The Pacific Northwest’s landscape is precarious and ever-shifting. Mount Rainier provides no shortage of proof.

A glacial outburst at about 6:50 p.m. Monday at the mountain’s South Tahoma Glacier sent debris and boulders as big as pickup trucks flowing down the mountain, said Mount Rainier National Park geologist Scott Beason.

The debris flow registered on seismic monitors and ran for more than 8 miles, Beason said.

Beason suspects warm, sunny weather filled the glacier with melt, rearranged the “internal plumbing” at the glacier’s base, caused water to blast a new channel through the glacier, and then flooded glacial melt into Tahoma Creek.

“The event lasted an hour and had four separate surges,” Beason said of the outburst flooding. “The outlet channel definitely shifted. It picked up a lot of loose material just below the glacier and carried it downstream and mobilized it into a debris flow.” 

As the world warms and Mount Rainier’s glaciers thin and retreat over time, these massive debris flows have become a common occurrence on the mountain’s south side. The park is building systems to forecast massive debris flows and send alerts to park staff when they’re triggered, Beason said.

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Debris flows are not new, he said, “but with a warming climate, you’d expect to see it more frequently.”

The park has recorded some 32 debris flows along Tahoma Creek. The South Tahoma Glacier that feeds the creek began to retreat in the 1960s, Beason said.

Robby Jost, an imminent threat technician with Mount Rainier National Park, surveys the flow caused by a glacial outburst. (Taylor Kenyon / National Park Service)
Robby Jost, an imminent threat technician with Mount Rainier National Park, surveys the flow caused by a glacial outburst. (Taylor Kenyon / National Park Service)

A debris flow in 1967 “obliterated” a campground near the creek, reported The Seattle Times that December. “Picnic tables and stoves were pushed every which way and many partly buried.” Luckily, the campground had been closed due to wildfire concerns.

No one was hurt Monday. Since the last big debris flow in 2015, park workers arranged large logs with root systems and boulders to protect a roadway near the creek.

The park closed its Westside Road, which saw minor damage after glacial floodwater overtopped it. The nearby Tahoma Creek trail suffered significant damage and Beason said it’s not passable. Beason expects the creek to run milky with sediment and higher flows for about five to seven days.