Glaciers are not known as fast-moving objects. Yet on Saturday, things started happening very quickly at a glacier high on the slopes of Mount Shasta.
At about 3 p.m. Saturday, wilderness rangers working for the Shasta-Trinity National Forest reported what they thought was a debris flow on the south side of the iconic mountain 220 miles north of Sacramento. Within three hours, Pilgrim Creek Road and Forest Service Road 31 were both buried in mud and rendered impassable. No structures were damaged, and no one was hurt.
Mud continued coursing down the mountain overnight, moving trees and giant boulders through the aptly named Mud Creek and eventually sending a torrent of dirt and debris into Lake McCloud and the McCloud River.
Forest Service officials think the cause is a glacier near the 14,180-foot summit of Mount Shasta. Known as Konwakiton Glacier, it sits on the peak’s southern slope and apparently melted or released a stored volume of water that had melted over some period of time.
They said that California’s prolonged drought, combined with hot summer temperatures, may have created a small snowmelt lake atop or within the glacier that eventually caused part of the glacier to collapse. The dammed water released in that collapse then triggered the debris flow that churned down the mountain.
Such events are known as a “jökulhlaup,” an Icelandic term for a glacial outburst flood.
Andrea Capps, a spokeswoman for the Shasta-Trinity National Forest, said an event of this magnitude has not occurred on Mount Shasta in at least 20 years.
“Glaciers are known sometimes to trap a certain amount of liquid water on them and create kind of a natural dam that helps trap all that water in place,” Capps said. “We’re talking about a small portion of glacier that melted enough to release the dam that was holding all that water.”
California’s three-year drought likely contributed to the event. Because of the prolonged dry spell, the mountain lacked an insulating layer of snowpack that would have protected the glacier from the hot sun. Without that protective layer, the south-facing glacier, located at an elevation of about 12,000 feet, likely melted faster than usual this summer.
Richelle Marley has lived in a rural subdivision near Mud Creek, at the base of Mount Shasta, for nearly 20 years. She said she has never seen anything like this in the area.
“We went outside and we could hear boulders rumbling down the creek,” Marley said. “I’d describe it as looking like chocolate pudding. It was just thick mud and boulders.”
Back at home, Marley decided to put a few important items in the car. The creek is about a quarter-mile away, but she wanted to be ready for a quick escape if necessary.
“We’re kind of uphill from the creek. But still, you don’t know,” she said. “It was pretty scary.”
Glaciers are the product of centuries of snowfall. Over time, snow accumulation creates pressure that gradually turns fallen snow into ice. Additional snowfall adds to the glacier and creates an insulating layer that protects the underlying ice from annual temperature changes. Mount Shasta has more glaciers than any other mountain in California.
It’s possible that climate change contributed to the weekend mudslide as well, said Slawek Tulaczyk, a geology professor and expert in glaciology at UC Santa Cruz.
Tulaczyk has spent years studying the glaciers on Mount Shasta. In 2003, he co-wrote a study that revealed that, despite global warming, Mount Shasta’s glaciers were actually expanding, and suggested that other high-elevation, northern-latitude glaciers also could expand. The cause is increased precipitation in some locations which slowly adds to a glacier’s mass.
But that was more than 10 years ago, and Tulaczyk says it may be time to take a fresh look at Mount Shasta’s glaciers.
“We know in glaciers, that when climate warming increases the meltwater on glaciers, they tend to fracture more and crack more,” Tulaczyk said. “That could be a mechanism.”
On the other hand, he said, mountain glaciers like those on Mount Shasta are thin compared to glaciers in Greenland and Antarctica. As a result, they are more likely to react rapidly to dramatic annual weather shifts, like a severe drought.
Tulaczyk noted that Mount Shasta is also a volcano, and that sometimes glaciers on volcanoes start moving or melting because the volcano is becoming active again.
Officials at the U.S. Geological Survey said there is no reason to worry about that in this case.
“There is no indication of volcanic unrest at Mount Shasta,” Carolyn Driedger, a hydrologist at the U.S. Geological Survey Cascades Volcano Observatory, said via email.
Mud Creek has been the site of numerous debris flows over the past century or more. This is partly because it lies on Mount Shasta’s south-facing slope, but also because the mountain’s steep sides are covered with historic volcanic rubble and ash, which is highly erosive. One of the biggest flows was in 1924, when a mudslide knocked out water lines serving the town of McCloud and muddied the waters as far downstream as the Sacramento River in Redding.
Steve Bachmann, a hydrologist at Shasta-Trinity National Forest, flew over the Konwakiton Glacier in a small plane on Saturday when the debris flow first was reported. He’s been working ever since to understand what happened.
So far, Bachmann said, there are no signs the glacier itself changed. But there was clearly a large stream of meltwater flowing from the bottom of the glacier.
A week before the mudslide, Bachmann said, a climber reported a large meltwater lake on an area of the mountain known as the Summit Plateau, a relatively flat area just above the Konwakiton Glacier. Normally, the plateau remains covered in snow all summer, and there is no standing water. Bachmann’s theory is that the snow cover melted, and the meltwater contributed to Saturday’s mudslide, which coursed more than 8,000 feet down the mountain.
“That water isn’t there anymore,” Bachmann said of the lake on Summit Plateau. “It’s possible it came through the glacier and it would have aggravated the melt, maybe even been temporarily suspended within the glacier itself. I would stress it’s a working theory.”
Officials are now concerned about a storm expected to arrive Wednesday and Thursday, the first of the fall season. Depending on how warm it is, and how much precipitation it brings, the storm could cause additional glacial melt and mudslides.
“Without its snow cover, it’s actually more vulnerable, more erosive,” Bachmann said of the glacier. “Some of our biggest erosive events come from fall storms.”