Mount Shasta, nearly devoid of snow, as seen from Montague (Siskiyou County) last month.
Mount Shasta, the widely recognizable face of California's far north, has lost almost all its defining snow cover for a second straight year.
Another summer of scorching temperatures, punctuated by the recent heat wave, has melted most of the mountain's lofty white crown, typically a year-round symbol of the north state's enduring wilds.
The lack of snow not only means unfamiliar views of the bare 14,000-foot-plus giant, it is hastening the demise of the mountain's glaciers. While the seven named ice sheets have been retreating for years, if not decades, the diminishing snow, which helps insulate the glaciers and keep them from thawing, has caused an unprecedented melt-off: About 20% of the glaciers' ice, and possibly more, is expected to have vanished over the past two summers.
The renowned Whitney Glacier - the longest glacier in California - lost up to 9 inches of ice depth a day during this month's record heat, according to Mauri Pelto, director of the North Cascades Glacier Climate Project and a professor of environmental science at Nichols College in Massachusetts.
Scientists say the current rate of decline is simply not sustainable.
"If we do this a few more years, there won't be any more glacial ice on the mountain," said Phil Dawson, a geophysicist with the U.S. Geological Survey's California Volcano Observatory, who grew up on the slopes of the now-dormant volcano. "Mount Shasta is a visual reminder that we're in different times now. The Sierra glaciers are almost all gone. ... Same going north, to Alaska, and the Himalayas."
The loss of snow and ice on Mount Shasta has created a raft of problems. The rocky slopes have become destabilized as the frosty cover has faded, unleashing dangerous mudslides down the mountain. Forests are drying out and inviting more wildfire. Water supplies have shrunk for local communities dependent on springs as well as for residents statewide who rely on the mountain's runoff to Shasta Lake.
This year, most of the mountain's snow was gone in August, slightly later than last year but still not staying into late summer or fall, which is usually the case, if not remaining year-round. The snowpack was already low after a dry winter, which marked a third year of drought, and was no match for the above-average heat that bore down in the second half of summer.
Some snow lingers on the north side of the peak, but the southern and western flanks visible from Interstate 5 have become mostly barren. Without snow, the glaciers are directly exposed to the sun and more prone to melting.
During this month's 10-day heat wave, the roughly 150-acre Whitney Glacier, which averages about 100 feet thick, lost 6 to 9 inches of ice depth a day, according to Pelto.
"All of a sudden that's several percent of the glacier melting away in a short span of time," he said. "It's like if you had a big pile of ice in your yard in the heat, it's going to melt at a pretty good clip."
Pelto estimates that Whitney Glacier will lose up to 10% of its total volume this year after surrendering 10% to 15% last year. The other two major glaciers on the mountain, Bolam and Hotlum, are probably receding at the same pace, he says.
"That's just not a sustainable rate for more than a few years," Pelto said. "And this story is being repeated. Maybe next summer it doesn't happen in California, or maybe not the same mountain range, but there's no mountain range that's escaping it over the span of two or three years."
The area that the glaciers on Mount Shasta cover is less than half what it was 20 years ago, according to Pelto.
Perhaps the biggest repercussion of the melt-off, or at least the most visible this year, is the debris flows on the south side of the mountain. As the snow and ice have disappeared, rocks and soil have become untethered.
The small town of McCloud (Siskiyou County), about an hour's drive north of Redding, faced a crushing water shortage this summer after repeated mudslides in the aptly named Mud Creek filled a drainage where a critical pipeline crosses. The above-ground line had to be shut down because of the debris pounding it.
"It takes us days to clear out the channel and just hours for it to fill back up," said Amos McAbier, general manager of the McCloud Community Services District, who led a futile effort to save the pipe. "The mud will come in pulses. Sometimes you'll see things relaxed and laid back, but then we'll get a pulse and we'll get a big mudflow."
Crews with heavy construction equipment spent weeks trying to clear debris from the creek where the town's water line runs. Fortunately, the community recently won grant money to install a new line in the ground beneath the drainage.
"It's just Mother Nature," McAbier said. "It's amazing and it's beautiful, but it just kind of makes us feel like little ants down here."
A handful of roads around the mountain and parts of the Shasta-Trinity National Forest remain closed because of the risk of slides.
The U.S. Geological Survey this year installed additional seismometers around the mountain, historically used for monitoring earthquakes and volcanic activity, but now for also detecting debris flows.
Dawson, with the agency, said the past several weeks have been quiet, likely because most of the snow on the mountain is gone and whatever debris was loosened has already come down. However, he worries about water in the glaciers. While glacial melt-off normally comes in a steady trickle, the ice sheets can store large deposits of water, which can break free at any time and cause flash flooding.
Mount Shasta, Dawson said, has just changed a lot since his childhood.
"It's very different in the West than it was," Dawson said. "I grew up with a white mountain. Now I see brown. It's depressing."
Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander