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Jonathan Dove was patrolling Mount Shasta on a clear, warm afternoon when a group of backpackers asked him if Mud Creek Canyon on the Northern California peak always flowed so heavily.

The U.S. Forest Service climbing ranger scrambled up a ridge to investigate. He peered into the canyon below and found a small creek flowing like a brown river. There was a loud roar, and then a wave of boulders, mud and water 4 feet high.

“It sounded like a freight train barreling down the canyon,” he said, and at times “like a thunder rumble. In my 10 years as a ranger on Mount Shasta, I’ve never witnessed anything of this magnitude.”

Government scientists say exceptionally hot, dry conditions and a lack of insulating snowpack primed Mount Shasta for the massive mudslide that rumbled down over the weekend after a pulse of water burst out from under an alpine glacier.

That a severe drought could cause flooding is the latest expression of a three-year dry spell that is afflicting California with increased wildfires, crop losses, water shortages and spikes in air pollution.

U.S. Forest Service scientists say they think the mudslide was triggered by water released from the Konwakiton Glacier, the fifth-largest of seven glaciers that sit high up on Mount Shasta, a 14,163-foot dormant volcano that towers over California’s northernmost counties.

The slide sent the thick slurry of mud, boulders and debris to pour through Shasta-Trinity National Forest Saturday afternoon, damaging roads below.

When water from melting ice becomes trapped beneath a glacier, “it’s almost like a cork in a bottle popping out suddenly,” said Steve Bachmann, a hydrologist with U.S. Forest Service.

He has made several helicopter trips to search for clues to the mudslide’s origin but has seen nothing unusual on the glacier’s surface. “The release must have been very rapid based on the conditions we saw below,” Bachmann said.

While debris flows can occur any time, he said, the heat and lack of precipitation have raised the odds considerably. With the drought, snow that usually acts to insulate Mount Shasta’s glaciers has been heavily depleted.

“In a year like this, all the snow is gone and it’s just the glacialized ice exposed to the sun day after day,” Bachmann said.

Such dry-weather events, known as “outburst floods,” are a common occurrence on Mount Shasta that happen whenever rapid melting causes water to pool behind glacier ice, said Jeffrey Mount, a retired University of California, Davis geologist who is now a senior fellow at the Public Policy Institute of California, a nonprofit think tank.

Glaciers respond slowly to temperature and precipitation changes, he said, so it takes a prolonged period of warming, dryness and low snowfall for such strains to build up.

“If you were going to get a rapid melting of a glacier, this is the year,” Mount said.

The mudslide that began Saturday gathered vast amounts of sediment and debris as it moved swiftly downhill. It slid through the night before starting to recede Sunday morning, the Forest Service said. Because the slide ran through a remote area of Siskiyou County, there were no injuries or major damage reported.

Glaciers on Shasta have unleashed similar dry-weather deluges before.

In 1924, a mudflow triggered by glacial melt covered an area five miles long, a mile wide and 10 feet deep, threatening the nearby town of McCloud and generating front-page headlines in San Francisco for weeks. Outburst floods occurred in 1926 and 1931, relatively dry years.

The last major mudslide on Mount Shasta happened in 1997 when a storm laden with tropical moisture dumped heavy rain on its northern side.