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CORVALLIS, Ore. - A newly published study by researchers at Oregon State University and two federal agencies concludes that high temperatures coupled with lower flows in many Northwest streams is creating increasingly extreme conditions that could negatively affect fish and other organisms.

The study, published in the professional journal Hydrobiologia, was funded and coordinated by the U.S. Geological Survey and the research branch of the U.S. Forest Service. It points to climate change as the primary reason for the extreme conditions.

"The highest temperatures for streams generally occur in August, while lowest flows take place in the early fall," said Ivan Arismendi, a research professor in OSU's Department of Fisheries and Wildlife. "Each period is important because it is a time of potentially high stress on the organisms that live in the stream. If they occur closer in time - or together - they could create double trouble that may be greater than their combined singular effects."

Arismendi, who was lead author on the paper, said climate change appears to play a role as snowpack levels lessen and snow begins melting earlier in the spring. Peak stream flows are coming earlier in the year, stretching out the amount of time when river flows are low.

"What results is that low flows are moving closer and closer to the time of the year when stream temperatures are highest," Arismendi said, "and that is not good."

The study looked at 22 "minimally human-influenced" streams from the period of 1950 to 2010, located in Washington, Oregon, California, Nevada, Montana and Idaho. The researchers found the hydrology of the streams was complex and differed among streams; while weather extremes affected all of the streams, the impact seems to be mediated by the influence of groundwater.

"Other studies have shown that high temperatures in streams lead to less oxygen and more thermal stress," said co-author Jason Dunham, an aquatic ecologist with the U.S. Geological Survey. "Low flows reduce the amount of suitable habitat and may lead to high density and overcrowding, more predation, changes in predator-prey relationships, and more competition - at least, among salmonids."

This study focused on the physical processes on the streams, Arismendi emphasized, and needs to be followed by biological studies.

"Coupling of low flow with high temperatures can have significant hydrologic implications in maintaining stream water quality," said Mohammad Safeeq, an OSU post-doctoral researcher in the College of Earth, Ocean, and Atmospheric Sciences and a co-author on the paper.

Arismendi said that over the years, weather and stream flow can be influenced by climate drivers like El Nino, La Nina, the Pacific Decadal Oscillation and other phenomena. But over the 60-year time frame covered by the study, the climate warmed appreciably, leading to lower flows and earlier peak flows.

"These streams have high natural variability," Arismendi said, "but the general pattern holds true."

Interestingly, Arismendi said that stream temperatures are not always higher on an annual scale despite a regional trend that has shown warming air temperatures. This could be because of increased snowmelt, he pointed out, or complex hydrological cycles.

"Even though our studies are showing that stream processes are much more complex than initially thought we are able to identify trends toward increasing synchrony in timing of low flows and high temperatures," Arismendi said.