When a series of powerful January storms dumped huge volumes of rain across the foothills of the Cascade Mountains, Washington's top geologist, Dave Norman, knew to expect landslides.
During the first two weeks of the month, the intense storms pelted the already saturated ground in western Washington, triggering at least 1,500 landslides that damaged or destroyed an estimated 200 homes, buried sections of 150 roadways and contributed to thousands of people seeking safety in emergency shelters.
President Obama designated 23 counties in the state as federal disaster areas, and FEMA to date has handed out more than $7 million to homeowners for temporary housing and home repair or replacement.
"This was definitely a big event," Norman said. "You're talking about a significant number of landslides across the state."
The destruction highlights the often overlooked danger posed by landslides, one of the nation's most costly natural disasters, responsible for as much as $3.5 billion in damages and 50 deaths each year. And, experts say, changing climatic conditions could make landslide risks worse, especially along the West Coast.
Landslides are most common in Washington, Oregon and the mountainous regions of California, but they occur in all 50 states, according to government experts who are dedicated to studying the causes and effects of landslides.
In Utah, for example, a landslide that began a decade ago continues to crack foundations and ruin homes in North Salt Lake. In North Carolina, where slides have killed six people in five years, state lawmakers have proposed legislation requiring real estate agents to warn homebuyers of slide risks, particularly in the state's mountainous western corner.
Yet despite landslides' great destructive power, the federal government does not maintain a national system to track landslide activity. Instead, data is collected piecemeal by each state, and there is no standard for how such events should be reviewed and learned from, said Lynn Highland, coordinator of the U.S. Geological Survey's National Landslide Information Center in Golden, Colo.
Highland and others have lobbied for years for a national landslide inventory to identify the most hazardous areas. Such a system could prove highly useful when plotting new developments, as well as siting and designing new highways, pipelines and power line rights-of-way. But getting states to track landslides is "like herding cats," Highland said, because state leaders fear the federal government could use the information to restrict building activities in slide-prone areas. In addition, she said, the information could lower property values in areas otherwise prized for their sweeping mountainside vistas.
Yet by ignoring the real risk of landslides, some state governments are turning a blind eye to serious public safety issues as well as potentially driving up the costs of building and maintaining billions of dollars' worth of public infrastructure.
That is what happened in central Utah, where record-breaking rainfall triggered a sudden landslide that stretched more than a mile long and was 200 feet thick. The 1983 slide dammed up Spanish Fork River, flooding the small town of Thistle, severely damaging two major U.S. highways and cutting off railroad traffic between Salt Lake City and Denver. The town was wiped off the map, but it cost nearly $400 million (in 2007 dollars) to reroute the highway and rail lines and repair other damage, marking it as the most expensive landslide in U.S. history.
Sudden storms trigger the most dangerous and spectacular landslides, called "debris flows," which are characterized by subsurface failures that loosen soil and everything on top of it, including rocks, trees and buildings. Such slides are capable of moving downhill at 100 miles per hour or faster, officials say.
Landslides can also be triggered by other natural events, such as earthquakes and fires, or by human activities such as forest clear-cutting, which exposes soil and can cause massive mudslides.
The Pacific Northwest has an abundance of the three most common factors that contribute to landslides, said Scott Burns, a geologist and landslide expert at Portland State University.
"I like to use the old baseball analogy: Three strikes and you're out," Burns said. "Most of the Northwest already has two strikes against them: steep slopes and weak soils and rocks. The third strike is water, which in this case is rain."
The combination of slides and flooding damaged 1,455 homes in Washington this winter alone, according to state estimates.
Things were almost as bad in neighboring Oregon, where a massive thunderstorm that dumped 3.25 inches of rain on the Portland metropolitan area on New Year's Day triggered 20 major landslides that damaged dozens of homes, Burns said.
One of those slides, in the Lake Oswego area south of Portland, injured five people and caused a home to slide 200 feet down a steep slope, trapping a family that escaped through a window. Another slide buried a large section of U.S. 26, closing the main route between Portland and Mount Hood for nearly a week.
The risks are likely only going to get worse as populations grow and more and more homes are built on steep terrain, Burns said.
"Landslides don't get as much attention as a major earthquake because it may only hit one or two houses at a time," Highland said. "But the damage adds up. The impact in totality is huge."
Global warming's role?
And scientists say landslide conditions could grow worse, particularly as climate change triggers more frequent and intense thunderstorms like those preceding this year's devastating Pacific Northwest slides.
Such concerns are supported by the findings of a study published in the journal Science that concluded climate models have underestimated the degree to which human-induced climate change will affect extreme rainfall events (ClimateWire, Aug. 8, 2008).
But it remains unclear whether global warming is a direct catalyst for more landslides, said Nathan Mantua, a climate scientist and co-director of the Seattle-based Climate Impacts Group, which is studying the effects of climate change on the Northwest.
Rainfall data in Washington show no clear evidence of an increase in intense precipitation events over the past six decades, according to a recent analysis done by the Climate Impacts Group.
However, computer models developed by the group forecast that extreme winter rainfall events will likely increase in Washington and Oregon as the climate warms, and more heavy thunderstorms mean an increased risk of landslides in the Pacific Northwest, Mantua said.
Other experts, like Ian Kraucunas, a climatologist with the Board on Atmospheric Sciences and Climate at the National Academy of Sciences in Washington, D.C., remain less certain about the link between warming and landslides.
"Events like more extreme rainfall are consistent with what we would expect from global warming," Kraucunas said. "But as far as [saying] that it played a significant role in a particular landslide event, the data are just not sufficient to do so at this time, and it may never be."
Nevertheless, Highland of USGS said the agency this summer will propose a study on the effects of a changing climate on landslides, noting "it's an area that needs more study."
California's immovable slide
Perhaps no event better illustrates the destructive -- and disruptive -- power of landslides than the now infamous April 2006 slide in the Sierra National Forest in central California.
With little warning, an avalanche of rocks as big as dishwashing machines came tumbling down the side of the Merced River Canyon and into the river below.
Within minutes, 6,000 truckloads of rock buried a 45-foot section of U.S. 140 -- one of three major routes into Yosemite National Park -- stranding tourist buses and cutting off the town of Mariposa, Calif., whose economic lifeblood is Yosemite park visitors.
"We've had about nine businesses that were forced to close," said Marilyn Lidyoff, the economic and business coordinator for Mariposa County.
But federal officials have more than the local economy to worry about.
If the tons of loose rock still clinging to the side of the canyon ridge fall into the Merced River, it could dam up the waterway and flood homes and towns miles upstream, said Alan Gallegos, a Forest Service geologist at the Sierra National Forest.
A 2007 study by USGS's Landslide Hazards Program concluded that that is exactly what would happen if the area were hit with a heavy rainstorm. The central California area is prone to what locals call "Pineapple Express" storms that originate from the South Pacific and can dump as much as 8 inches of rain within two days.
"The landslide really has not been tested yet by this max-type of a precipitation event," Gallegos said.
The Forest Service used helicopters to place global positioning system monitors on three sections of the landslide. The readings indicate that the landslide continues to move -- as much as three-quarters of an inch during a rain shower, and about a quarter-inch a day most other times, Gallegos said.
In addition to the GPS monitors, the Forest Service has two real-time water level meters in the river, one immediately upstream of the landslide and one downstream. If the upstream monitor shows a sudden rise in water level while the downstream monitor drops, they will know that the worst has happened, Gallegos said.
While Gallegos said the landslide "appears to be settling down," there are no guarantees that it will stabilize on its own.
"We think there's a low likelihood the landslide could fail," Gallegos said, "but we are being very conservative to ensure public safety."
Meanwhile, the Merced River Canyon slide continues to block U.S. 140 due to the presence of the limestone salamander -- a rare amphibian barely 3 inches long that has taken up residence in the rock pile's cracks and crevices.
The salamander is listed as threatened under California law, and the slide area lies within a federal Area of Critical Environmental Concern, so harming even one salamander is illegal.
The combination of problems has vexed highway engineers trying to repair the blocked portion of U.S. 140. Shortly after the landslide, the California Department of Transportation installed two temporary bridges across the Merced River to detour motorists around the rock pile -- a move that helped save Mariposa.
One emerging idea for a permanent solution is to burrow a tunnel through the landslide pile to restore traffic. But CalTrans, in part to avoid harming the salamander, favors building two large permanent overpasses that would cross the river at two points. The Forest Service and some environmentalists fear this would ruin the the scenic beauty of the Merced River, which is designated a National Wild and Scenic River.
Knowledge reduces risk
Experts say difficult situations like that in the Merced River Canyon will continue until efforts to map landslide activity and areas of high risk are ramped up at the state or federal level.
Some states have begun doing this, including Washington, Utah and North Carolina. USGS is working on a landslide inventory pilot project with these states, as well as with California, Kentucky, New Jersey and Oregon.
One of the principal components of the effort is to map areas where landslides have already occurred. That is significant because "if the land has moved once, it has a higher potential of moving again," said Scott Burns, the Portland State University geologist.
For example, geologists believe the origins of the Merced River Canyon landslide date back more than 1,000 years, meaning the area is highly susceptible to further landslide activity.
In Oregon, researchers have discovered that "there's literally thousands and thousands more landslides than we originally thought," said Bill Burns, an engineering geologist with the Oregon Department of Geology and Mineral Industries.
Following catastrophic landslides in 1996 and 1997, the city of Seattle obtained federal grants allowing USGS to map hazardous landslide areas. City and the federal agents also developed an early-warning system that alerts residents when conditions are right for slides, said Bill Benzer, landslide mitigation manager for Seattle Public Utilities.
The city has used the data to steer new development away from trouble spots, or to mandate soil stabilization measures in at-risk areas where building does occur.
But most of the rest of the country lives with the incalculable risk of a catastrophe.
"When someone applies for a building permit, they assume that if they get it, the house is safe," Burns said. "But there's no information on landslides. There's no way to know whether there's a chance the house you're building is going to slide down a hill, or that something one day is going to slide into it."
Scott Streater is a freelance journalist based in Colorado Springs, Colo.