If you take a look at Utah's big landslide years, says M. Lee Allison, director of the Utah Geological Survey, "they've been wet years."
Two years ago it was wet. Last year was even wetter, the moistest since the "flood years" of 1982-84 -- the era of the infamous Thistle slide, a river down State Street and the Davis County debris flows.1997-98's precipitation was "the main natural factor contributing to recent landslide movement" along the Wasatch Front, said UGS geologist Francis Ashland.
Layton, North Salt Lake, Salt Lake City, Provo and other communities all saw significant, even neighborhood-threatening slides.
That raises the question: What's going to happen in the spring of '99?
"If there hadn't been landslides in '97-98, we wouldn't have many concerns for this year," Ashland said. However, with an already high water table in some areas, "there's a good possibility we'll see movement in the landslides this year even if we have only normal precipitation."
So far the winter months have been mild and the water year unimpressive, with precipitation in some areas near average or significantly below the rates of the previous two years.
If this spring's high-mountain snowmelt is gradual, "the ground can take it," Allison said.
But in some still-saturated places, merely watering the lawn on a precarious slope could result in slippage, he said.
When we glance to the now-snowcapped Wasatch peaks, most of us see a beautiful backdrop for our cities, playgrounds, serene havens for our spirits.
UGS geologist Gary Christenson notices something else.
Those mountains, he said, "long to be flat." All summits and ridges are -- thanks to erosion and other forces of nature -- wearing away, destined to make their way bit by bit to the plains and to the seas. "That's what they're trending toward," he said.
Slipping hillsides only occasionally get the public notice they deserve, Allison said.
"Earthquakes are like a 747 crash -- they get worldwide attention," he said. "But landslides are like a car wreck."
All told, car accidents cause more damage and claim more lives than the crash of an airliner. The same can be said of landslides, Allison added.
Slides occur when soil, rock or debris shifts downslope due to gravity, oversaturation and the motion of earthquakes, said Francis Ashland of the Utah Geological Survey.
"They can move at very slow, almost imperceptible speeds, or very fast," he said, though most of those in Utah are of the more gradual sort. Rarely will Utahns see houses dramatically tumbling off cliffs on the evening news, as seems annually to be the case along the West Coast. Nor do we get the killer mountain slides of the Andes or the volcano-spawned mudflows that swallowed 20,000 Colombians in 1985.
Still, in areas occupied or used by man, landslides can cause significant property loss -- obstructing or damaging roads and severing such utilities as water, gas and sewer lines or damaging or destroying buildings and homes.
The U.S. Geological Survey estimates landslides and other ground failures annually claim more than 25 lives in the United States and cause an economic loss of $1 billion to $2 billion. That, adds the National Research Council, is more than all other natural hazards combined.
"Landslides commonly occur with other major natural disasters such as earthquakes and floods that exacerbate relief and reconstruction efforts," a USGS Web site notes, "and expanded development and other land use has increased the incidence of landslide disasters."
Since Utah has so far escaped the havoc of a major earthquake, slides are the most persistently damaging geologic hazard in the state. In just in the past few years, several houses -- in Layton, North Salt Lake and Provo -- have been condemned and two demolished because of landslide damage, even as neighbors continue to battle the phenomenon. Other newsmaking sites have ranged from Heber City to Springdale.
A historic footnote
The Beehive State's most-often referenced contribution to the nation's landslide literature is, of course, the Thistle disaster of 1983. Although it caused no casualties, the slide remains Utah's -- and North America's -- single most destructive slope failure, Robert L. Schuster noted in a report for the National Research Council.
"It is basically a slurry of debris and rocks and mud," cut loose by a slope failure high on the mountain, Christenson said. And the entire slide was on the move again, though slowly, in 1998.
In 1983 the slide dammed the Spanish Fork River, drowning the small community of Thistle, destroying 10 homes, 15 businesses and the Denver & Rio Grande Western Railroad switching yards. It severed D&RGW's rail line as well as U.S. Highways 50-6 and 89. The transcontinental routes had to be rerouted and rebuilt. An economic analysis prepared by the University of Utah in 1984, Schuster said, pegged the direct costs of Thistle alone at more than $200 million.
"In addition, numerous indirect costs were reported," Schuster wrote. "Most of these involved temporary or permanent closure of highway and railroad facilities to the detriment of local coal, uranium and petroleum industries; several types of businesses; and tourism. . . . Perhaps the largest single loss due to the Thistle landslide was $81 million in revenue lost by the D&RGW in 1983 as a result of the slide."
Despite such numbers, nationwide coordination of reports and research has been haphazard, Allison said.
State geologists across the country, working with the U.S. Geological Survey, are looking at ways to better disseminate research and details about landslide mitigation efforts, Allison said.
"One year from now we hope to have a national plan with strong state and local components to it," wherein scattered agencies will be able to share data, Allison said.
For instance, he said, "What they use in Colorado, we can bring here," and vice versa, because of similar land formations.
The USGS administers the National Landslide Hazards Program, an effort under way since the 1970s to gather information, conduct research, respond to emergencies and produce scientific reports for geologists, planners and consultants.
In addition to working more efficiently together, geologists also hope to help local governments and the general public learn more about landslides -- and the roles both nature and man play in causing them.
"I'm hopeful landslides will become more visible on people's radar screens," Allison said.
The Utah Geological Survey reports on specific landslides and other issues on a Web site at (www.ugs.state.ut.us). The U.S. Geological Survey's National Landslide Hazards Program has a Web site at (landslides.usgs.gov/landslide.html).
Next: Getting to know landslides