All eyes on skies — and Utah hillsides

Usually geologists speak of time in 1,000-year chunks, or million-year eras.

A couple of scientists at the Utah Geological Survey are concerned just about the next few weeks.

The immediate calendar will determine the risk Utah residents face for landslides and debris flows like the slide at Cedar Hills at the end of April and larger flows such as the Thistle Junction landslide in 1983 that blocked the Spanish Fork River and flooded the small town.

The rest of May and the first few weeks of June are a critical melt window for the snowpack in Utah, which is above average in many places throughout the state, but especially in southern Utah, where floods and landslides have already threatened the breathtaking landscape and communities. The longer the weather stays cool, the more rapidly it will warm later in the season, creating a steep drop in the snow levels and a substantially greater risk for floods, landslides and debris flows.

"We haven't melted a lot of snow. We've been lollygagging around here for a couple of weeks, since the middle of April," said Richard Giraud, a geologist who specializes in debris flows at the Utah Geological Survey. "That window of opportunity gets narrower and narrower."

Areas most at risk for debris flows are homes near the mouths of washes or canyons in Davis County, at the base of all faces of Mount Timpanogos, near the south flank of the Uinta Mountains and the plateau east of Cedar City in Iron County. Giraud and other geologists think those areas are threatened because the mountains above them have above-average snowpack and steep slopes that may slough off dirt and debris.

Debris flows are like thick, muddy flash floods. Water running off snowpacks soaks the dirt underneath until it cannot hold any more water. The dirt breaks off, plunges down a slope and collects trees, rocks, boulders and anything else in its path. Usually, the flow heads for a pre-existing stream channel, but the rocks and trees in the mud clog that channel, forcing the quickly moving sediment out of the channel and into whatever is in its way — including homes and people.

Fourteen deaths from debris flows in approximately 155 years are a testament to the speed and deadliness of debris flows, Giraud said.

"You have to move quick," he said.

A debris flow in Rudd Canyon in Davis County in 1983 pushed houses off their foundations and caused "a lot of damage to landscaping, patios and porches. If you have basement windows, the mud and rock flows into the basement window well, breaks out the basement window and flows into the basement of the house."