Officials with the Utah Geological Survey and the National Weather Service warned Friday that, in addition to potential flooding, landslides pose what they call "a moderate threat" along bench areas from Weber to Utah, and are an even bigger danger in pre-existing slide areas.

"If you're in one of those areas, the potential for movement is high," said Francis Ashland, a senior geologist with the Utah Geological Survey. "What we're seeing in terms of movement is that most of the historical landslides have already reactivated this year. There were four damaging landslides in Davis, Salt Lake and Utah counties in 1998, and all four have now reactivated. That gives us an indication of where we're at in terms of landslide potential."

Setting the stage for such conditions: a second consecutive year of normal to above-normal precipitation in northern Utah that has raised groundwater tables all along the bench areas. Typically, the second year of such a weather regime is when slides start to occur.

"The soil moisture and infiltration of water is based on precipitation over a long period," said Brian McInerney, a hydrologist with the National Weather Service. "If you look at the last two months, we've been over 150 percent of normal across northern Utah. This is a phenomenon that occurs over time."

Weber County residents, of course, have already endured one slide.

A slope above a South Weber home failed on April 9, rumbling down through a dirt road, a canal and a rock wall before slamming into a home, injuring a child inside and causing significant damage.

State geologists consider that slide something of a one-time event, since human- placed fill was at the top of the 60-foot by 800-foot flow. The upper part of the slope had been modified over time by gravel pit operations on the bluff top.

"It was more of a debris flow," said Ashland.

But the slope is also characteristic of the most slide-prone areas along the Wasatch Front, which consist primarily of silt and clay. Less-prone slide areas are gravel- and sand-based.

That, Ashland explains, is why the ancient Lake Bonneville shoreline is more prone to sliding in Davis County - where "hundreds and hundreds" of slides are mapped - than Salt Lake County, where only 60 to 80 have been documented.

The Utah Geological Survey currently is monitoring 50 slide areas in northern Utah, collecting data on when they move, by how much, how much moisture is in the soil and where the water table is.

Unlike California, where a heavy rainstorm can trigger the collapse of a hillside, Utah slides are more the result of long-term precipitation, with the critical period spanning from March 1 to June 1.

With a dry May, Ashland says slopes would stabilize quickly. But if the current wet cycle continues, the landslide watch would likely continue into the summer. As things stand, state geologists are looking at some of the most unstable slopes they've seen since 1999.

"Based upon where the water table is now, we expect the potential for slides to remain high for the next 60 days," he said. "Last year we saw slides throughout the year. If we're still getting landslides into the summer, that scenario could repeat itself."

jbaird@sltrib.com

For more information on landslides in Utah, go to the Utah Geological Survey at http:// geology.utah.gov