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Reverse Effect: Wildfire Smoke Can Cool Rivers

One of Will Harling’s first memories is the 1977 Hog Fire in Northern California. Fire burned the forest behind the cabin where he was born.

Ten years later, in the Siege of ’87, nearly 200,000 acres burned through the footprint of the Hog Fire and beyond. Amid all the flames and smoke, Harling remembers how cool it felt.

The smoke inversion dropped both air and water temperatures below 60 degrees. “That was the first summer we didn’t even want to swim in the river it was so cold,” Harling said.

Wildfires typically call to mind flames, smoke, fire lines cut by firefighters in leather boots.

But it turns out that wildfires may also help cool water.

Harling, now executive director of the Mid-Klamath Watershed Council, said wood smoke deflects sun rays and lowers the temperature of air and water.

Case in point: Earlier this year, chinook were dying in Klamath River tributaries, Harling said. But near the end of July, two large wildfires ignited, cooling the water. Fish stopped dying at such high rates.

“In addition to stopping spring chinook from dying, lower water temperatures triggered fall chinook to come into the mainstem Klamath River and then into the tributaries,” Harling said.

Cooler water triggers salmon to swim upstream from the Pacific Ocean and seek out spawning grounds.

Taz Soto, lead fisheries biologist for the Karuk Tribe, says fires near the Klamath River have dropped the water temperature by about 10 degrees. Ten degrees can make a big difference for salmon. And that change could be partly because of wildfire.

“One positive thing that’s come out of all the wildfires this year is it’s cooled off the river. The media rarely talks about the positive things about fires and that’s one of them,” Soto said.

This smoke is so heavy Soto compares it to fog. He says you can’t see your shadow and the sun can’t shine through.

“It’s a relief for the fish but not for the people who have to breathe it in,” he said.

Michael Hughes, who directs Oregon Institute of Technology’s environmental sciences program and formerly worked for the Klamath Tribes, says shade helps cool water, the most common being clouds, vegetation hanging over streambanks, or smoke.

Volcanic eruptions are an extreme example. Hughes says major eruptions cause major cooling. Mount Pinatubo erupted in the Philippines in 1991 and global temperatures dropped, bucking a warming trend.

Hughes describes even stronger cooling effects caused by the Mount Mazama eruption, which created Crater Lake.

“Mount Mazama would have had more intense impacts on temperature and climate. It would have likely induced a global dip in temperature by a degree or two for a year or two,” Hughes said.

To be clear, Hughes is not advocating lighting wildfires to fix warm streams and rivers. Nor does he think the next massive volcanic eruption will do the trick.

So what can be done in the short term? Hughes says re-vegetating streams and rivers can shade the water and help cool it down.

Soto says watersheds have evolved around a natural fire cycle, and fire provides more benefits than drawbacks.

Soto acknowledges drawbacks of human- and lightning-caused fires, but advocates for controlled burns and more active forest management.

And he says more research is needed on how smoke affects water temperature and salmon spawning.

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