If you take a stream ecology course, you are generally taught that as a stream winds down from its headwaters at higher elevations, the water temperature will increase fairly steeply at first, and then gradually—and predictably—approach air temperature as the stream levels off at lower elevations. But several researchers at the School of Environmental and Forest Sciences (SEFS)—including doctoral student Aimee Fullerton and Professors Christian Torgersen and Josh Lawler—have recently published new findings in Hydrological Processes that could change the way we think about stream ecology and temperature dynamics.
The paper, “Rethinking the longitudinal stream temperature paradigm: region-wide comparison of thermal infrared imagery reveals unexpected complexity of river temperatures,” is a meta-analysis of thermal data from 53 rivers across the Pacific Northwest. Torgersen, a research landscape ecologist with the U.S. Geological Survey and an affiliate professor with SEFS, started building this massive data set in 1994. He partnered with long-time colleague Russ Faux at Watershed Sciences, Inc. (now part of Quantum Spatial), to collect thermal information from Faux’s aerial surveys of hundreds of rivers in Oregon, California, Idaho, Washington and a few other states.
Thermal infrared imaging is usually accurate to within a half degree, so these readings provided a trove of high-level, high-quality spatial data to explore. “This is the first time we’ve had the kind of spatial data over many, many rivers—and at a fine resolution—to even look at these patterns,” says Torgersen. “It was my dream project.”
Using these data, the researchers set out to map spatial patterns in river temperature during the summer, when fish are most stressed. They wanted to determine, among other information, whether they could predict the location of cold patches, which provide useful “cold refuges” for fish as they migrate up a stream. And though the authors expected to find geographical indicators for how a stream’s temperature would behave, the actual results surprised them.
Rather than finding predictable patterns, they discovered a great deal of variability and complexity in the streams. About half of the rivers behaved as expected, with temperatures steeply warming from the headwaters, and then gradually tapering off as the stream progresses. With other streams, though, the pattern was more gradual and linear, or the temperature stayed the same; and then in other cases, the temperature actually decreased or fluctuated over lengths of 50 kilometers or more—starting out cold, warming a bit, and then getting cold again.
“I think most people would say it’s not super surprising that there’s variability in these patterns,” says Torgersen. “But at this broad scale to see some of these odd-ball patterns was kind of a humdinger. We just know a lot less about river temperature than we do about air temperature.”
That’s why the mapping of water temperature in this study was so valuable. Most mathematical models of stream temperature, while largely accurate, aren’t able to account for fine-scale variations. Yet there are so many factors that can impact temperature variability, says Fullerton, such as tributaries, groundwater and nearby vegetation, or even coastal fog deflecting solar radiation. So this research provides a crucial perspective for what is actually happening in the water—and, ultimately, how those variations impact all of the species depending on the stream.
Fullerton says an important caveat with these findings is that the researchers only studied a snapshot in time. Their data came exclusively during the summer, so they weren’t capturing temperatures during different seasons, or overnight.
Still, these results can already reshape how researchers think about stream restoration projects, and how they determine the “natural” template for a certain section of river. It will be vital to examine the broader context of any stream segment—what’s happening directly up- and downstream, or along the riverbanks—to get the most comprehensive and accurate reading.
The diversity of thermal habitats in these streams, moreover, could be good news for the long-term survival of existing species, especially salmon. It appears that species may already be accustomed to navigating through a variety of conditions, and coping with a range of temperature tolerances, which could make them more resilient and less susceptible to future land use and climate impacts. “That’s going to help them with whatever comes next,” says Fullerton.
An important aspect of the climate analysis came from Professor Lawler, who is a coauthor on the paper. He played a key role in developing the approach for comparing patterns of water temperature among streams. “He was essential as a reality check to make sure our assertions were valid,” says Torgersen. “He also helped us couch these results in the context of climate change, and what the implications of this work are for understanding how species respond to a warming climate.”
Fullerton has worked as a research scientist with the National Oceanic and Atmospheric Administration (NOAA) since 2002. Now into her fifth year of doctoral study—working with Torgersen and Lawler—she can’t wait to dive back into the data and expand their analysis.
This first paper focused on a broad-scale perspective, and the next step is to key in on a finer scale and begin to look at how these spatial patterns might be affected by climate change, and therefore might affect the vulnerability of salmon. Specifically, the researchers will be quantifying the size, location and distance between cold water patches that salmon use, and considering how those patterns might change under future climate scenarios. After that, a third component of this research will be to look at drivers of these patterns, and whether we can predict where colder patches will occur in the landscape.
Which is to say, there’s much more to come from this exciting research, which has already challenged a number of long-held assumptions. “My hope is that stream ecologists will be reading this paper and then teach students that you can’t assume the temperature will increase,” says Torgersen. “It could change the way people think about basic stream ecology questions, and how to develop their models.”
Photos © Aimee Fullerton and Christian Torgersen.