YOSEMITE — Rockfall activity in 2016 was slightly lower than in previous years, with 58 documented events (rockfalls and rockslides) and a cumulative volume of about 5,000 cubic meters (roughly 15,000 tons). Nevertheless, many of these rockfalls were consequential, impacting park infrastructure and affecting park visitors, employees, and residents.

The year got off to a quick start with a rockfall on Jan. 9 from “Little Windy Point” near Dog Rock. Roughly 90 cubic meters (270 tons) of rock slid onto the El Portal Road, blocking both lanes. The road was closed for several days as the cliff was assessed and the road cleared. This same location was active again almost exactly one year later.

Later in the year, another larger rockfall in the Merced River Gorge impacted the El Portal Road on the rainy afternoon of Oct. 31 (Halloween). A huge boulder (approximately 1,000 cubic meters, or nearly 3,000 tons) perched on the canyon rim above and west of Kat Pinnacle slid out along saturated soil and tumbled down the slope toward the river. Fortunately the boulder stopped against a bedrock outcrop midway down the slope, but another 80 cubic meters of associated rock debris landed on the road. The road was closed for a day as the rocks were blasted and cleared.

The most interesting rockfalls of 2016 happened at Middle Brother. Reminiscent of the 2009-2010 rockfalls from the Rhombus Wall, a series of rockfalls occurred from the lower part of the Middle Brother cliff over several months. The first occurred sometime in early February, as a roughly 1,000 cubic meter (3,000 ton) slab of rock exfoliated from the cliff, decimating the live oak forest at the base of the cliff and sending large boulders to the edge of the talus slope near Wahhoga.

Smaller slabs fell sporadically throughout the spring and summer. On the afternoon Aug. 3, hundreds of park visitors witnessed two large rockfalls in quick succession that produced large dust clouds. Another rockfall occurred on Aug. 4, and four more occurred on Aug. 5. In all, some 2,000 cubic meters (nearly 6,000 tons) of rock were shed from the cliff in 2016. This “progressive” rockfall behavior is occasionally displayed in exfoliating landscapes and is an area of vigorous scientific research.

Other substantial rockfalls in 2016 occurred from Sunnyside Bench, the East Ledges of El Capitan, Panorama Cliff, Little Yosemite Valley, the Merced River Gorge, and Hetch Hetchy.

It is very likely that there were additional rockfalls and rockslides in 2016, but these events either were not witnessed or went unreported.

If you witness a rockfall of any size, encounter fresh rock debris, or hear cracking or popping sounds emanating from the cliffs, please contact park geologist Greg Stock at greg_stock@nps.gov, or contact Park Dispatch by dialing 911 within the park. Documented rockfalls are added to the park database, enabling long-term evaluation of rockfall activity to improve public safety.

Source: Greg Stock, National Park Service