Forest closes Greys River, too unstable after landslide

STAR VALLEY, WYO – After a risk assessment, Bridger-Teton National Forest announced today it is closing the Greys River Road (FR 10138) to all activity until further notice. The closure is in effect for the trailhead at the parking lot outside of Alpine to the area of the landslide at Porcupine Creek—about 17 miles down the road.

A section of earth began moving in early February. The slide ran over parts of Greys River Road near its junction with Porcupine Creek. A makeshift alternative route around the debris was created for snowmobilers weeks ago but after reassessing, the Forest Service has decided any travel in the drainage north of Porcupine Creek is not safe.

“We have finished analyzing the modeling that has been completed on a potential flooding scenario,” said Greys River District Ranger Justin Laycock. “While the model shows that the flooding risk to homes or property in Alpine, Wyoming is considered ‘low’ and limited to two structures, the risks in the corridor itself are much different.

U.S. Geological Survey technicians installing one of two emergency streamgages to monitor the level of the Greys River. credit Jay Bateman, U.S. Geological Survey

Engineers are concerned with a potential 40-foot high natural dam that models indicate could completely occlude the Greys River. In that event, water could back up behind a natural dam and if the dam were to later breech, “low lying areas of the Greys River corridor could experience up to a 10-foot wall of water,” Laycock said.

“Given the uncertainty of when or if the scenario could take place, and the fact that the ability to notify and clear the acres of affected area in the modeled scenario would take longer than the four hours [models show we would have] we would have to act, we must close the area to human presence until the situation stabilizes,” Laycock added.

The landslide is a quarter of a mile wide and is still slumping towards the Greys River. The Forest worked with the US Army Corps of Engineers (USACE) on modeling the flood scenario should the landslide create a natural dam in the Greys River.

“The predicted rain and warm up that is characteristic of the beginning of spring runoff has reached one of our established trigger points and is the reason for my decision to close the corridor immediately” Laycock said.

With the closure order, two of the Forest’s Guard Stations, which are usually available to rent, the McCain and Deer Creek Guard Stations, will be unavailable for reservations until further notice.

 Although the Alpine access and the Greys River Corridor is closed through Porcupine Road, visitors still may access the Forest beyond the closure by three other routes:

  • either by the Smith’s Fork, Forest Service Road 10072, road located South of Afton or
  • through forest service access from Big Piney, WY (proceed to Ryegrass Junction to Forest Service Road 10125, also known as N. Cottonwood, then over McDougal Gap to Greys River Road) and
  • The La Barge Road accessed from the town of La Barge, Wyoming.

“There is quite a bit of country still open and accessible upstream of the landslide but the area downstream is going to remain closed until further notice,” Laycock said.

NOTE: Greys River runs south-to-north in the drainage.

Unbelievable aerial footage of the Greys River landslide on March 10 by Sam Cook, Contour Aerials.

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