Falling boulders are the single biggest force shaping Yosemite Valley, one of the most popular tourist destinations in the national park system. Now swaths of some popular haunts are closing for good after geologists confirmed that unsuspecting tourists and employees are being lodged in harm's way.
On Thursday, the National Park Service will announce that potential danger from the unstable 3,000-foot-tall Glacier Point, a granite promontory that for decades has provided a dramatic backdrop to park events, will leave some of the valley's most popular lodging areas permanently uninhabitable.
"There are no absolutely safe areas in Yosemite Valley," said Greg Stock, the park's first staff geologist and the primary author of a new study that assesses the potential risk to people from falling rocks in the steep-sided valley. The highest risk area is family friendly Curry Village, which was hit by a major rock fall several years ago.
A newly delineated "hazard zone" also outlines other areas, including the popular climbing wall El Capitan, where the danger posed by the rock falls is high but risk of injury is low because they aren't continuously occupied.
"Rock falls are common in Yosemite Valley, California, posing substantial hazard and risk to the approximately four million annual visitors to Yosemite National Park," reads the ominous opening line of the report.
The move to close parts of historic Curry Village, a camp of canvas and wooden cabins, comes four years after the equivalent of 570 dump trucks of boulders hit 17 cabins, flattened one and sent schoolchildren scrambling for their lives. The park fenced off 233 of the 600 cabins in the village.
The new report, obtained by The Associated Press, now identifies 18 more that will be closed Thursday.
An examination by the AP after the 2008 fall found park officials were aware of U.S. Geological Survey studies dating back to 1996 that show Glacier Point behind Curry Village was susceptible to rock avalanches. Yet visitors were not warned of the potential danger, and the park service repaired and reused rock-battered cabins.
Rock falls in and around the century-old Curry Village have killed two people and injured two dozen others since 1996. Since officials began keeping track in 1857, 15 people have died throughout the valley and 85 have been injured from falling rocks.
This new study, prompted by the 2008 Curry event, is the first to assess risk to people. Officials say dangers exist in nearly every national park but they are particularly acute in Yosemite given its unstable geology, which causes rock falls weekly. Park officials will use the study to develop policy that guides future planning.
Yosemite Valley is ringed by 3,000-foot walls of granite. Since the last glacier retreated 15,000 years ago, the biggest factor shaping the most popular tourist destinations in the park has been the sloughing of rock when granite heats and cools and eventually breaks along fissures and cracks.
Stock used laser mapping to create the first detailed look at the cliffs, which ultimately could identify which formations are most vulnerable.
The report shows the greatest dangers are within 180 feet of the base of the cliffs. However, there is a 10 percent chance a potentially deadly boulder will fall outside of the zone every 50 years.
With the removal of lodging from highly problematic areas and increased awareness, risk can be reduced by up to 95 percent, Stock said. "That's a huge reduction, but it's not possible to reduce all risk in the park."
Part of Yosemite's charm is the guest cabins and other structures built around boulders, some the size of houses. It was widely assumed that they could have fallen in one cataclysmic event. The new study concluded that the boulders had fallen over time, and the information was used to delineate the most potentially dangerous areas of the valley.
"It's easy now to look around and see all of these rocks and know there's a hazard here, but that hasn't always been the case," said park spokesman Scott Gediman.
In November 1980, falling rocks killed three people and injured 19 more on the trail to Yosemite Falls, the icon of the valley and one of the most popular visitor hikes.
The biggest modern-day rock avalanche occurred in 1987, when an unstable formation called Middle Brother on the north side of the valley launched the equivalent of more than 22,000 dump truck loads of rock onto the main road.
Last year 53 rock falls occurred, including a six-ton boulder that fell in September from the upper Yosemite Falls Trail onto an amphitheater. Fragments hit a footbridge where tourists take photos, but no one was injured.
Park officials said two employee dormitories and parts of three others built in 1999 would be closing, which will further exacerbate a critical staff housing shortage.
Also on the closure list: a half-dozen sites at Camp 4, a $5-a-night camping bargain near El Capitan used mainly by climbers.
A representative of the park's concessionaire said visitors with reservations in Curry Village this summer will still get rooms, and that some cabins can be moved to safer areas.
Rooms at the Yosemite Lodge and the historic Ahwahnee Hotel are not in the danger zone. But the Sierra Club's Tudor-style rough-hewn granite LeConte Memorial, a library and educational site, are.
Park interpretive exhibits explain the rock fall phenomenon, but no signs warn visitors about potential danger.
"We are about as close to predicting these events as others are at predicting earthquakes _ which is to say not very close," Stock said.
Follow on Twitter: http://twitter.com/TConeAP