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It’s a simple fact of life across the rural West, as it is here in Montana’s mountain-ringed Big Hole River Valley. Flooding river bottoms to grow hay sustains the economy but means less water in the river for the prized wild trout population

The competition for water is not new, but it is intensifying as the climate here gets warmer and drier.

“The biggest worry for trout is that smaller streams will simply run dry in late summer and temperatures in the remaining pools will exceed lethal levels,” said Steven W. Running, a climate scientist at the University of Montana in Missoula who is a member of the Intergovernmental Panel on Climate Change. “Even if the stream has good flow 11 months of the year, fish have to survive the highest stress conditions in late summer. We could lose the populations in these smaller streams, and they won’t come back.”

By all accounts, these kinds of changes in the West’s celebrated trout fisheries are happening quickly — faster, experts say, than in other parts of the country. A new report by the Rocky Mountain Climate Organization, based on research by the National Oceanic and Atmospheric Administration, shows temperatures in the West the last five years increased by 1.7 percent, compared with 1 percent elsewhere.

In a February paper in the journal Science, researchers said they expected the changes to accelerate.

“We’ll see less snow, even if there is the same amount of moisture,” said the lead author, Tim Barnett, a researcher at the Scripps Institution of Oceanography in San Diego. “The snow there will melt sooner and the flow will end sooner. It’s not a nice picture that emerges.”