NPS just came out with an estimate of how large the March 28 Ahwiyah Point rockfall was, and I was surprised to learn that it was nearly 50% larger than the major rockfall in 1996 that fell near Happy Isles. Turns out, there has been a really interesting conversation going on over at Supertopo about the rockfall, and I picked up some more interesting information as well, and would recommend that as a good source for pictures and analysis about where the rockfall came from.
The main impact of the Ahwiyah Point rockfall is the closure of the southern part of Mirror Lake Loop Trail – part of the trail that often used by the Yosemite Valley Stables. With the Stables scheduled to open for business on April 10, I was a little worried about where they would end up going, and how that would impact their trail rides. However, in a case of making lemonade from lemons, (or maybe in the habit of not making a big to-do about little things) I was told that they planned to change their route just a little and incorporate more information about the geology of the Valley and educate people about rockfall. The story of rocks in Yosemite is a pretty interesting one, so people going on the 2-hour trail ride will be in for a treat.
One important note for the climbers: it sounds like the Slabs Approach to Half Dome has not been affected by the Mirror Lake Loop closure. According to the lead climbing ranger (JesseM – again from the Supertopo thread), that approach is still ‘open’, but is also still ‘active’ in terms of rockfall in its own right. Go places at your own risk.
Anyway, here’s the text from the NPS page…
On March 28, 2009, a large rockfall occurred from Ahwiyah Point near Half Dome. Rocks fell roughly 1,800 feet, knocking down hundreds of trees and burying hundreds of feet of trail on the southern portion of the Mirror Lake Loop Trail. The impact generated ground shaking equivalent to a magnitude 2.4 earthquake. Numerous smaller rockfalls have occurred from Ahwiyah Point since the initial rockfall on March 28. There were no injuries or structures affected.
The approximate volume of the initial rockfall is 43,000 cubic meters, or 115,000 tons. This is considerably larger than the 1996 Happy Isles rockfall, which was estimated at 30,000 cubic meters in volume. Therefore, this rockfall is the largest one in Yosemite National Park since the 1987 Middle Brother event.
Due to the debris and trail coverage, the southern portion of the Mirror Lake Loop Trail is closed to hikers indefinitely.
Because of the most recent rockfall activity around Yosemite Valley, there has been speculation that rockfall has become more frequent. Based on historical databases and recent events, park geologists are unable to discern a geologically significant increase in rockfall activity in Yosemite Valley.
Rockfalls are a natural and dynamic geologic process. Due to its steep, glacier-carved cliffs, Yosemite Valley experiences many rockfalls each year. Natural processes like rockfall help to create the beautiful and changing scenery in Yosemite National Park.