A slurry of rocks and mud sounded like a freight train when it ripped through a popular Mount Rainier hiking destination in 2001 and scared some television viewers who believed their homes were in the path.
As it turned out, the debris flow at Comet Falls proved less dangerous than initially believed, but it gave scientists insights into a phenomenon that continues to mystify.
Such a debris flow likely added damage to Mount Rainier National Park when a flood – sparked by nearly 18 inches of rain in two days – shut it down in November 2006. Experts are concerned that the level of flood danger is increasing as sediment builds in glacier-fed waters like the Nisqually River.
Scientists suspect that climate change – specifically, shrinking glaciers that leave unstable rock behind – is adding to the risk of debris flows that help clog river channels downstream.
This summer, a team of researchers is gathering information at Mount Rainier that could help provide answers. One of the leading scientists is Gordon Grant, a U.S. Forest Service hydrologist and Oregon State University professor of geosciences.
“Geological record documents debris flows for as long as the mountains have been around,” Grant said. “But given well-documented glacier retreat here and elsewhere, now is a good time to ask whether glacial retreat is changing the risk.”
Among the scientists’ questions: Have debris flows become more frequent? Does this add to the dangers around the Nisqually River and Mount Rainier’s other glacier-fed rivers, making them more likely to jump their banks?
Consider this: At Longmire, the river is nearly 30 feet higher than most of the national park compound, including the popular National Park Inn, ranger housing, maintenance shops and other historically important buildings.
SEDIMENT BUILDING UP
Glaciers on Mount Rainier and elsewhere are shrinking, and glaciologists have blamed climate change. Debris flows typically begin where glaciers run out.
Moreover, sediment has been building – up to a foot per decade – on the bottoms of rivers such as the Nisqually, increasing the likelihood of future floods.
The process, which river experts call aggradation, is typical of glacier-fed rivers of this type.
But Scott Beason, a Mount Rainier National Park ranger who measured the river beds before and after the flood and wrote his master’s thesis about them, suspects that climate change has added to the risk of debris flows which help clog the channels.
During the 2006 flood, Beason used a real-time Internet connection to monitor a U.S. Geological Survey gauge in the Nisqually River. He noticed that water line didn’t rise smoothly. Instead the picture was interrupted by spikes, small jags representing pulses of debris, rather than water, which boosted the flow.
“The power of water to sculpt the land is just amazing,” Beason said. “It doesn’t take much to start a debris flow. … They’re just a completely different kind of monster than a river.”
BIRTH OF A DEBRIS FLOW
In 2001, scientist Carolyn Driedger and fellow U.S. Geological Survey volcano experts went up in a helicopter to check on reports of the debris flow at Comet Falls.
Driedger and her colleagues got a look the following day, when a second mass cut loose. “It was one of the most spectacular things I’ve ever seen in my whole life,” Driedger said.
A 6-foot-wide stream of melt water slipped out of one glacier basin and into another, then became a muddy slurry that bulked up as it enveloped a mass of rocky refuse and charged downhill.
Before that helicopter flight, few scientists had observed the birth of such flows.
The Comet Falls incident took place in summer and was unrelated to flooding. Even so, Grant suspects a link between debris flows and floods is common to volcanoes throughout the Pacific Northwest. “It’s a coupled phenomenon,” he said, meaning that volcanic debris flows are frequently associated with floods.
One theory is that climate change has increased the incidence of extreme storms. Scientists plan to look for a correlation between bad weather and debris flows, Grant said.
Also, Grant said his team plans to take a closer look at how debris flows come about. “We don’t really know the mechanism by which they begin,” he said. “A key issue is how they bulk up.”
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