YOSEMITE NATIONAL PARK, Calif. - Yosemite Valley is one of the most popular U.S. national parks, but the risk of falling boulders is forcing parts of it to close for good. Geologists say unsuspecting tourists are in harm's way in a valley ringed by 3,000-foot (914-meter) walls of granite.
"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 from falling rocks. The park gets about four million visitors a year.
The National Park Service announced Thursday that the danger leaves some of the most popular lodging areas permanently uninhabitable. The highest risk area is Curry Village, where equivalent of 570 dump trucks of boulders hit 17 cabins, flattened one and sent schoolchildren scrambling for their lives.
An examination by The Associated Press after that 2008 incident 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.
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 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.
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.
A newly delineated "hazard zone" outlines other risky 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.
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 amphitheatre. Fragments hit a footbridge where tourists take photos, but no one was injured.
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.