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CSI professor talks about difference between a rock slide and a rock fall

Published: Jul. 12, 2020 at 2:45 PM MDT
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TWIN FALLS, Idaho (KMVT/KSVT) -US 95 near Riggins is closed indefinitely, after another rock fall event Friday, after the initial one on July 3rd.

But how common are they?

As it turns out, they're very common, not just near Riggins, but through out all of Idaho, explains College of Southern Idaho geology professor Shawn Willsey

Willsey also explained that the event near Riggins is a rock fall, and not a rock slide.

The difference is that in this case, the rocks became unstable and collapsed off the slope, rather than an entire mass of rock and land sliding

“Up near Riggins, rock falls are an incredibly common process, because we have a couple of factors. We have these very deep narrow canyons with the Salmon river and it’s tributaries moving through them. So we have very steep slopes and topography. And then compounding that, we have really weak rocks, so rocks that don’t hold up really well,” Willsey said.

Geologists can't exactly predict when one will happen, but they can identify problem areas.

“We can look at places that have had problems in the past. I was actually looking at some geologic maps that I found online, and they actually mapped some slides between Riggins and Pollock right where this rock fall took place,” Willsey said.

And if you're out and about, there are a couple things to look out for, including heavy rainfall events.

“Any sort of rock falls, even if its small rocks, pebbles coming down, could be a precursor to a bigger event as that slope becomes destabilized. So certainly if you see something falling, you should try to get out of the way as quickly as possible,” Willsey explained.

There’s one other thing that could lead to rock falls as well, an earthquake.

“I checked to see how far the rock slide near Riggins was to the epicenter, it’s about 85 miles. So it is possible that that earthquake in March you know shook enough of that area that it just slightly destabilized the rocks enough that were getting a rock fall event sooner than we might have otherwise. There’s no way to really know that and prove that per se, but its possible given the proximity that its caused some of these rock fall events to happen,” Willsey explained.

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