A new analysis of streams in the western United States has found that despite a general increase in air temperatures over the past several decades, western streams are not necessarily warming at the same rate. Several factors may influence the discrepancy, including snowmelt, interaction with groundwater, water flow and discharge rates, solar radiation, wind, and humidity. But even after factoring out those elements, the scientists detected cooler-than-expected maximum, mean, and minimum stream temperatures. Looking at streams individually, they found that some seemed to be getting warmer, some cooler, and others showed little change at all.
Results of the research, which was supported by the U.S. Geological Survey, the U.S. Forest Service, and Oregon State University, are published in Geophysical Research Letters, a journal of the American Geophysical Union.
Cold, clean water is one of the most important ecosystem services in the western United States. Many species of economically and culturally important salmon, trout, and other species depend on these cold waters to thrive and survive, so naturally, the potential loss of cold water in the face of climate change is a concern.
These findings show that changes in stream temperatures will not simply parallel changes in warming air temperatures, as commonly assumed or
projected. They point to the likely importance of local factors, such as land form, land cover, and geology, in influencing climate sensitivity of stream temperatures. A second key finding is that the vast majority of streamgages in the region either lack long-term data or are too strongly influenced by local human activities to provide a clear evaluation of climate effects.
For this study, researchers used long-term records from USGS and U.S. Forest Service gaging stations. Of the more than 600 stations evaluated from California, Nevada, Oregon, Idaho, Washington, and Alaska, less than 20 were suitable for analyzing climate effects. This set included streams with minimal human influences and sufficiently long-term records of temperatures for an evaluation of trends.
These results highlight the fact that stream temperature is one of the most important, yet least understood aspects of climate change. The researchers caution that the findings do not mean that climate change will not affect stream temperature, which is a fundamental driver of ecosystem processes in streams. However, the relationship between air temperatures and stream temperatures may be more complex than previously realized and require additional monitoring.
The paper, The paradox of cooling streams in a warming world: regional climate trends do not parallel variable local trends in stream temperature in the Pacific continental United States, was authored by Ivan Arismendi, Oregon State University; Sherri Johnson, U.S. Forest Service; Jason Dunham, USGS; Roy Haggerty, Oregon State University; and David Hockman-Wert, USGS.
Reconstructing Thermal Regimes in Streams from Sclerochronology of Freshwater Mussels (link to a paper, a USGS podcast, and an Oregon Field Guide television episode about the research)
Climate Impact on Streamflows, Thermal Regimes, and the Changing Distribution of Trout in the Great Basin
USGS Forest and Rangeland Ecosystem Science Center Aquatic Ecology Laboratory