A large rock buttress above the Hidden Falls viewing area on the west side of Jenny Lake in Grand Teton National Park that the park has been monitoring has fallen.

The extent of the debris and damage from the fallen rock was aligned with a risk assessment and modeling that the park conducted earlier this summer.

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.”

He adds, “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 the west side of Jenny Lake on Sunday, November 11 after receiving a report of the fallen rock from an employee of park concessionaire 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 believe that recent seasonal weathering contributed to the rock fall.

Summer 2018 Photo Courtesy: Grand Teton National Park
Summer 2018 Photo Courtesy: Grand Teton National Park

According to snow-cover, park staff believe that the rock fall 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.

November 2018 Photo Courtesy: Grand Teton National Park
November 2018 Photo Courtesy: Grand Teton National Park

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

The park has been monitoring this same rock buttress since mid-summer when the expanding crack in the rock was identified.

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.

Noojibail said, “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.”

He adds that any additional actions that may take place in the area will be based on additional assessment, most of which will occur in the late spring or early summer.

Rock fall 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.

Information Courtesy: Grand Teton National Park

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