Oregon State University

College of Earth, Ocean, and Atmospheric Sciences

Climate Change and Streams

Flowing stream

May 16, 2012

Two recent studies of the effect of rising temperatures on streams yield unexpected results.

Stream temperatures do not parallel warming climate trend

An analysis of streams in the western United States with long-term monitoring programs has found that despite a general increase in air temperatures over the past several decades, streams are not necessarily warming at the same rate.

Several factors may influence the discrepancy, researchers say, including snowmelt, interaction with groundwater, flow and discharge rates, solar radiation, wind and humidity. But even after factoring out those elements, the scientists were surprised by the cooler-than-expected maximum, mean and minimum temperatures of the streams.

“Individually, you can find streams that seem to be getting warmer and others that are getting cooler,” said Ivan Arismendi, a post-doctoral researcher and lead author on the study. “Some streams show little effect at all. But the bottom line is that recent trends in overall stream temperature do not parallel climate-related trends.”

The researchers caution that the findings don't mean that climate change will not have an impact on 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.

Alternatively, there may be a time lag between air temperature and stream temperature, they say. [More findings at the link below.]

Impact of warming climate doesn't always translate to streamflow

An analysis of 35 headwater basins in the United States and Canada found that the impact of warmer air temperatures on streamflow rates was less than expected in many locations, suggesting that some ecosystems may be resilient to certain aspects of climate change.

The study was just published in a special issue of the journal BioScience, in which the Long-Term Ecological Research (LTER) network of 26 sites around the country funded by the National Science Foundation is featured.

Lead author Julia Jones, an Oregon State University geoscientist, said that air temperatures increased significantly at 17 of the 19 sites that had 20- to 60-year climate records, but streamflow changes correlated with temperature changes in only seven of those study sites. In fact, water flow decreased only at sites with winter snow and ice, and there was less impact in warmer, more arid ecosystems.

“It appears that ecosystems may have some capacity for resilience and adapt to changing conditions,” said Jones, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences. “Various ecosystem processes may contribute to that resilience. In Pacific Northwest forests, for example, one hypothesis is that trees control the stomatal openings on their leaves and adjust their water use in response to the amount of water in the soil.”

“So when presented with warmer and drier conditions, trees in the Pacific Northwest appear to use less water and therefore the impact on streamflow is reduced,” she added. “In other parts of the country, forest regrowth after past logging and hurricanes thus far has a more definitive signal in streamflow reduction than have warming temperatures.” [More findings at the link below.]


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