BIRDSEYE, Utah County -- Looking around the gun range nestled into the hillside in Spanish Fork Canyon it is readily apparent that the Thistle slide continues to rumble and roll.
Fence posts in the northwest corner of the Utah County Sheriff's gun range need almost constant repair because the ground keeps rising, causing the wire fencing to pop and break."We've actually seen quite a bit of movement this year," said sheriff's deputy Danny Curtis. "It's raised about 9 feet."
"I think it's moved more than that," said Sheriff David Bateman.
The gun range is near the area where a 1983 landslide buried part of the town of Thistle, displaced railroad tracks and dammed the Spanish Fork River, causing the remainder of Thistle to disappear under the resulting lake. New movement along the slide was detected in 1997 and again in 1998.
Francis Ashland, an expert with the Utah Geological Survey Association, said he isn't surprised to hear about the impact at the gun range.
"Yes, the landslide is moving again," Ashland confirmed. "In 1997, we noticed the upper part appeared to be moving in the winter but not affecting the lower part."
Researchers flew over the area and made the trek to the site in May 1998. Ashland says significant movement occurred -- and was continuing to occur -- in the area visited.
In all, more than 100 feet had moved, he said.
"The land piggybacked on top of the blockage from the 1983 slide," he said, "and a small finger moved through the county sheriff's gun range."
Ashland said a U.S. Geological Survey team paid for an aerial survey by a local company in 1998. The research verified that a large block of mountain broke away and bulldozed the original landslide.
"We think what happened is excessive precipitation caused a rise in the groundwater, which triggered the landslide movement," Ashland said.
Ashland believes if the major slide had not occurred in 1983, it would likely have been triggered during the wet years of 1997 and 1998.
"Thistle would have had only another 15 years," Ashland said.
Since 1998, the area has dried out and there's been no significant movement that he's aware of -- but a small threat remains. Precipitation in 1999 and during the first few months of this year has been below normal.
The Thistle slide is listed in a National Research Council report as North America's single most destructive slope failure. More than $200 million in direct costs are attributed to it.
"People come from all over, as far as Japan, to study the slide area," said Ashland.
"It's a virtual living laboratory. We monitor it to learn about slides in clay soils, which is a problem all the way up and down the Wasatch Front. It's a world class landslide."
Ashland said people have mistakenly assumed that the land involved in the 1983 slide moved as far as it could and eventually buttressed itself against the other side of the valley.
Pent-up pressure from the movement, however, continues to make it tough for drilling companies or surveyors to pull pipes and drills back out of the ground once they've penetrated the soil.
Those pressures are also pulling apart the fencing at the gun range.
A water line laid across the slide area by Spanish Fork City after 1983 is at risk, Ashland said. It needs to be moved, either to the sandstone rock on the canyon side or through the emergency spillway built to drain the dammed area of the river.
Transportation corridors have been moved, and the town of Thistle has been abandoned.
"We consider it (a new slide) very low-risk but it (the old slide area) is still moving. It would have to move a considerable distance and would take several years of movement to be a problem again, given the volume of material," Ashland said. "It's not going to happen overnight but it could affect the river again and that would bring flooding that could be a problem both upstream and downstream."
You can reach Sharon Haddock by e-mail at haddoc@desnews.com