A large patch of rock at Mount Adams’ upper cone is physically weak, the result of years of cold and hot water circulating through minerals, he said. The weathered mass of rotten rock — sitting thousands of feet above Trout Lake — remains under the influence of gravity, ready to tumble with an earthquake or further erosion.
The 1980 Mount St. Helens eruption unleashed the region’s most recent and destructive lahar. More than 2 billion cubic yards of material wiped out bridges and buried forests across the upper 17 miles of the North Fork Toutle River valley, according to USGS. However, an eruption isn’t necessary to set off a lahar. These liquid landslides can be triggered by erosion and rapid snowmelt.
A lahar can grow more than 10 times its original volume as debris rushes downhill. Depending on the steepness of a slope, lahars can surpass 120 mph before slowing in lowland areas, burying everything in their path.
Once Arnold learned about the geological threat at her doorstep, she began making a plan. She said she would have to abandon her multiple chickens, barn cats and donkeys, Minnie and Pearl. She wouldn’t have much time because Mount Adams would already be closing in. If she had the 15-minute warning that could come with the planned USGS detection system, she would scoop up her elderly beagle and drive to a hill nearby, from which she would watch water and debris swallow her property.
“We’ve been through major wildfires, but there’s something about the lahar that feels ultimately more fatal,” she said, glancing behind her at the snow-capped peak.
Multiple 3-by-5-foot aluminum boxes hold the solution for saving communities nestled within Mount Adams’ hazard zone, including Trout Lake, BZ Corner and Husum.
Ben Pauk, a geophysicist leading the monitoring project, poked at a prototype’s contents — multicolored wires, infrasound sensors and GPS receivers. The instruments are like a trip wire that detects ground movement and, if fitted with a notification system operated by county and state agencies, pings those nearby.
From the time it’s detected, a lahar could flow down the White Salmon River valley and reach Trout Lake within 20 minutes, Pauk said.
The grooves carved in the White Salmon drainage also tell the story of massive avalanches of clay and other debris that swept through the valley.
About 6,000 years ago, the Trout Lake mudflow dammed Trout Lake Creek, forming the titular lake, and scattered large boulders throughout the flat landscape. And 300 years ago, another large flow, the Salt Creek lahar, scoured the valley. These massive lahars deposited the material that underlies Trout Lake.
Significantly smaller lahars of snow and ice continued to fall down Mount Adams’ flank. The USGS observed one as recently as 2017, according to a 2018 report. Although these smaller flows didn’t reach the Trout Lake valley, they are a reminder that rock on the volcano’s steep cliffs is unstable.
“We might not have any precursor earthquakes or volcanic activity,” Trout Lake resident Rachel Haymon said. “It could just happen.”
“Putting instrumentation on the ground isn’t the end of it. That’s only the beginning,” he said. “It’s just one part of a much broader equation that has to be solved.”
Until this happens, Haymon continues to sound the alarm. Arnold said she didn’t know about the risk Mount Adams posed when she moved to Trout Lake in 1991. At least not until she befriended Haymon.
Residents in communities surrounding Mount Adams sit within a hazard zone, and many seem unaware of the volcano’s threat, Haymon said. A lahar’s damage isn’t preventable, but USGS’s monitoring system and county’s alert system can minimize loss.
“Some people might still not escape,” Haymon said. “But you can save a lot of people.”
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