regarding the roles of climate change versus changes in other factors, such as changes in land or water use and land cover.  To evaluate evidence for potential climate impacts in locations minimally impacted by these factors, we analyzed long-term trends in stream temperatures in 18 least-disturbed watersheds across western North America. Based on hypothesized influences of observed trends in air temperatures and stream flows, we predicted climate impacts to be manifested as increasing magnitudes (warming) and variability of stream temperatures over time.  Although we found warming trends for some sites during 1951-2009 there was an opposite cooling trend in more recent time series (1987-2009). Lack of coherence in temperature trends that we evaluated may be related to complex interactions among non-climatic and climatic variables or time lags in responses of water temperatures.  Regardless of the process driving these patterns, our results suggest that climate impacts on air temperatures and stream hydrology cannot be simply extrapolated to infer future responses of stream temperatures.