Rock buttress as it appears now. (NPS)

JACKSON HOLE, WYO – So, remember that cracked boulder the park was worried would fall onto people visiting Hidden Falls in Grand Teton NP last summer?

It fell.

Back in July, park authorities became aware of a rock buttress high above the Hidden Falls on the west side of Jenny Lake that was experiencing unusual fissuring. There was some concern the rocks could come loose and potentially fall onto visitors of the popular viewing area.

That story morphed into ‘fake news’ about the Yellowstone caldera of all things but in reality, a closure of Hidden Falls was ordered on July 10. That lasted a month as park officials conducted an extensive risk assessment. It wasn’t until August 10 that closures were lifted after it was determined any rockfall would likely not reach the Hidden Falls viewing area.

It turns out that assessment was exactly right. And credit the park for exercising caution on behalf of public safety. They caught some flak for closing the popular tourist attraction for a month during prime time visitation.

Park authorities today announced that the large rock buttress above the Hidden Falls viewing area on the west side of Jenny Lake in Grand Teton National Park—the same one the park has been monitoring since July—has fallen.

Rock buttress as it appeared in August 2018 with red shaded area the part officials were worried would break off. It did, sometimes earlier this month. (NPS)

The extent of the debris and damage from the fallen rock was exactly what the a risk assessment and modeling that the park conducted earlier this summer said would happen.

Grand Teton National Park Deputy Superintendent Gopaul Noojibail said, “It appears that our risk assessment and modeling accurately aligned with the rock fall event that recently took place above Hidden Falls. Human safety is always the priority and our abundance of caution and the risk assessment served us and our visitors well.”

Park staff surveyed the site on Sunday, November 11 after receiving a report of the fallen rock from an employee of Exum Mountain Guides. After investigating the site and consulting with a University of Utah seismic expert who confirmed there has been no major seismic activity in the area in the last few weeks, park staff believes that recent seasonal weathering contributed to the rock fall.

Hidden Falls, Inspiration Point area under emergency closure orders for safety reasons. (NPS)

Judging from snow cover, it is estimated the rockfall event happened sometime before a snowstorm on Sunday, November 4.

The crack in the rock identified earlier this summer was approximately 100 feet long and it appears that the entire length of the crack broke off or calved from the mountainside. After initial observation, it appears that the Hidden Falls Overlook did not receive any damage. Large rock debris is located about 50 yards from the overlook with small rock and tree branch debris closer to the overlook area.

Park staff hopes to retrieve data from electronic monitoring equipment that was installed earlier this year to learn more about the incident.

In July an emergency closure was implemented in the Hidden Falls area for human safety due to the expanding crack in the large rock buttress above the Hidden Falls viewing area.  During the closure, National Park Service staff implemented multiple methods to monitor the situation and developed a risk assessment for potential rock fall.  Subject matter experts from the National Park Service Geologic Resources Division, Yosemite National Park, and United States Geological Survey Landslide Hazards Program were consulted.

Based on the risk assessment that used field observations and modeling regarding what would happen if the rock buttress were to come loose and fall, most of the Hidden Falls viewing area was reopened to the public in early August.  The modeling indicated that rock fall, if it did occur, would be unlikely to reach the viewing area due to distance and terrain.  A small closure remains in effect west of the viewing area.

“I am pleased that our risk assessment was accurate regarding the extent of debris and that we took subsequent actions to reduce the risk of possible injuries in a very popular area of the park,” Noojibail said. He added that any additional actions that may take place in the area will be based on further assessment, most of which will occur in the late spring or early summer.

Rockfall is a part of the naturally dynamic environment of mountains, and is always an inherent risk when traveling in the Teton Mountain Range. As a relatively young mountain range, the Tetons are still rising and actively eroding.