A Slippery Slope

Warm rains and glacial melting trigger dangerous debris flows

Grinding over ancient layers of lava and ash, the glaciers of the Cascade Range act like supersized sheets of shrinkwrap. Stretched taut across tons of pulverized rock, these blankets of frozen snow hold sand, gravel and boulders in place — that is, until they start to melt. Then the sediments, unlocked from the glaciers’ icy grip, are vulnerable to gravity. The steeper the slope or gully, the more likely they are to break loose, especially when pounded by warm rainstorms blowing in from the sea.

Three Sisters in the Oregon Cascades

Three Sisters in the Oregon Cascades (Photo: University Marketing)

That’s what happened in early November 2006, says OSU geoscientist Anne Nolin. On virtually every Cascade peak from Mt. Rainier in Washington to Mt. Hood in Oregon, a “perfect storm” of driving rain, balmy temperatures and receding glaciers sent torrents of rock and mud tearing downhill.

“It was raining almost to the top of Mt. Hood,” recalls Nolin, an internationally known expert in mountain hydroclimatology. On her laptop, she clicks open a photo of Mount Hood with one of her graduate students standing beside a jumble of debris that had spewed out of Eliot Creek into a grove of evergreens during the storm, which dumped over 13 inches of rain on Mt. Hood in 36 hours.

“This area used to be soft forest duff,” Nolin explains, pointing to the photo. “Now it’s full five feet in boulders and logs.”

Collecting data with sophisticated technologies (satellites, lasers and computer models), as well as traditional methods (boots on the ground), Nolin is leading an investigation that will more fully describe the forces energizing alpine debris flows.

“There’s an enormous amount of sediment up there — pyroclastic debris from volcanoes, till ground up by glaciers,” she says. “Once it’s no longer held in place by the ice, it becomes unstable. Add water, and these unstable sediments are mobilized.”

The study, supported by more than $350,000 in National Science Foundation (NSF) stimulus funds, also will help foresters, park managers and mountain communities better predict events like the 2006 deluge, which washed out bridges, swept away campgrounds, closed roads and set the stage for future floods by choking river channels.

Pineapple Express

Snow is Nolin’s medium. Practically born with skis on her feet, she has plied the slopes from Killington Mountain in Vermont, near where her family has a home, to Mt. Hutt in New Zealand, where she spent three and a half months of her 2009-2010 sabbatical. The other eight months she lived (and skied) in the Vaud and Valais regions of Switzerland. (The Northern and Southern Hemispheres together gave her back-to-back winters — something only a lifelong snow lover would deem delightful.) While overseas, she gave a flurry of presentations about debris flows, as well as conferring with fellow researchers at the University of Canterbury in Christchurch, the Ecole Polytechnique Federale de Lausanne and the University of Zurich.

The Cascades will see more rain, less snow and changing water flows as climate shifts precipitation patterns, says Anne Nolin of OSU’s Dept. of Geosciences. In addition to analyzing debris flow risks, Nolin focusing on snowpack and water availability in the McKenzie River Basin. (Photo: Karl Maasdam)

The Cascades will see more rain, less snow and changing water flows as climate shifts precipitation patterns, says Anne Nolin of OSU’s Dept. of Geosciences. In addition to analyzing debris flow risks, Nolin focusing on snowpack and water availability in the McKenzie River Basin. (Photo: Karl Maasdam)

All of these scientists are seeing the same thing on their local mountaintops: a steady nibbling away of glacial edges. Satellite images of Hood and Rainier show glaciers shrinking by 14 percent between 1987 and 2005, Nolin reports. That’s a loss of nearly 1 percent ice volume per year.

It is at this ragged glacial edge, where ice is fragmented and meltwater is leaking down the ultra-steep terrain of towering peaks, that most debris flows begin. Nolin and her team are trying to pin down the triggering mechanisms. One culprit could be the so-called Pineapple Express — those notorious storms nicknamed for the warm temperatures and monsoon-like quantities of rain they bring from their origins in the tropical Pacific. They are examples of “atmospheric rivers” — airborne water plumes that shoot extraordinary amounts of vapor through the atmosphere. Nolin describes them as “laser beams of moisture,” which blast into the Northwest from time to time, including the 2006 storm that ranked as the decade’s worst.

“We’re trying to understand the character of these storms and their impact on mountain sediments,” she says. “Basically, we want to know how climate change affects rain-induced debris flows in the Northwest and other mountain regions worldwide.”

After Year One of the three-year study, Nolin and her team of colleagues and graduate students have found a clear link between debris flow events and unusually high freezing levels — the elevation where precipitation falls as snow instead of rain.

“The freezing altitudes of nearly all the storms that caused debris flows are at least one standard deviation higher than other significant rainfall events occurring in the same season,” Nolin writes along with her co-investigators Stephen Lancaster, an OSU geomorphologist, and Gordon Grant, a courtesy professor from the U.S. Forest Service, in their annual report to NSF. “Further, nearly all debris-flow events were coupled with … atmospheric river-like conditions.”

Yet because of the complex interplay of mountain systems, storm dynamics and debris-flow mechanics, Nolin says, “the conclusive story continues to elude us.”

Upslope, Downslope

“Water flows downhill, but policy flows uphill,” Nolin told members of the international Mountain Research Initiative in Perth, Scotland, last fall.

On the “upslope-downslope continuum,” it’s the big population centers in the valleys and on the coasts that pass the laws and set the agendas for timber harvest, land use, energy resources, air quality, water allocation and just about everything else that affects the highlands, she explained.

Policy isn’t the only thing that rises. Greenhouse gasses produced by cities and by fossil fuel users in the lowlands have caused temperatures to rise in the mountains. Research reveals that this warming is altering the foothills and forests of Oregon’s Cascades in measurable ways. Spring is arriving a full month sooner than it did 50 years ago in some parts of the H.J. Andrews Experimental Forest, Nolin says, citing the research of OSU atmospheric scientist Christoph Thomas. Winters’ final frosts, he found, are falling ever earlier on the calendar. Water levels in the McKenzie River are dropping. Lower elevation snowpack — accumulated layers of snowfall that build up and compact during the winter — is disappearing.

“When snow melts earlier, we lose water storage,” says Nolin. “Snowpack is a reservoir for us.”

In Oregon’s Hood River Valley, 50 to 80 percent of the water that irrigates crops comes from Mt. Hood’s glaciers and snowpack. If early melting trends continue, that priceless meltwater is in danger of dwindling by early- to mid-summer, leaving farmers in short supply during the hottest months when they need it most.

“Climate change,” Nolin says, “disproportionately affects mountain regions.” One reason is found in the physical properties of light and frozen H2O — properties she studied along with satellite remote sensing as a Ph.D. student at U.C. Santa Barbara. After having previously worked as a soil and water scientist, she became entranced by the elegant physics of light interacting with ice particles.

“Soil and snow are both particulate, porous substances,” she says. “But snow is so much more simple and clean. Radiative transfer theory is a very straightforward way to monitor snow from satellites.”

In fact, the glittering white of snow and ice is what explains the vulnerability of mountains to climate change. Whiteness, Nolin explains, reflects sunlight back into the atmosphere. As light-reflecting snowcaps and ice sheets shrink, more sunlight gets absorbed into the earth instead of bouncing off.

Melting accelerates as ever more light and heat are captured and held. Scientists call this phenomenon the “ice-albedo feedback.” As a vicious cycle, it causes temperatures to actually rise faster in ice-laden places than elsewhere on the planet.

Those ice-laden places include the North Atlantic island of Greenland, where as an early-career scientist, Nolin spent several summers studying polar climatology.

“It’s flat and white as far as you can see,” she recalls. But if that sounds like a complaint about the frozen landscape, she quickly sets the record straight. “It glitters,” she says. “It’s very pretty.”

On the Web: See more about OSU’s Mountain Hydroclimatology Research Group.

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