The Search for the Riverhound
Late in the day on Wednesday 7/14/2021, Annie Crowley and I gathered our supplies. Ice axes, a tent, layers to combat the cold temperatures, and most importantly fancy Ramen and...
On the afternoons of September 27 and 28, 2017 a series of rockfalls occurred from the southeast face of El Capitan, near the path of Horsetail Fall. Seven rockfalls occurred on September 27, and another much larger rockfall occurred on September 28. Rockfall in the area continues to be reported.
Initial September 27th rockfall, outlined in yellow. NPS
The initial rockfall on September 27 struck two climbers who were walking along the base of the cliff after retreating from the East Buttress route of El Capitan. YOSAR extracted the two climbers shortly thereafter. Andrew Foster of Wales, 32 years old, was found deceased, and his wife was transported by shorthaul to El Cap meadow and then an area hospital for treatment. Six more rockfalls occurred over a four-hour time span, for a total volume of about 450 cubic meters (a cubic meter is about the size of a washing machine), or about 1,340 tons.
There was another rockfall from the same area on September 28, 2017. This one was much larger than the rockfalls of the previous day. It measured at 10,250 cubic meters in volume, or about 30,500 tons. The rockfall generally propagated upward and outward from the origin of the September 27th rockfalls, greatly expanding the overall source area. Rock debris traveled along the same path as before, but because this rockfall had greater mass it had more energy for runout. Boulders reached the valley floor, and smaller fragments – termed “flyrock” and similar to shrapnel – traveled beyond the base of the talus slope to Northside Drive. A piece of flyrock struck a vehicle, hitting the driver and causing a head injury.
In many ways the rockfalls were typical in their volume and behavior. There have been larger historical rockfalls elsewhere in the park, including the 1872 Liberty Cap rockfall, the 1981 Elephant Rock rockfall, the 1982 Cookie Cliff rockslide, the 1996 Happy Isles rockfall, and the 2009 Ahwiyah Point rockfall. The September 28, 2017 rockfall ranks as the 29th largest rockfall in Yosemite on record. Also, the “progressive” nature of these rockfalls, with several events from the same location, has also been seen at other locations in Yosemite, including Middle Brother and the Rhombus Wall. An important take-away is that exfoliation-type rockfalls like this are commonly active over hours and days, and sometimes longer.
The “progressive” nature of these rockfalls, with several events from the same location, has also been seen at other locations in Yosemite, including Middle Brother and the Rhombus Wall. An important take-away is that exfoliation-type rockfalls like this are commonly active over hours and days, and sometimes longer.
The consequences of these rockfalls were less typical. It has been 18 years since the last rockfall-related fatality in the park, when rock climber Peter Terbush was killed by a rockfall from Glacier Point on June 13, 1999. There have now been 16 fatalities and more than 100 injuries from rockfalls since records began in 1857.
Comparative photos of September 27th and 28th rockfalls. NPS
What Causes Rockfall?
After rockfalls like the recent ones from El Capitan, a common question is “What causes rockfall?” The question can be answered on two levels. The first, and relatively simple level, involves those processes that destabilize the rocks. A number of geologic processes set the stage for rockfalls, including glaciation, weathering, and bedrock fractures. Tectonic stresses and erosion cause granite rock to fracture. Rockfalls later occur along these fractures. Fractures that develop parallel to the surface are called sheeting or exfoliation joints. Sheeting joints create large slabs of rock that ultimately fall away in a process known as exfoliation. In Yosemite Valley, Royal Arches and the face of Half Dome are examples of landforms that have resulted from this process. Over long periods, water flowing through fractures decomposes the bedrock in a process called weathering. Weathering loosens bonds that hold rocks in place.
Rockfall activity associated with seasonal timing. NPS
At the second level, we often think of rockfalls as being “triggered” by a specific force such as water, ice, earthquakes, or vegetation growth. If water enters fractures in the bedrock, it can build pressure behind unstable rocks. Water also may seep into cracks in the rock and freeze, causing those cracks to grow. This process is called “frost wedging” or “freeze-thaw” and can incrementally lever loose rocks away from cliff faces. Recent research suggests that daily temperature variations and extreme heat can also cause rock slabs to become unstable. Ground shaking during earthquakes often triggers rockfalls. Additionally, a variety of vegetation grow into the sheer rock faces where their roots expand and pry apart joints in the granite. Most rockfalls in Yosemite occur in the winter and early spring, during periods of intense rainfall, snow melt, and/or subfreezing temperatures, but many large rockfalls – like the recent ones from El Capitan – have also occurred during periods of warm, stable weather.
Recent research suggests that daily temperature variations and extreme heat can also cause rock slabs to become unstable.
For any given rockfall, there is always a large degree of uncertainty about what exactly triggered it. Historical records indicate that more than half of all documented rockfalls in Yosemite were not associated with a recognizable trigger. Recent research is exploring how cyclical processes, such as daily, seasonal, and annual temperature changes, can weaken rocks through stress fatigue and cause them to fall without a specific, recognizable trigger. For these reasons, predicting rockfall events is not yet possible, but understanding the forces that trigger rockfalls is an important step toward this goal.
September 28th rockfall seen from high on the Waterfall Route, El Capitan. Photo Pete Zabrok
What Are The Primary Rockfall Related Concerns Rock Climbers Should Be Aware Of?
Visitors to Yosemite face rockfall risk whenever near any vertical wall in Yosemite. Rock climbers tend to be more at risk because they are in closer proximity to the cliffs. How can climbers take responsibility for addressing rockfall risk? Unfortunately there are no hard or fast rules, but here are a few things to keep in mind. Rockfall areas are often active for many weeks or months, so avoid climbing beneath these areas; the southeast face of El Capitan is presently a good example of this. Reports of continued rockfall coming from from the southeast face of El Capitan show that the area remains an active rockfall zone. Fresh talus and/or damaged vegetation at the base of your intended climb are good indicators of recent activity. Be especially aware of cracking or popping sounds emanating from the cliffs, as these sounds have preceded many rockfalls. A helmet may not save you from a large rockfall, but it could offer protection from flyrock that accompanies most rockfalls. Minimize your time approaching or bivying at the bases of cliffs, particularly those in known active rockfall zones. Loose rock is also often encountered on Yosemite climbs. Knock on questionable blocks or flakes to assess their stability and do not continue if not comfortable with the terrain. Lastly, if loose or hollow holds must be used, focus on pulling down on them and not out.
How Does The Park Research Rockfall?
The National Park Service in Yosemite is responding to rockfall in a variety of ways. Park scientists, in collaboration with the U.S. Geological Survey (USGS) and academic researchers, are actively studying rockfalls through the use of new technology, such as high resolution digital photography, laser mapping of cliffs, rockfall runout modeling, and exposure dating of ancient rockfall boulders. These tools offer vast improvement in resolution from previously available data, allowing geologists to accurately map rockfall zones and to study rockfall source areas. The park is also actively investigating methods for monitoring rockfall activity. Links to publication on Yosemite rockfalls can be found on the park’s webpage: https://www.nps.gov/yose/learn/nature/rockfall.htm
What Should You Do If You Witness Rockfall?
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 209/379-1420 or by email at greg_stock@nps.gov, or contact Park Dispatch by dialing 911 within the park. Documented rockfalls are added to the park database (http://pubs.usgs.gov/ds/746/), enabling long-term evaluation of rockfall activity to improve public safety.