Spectacular granite ramparts with their wispy waterfalls that draw visitors the world over to the Yosemite Valley of Yosemite National Park are not static fixtures, but rather geologically active, capable of raining hundreds of tons of rock down with little notice.

You need not be present during such an event to know they occur. Walk about the valley floor and you'll find massive boulders interspersed with trees in many areas. One boulder in Camp 4, a campground near El Capitan popular with climbers, has a volume of nearly 3,300 cubic yards. Other massive boulders that have come down in centuries past can be seen in and around Curry Village.

More recently, a massive rockfall from Glacier Point in October 2008 led the Park Service to permanently close 233 buildings in a portion of Curry Village due to the continuing danger of rockfalls. While that closure greatly reduced the risk of visitors or staff being injured by rockfalls, more structures and even campsites need to be closed for safety's sake, a new assessment concludes.

The decision to relocate 18 cabins, close two dorms, and move eight campsites from Camp 4 to another campground is not necessarily indicative of a greater than typical number of rockfalls in the Yosemite Valley, but rather due to the greater numbers of visitors who want to explore the valley.

Evidence of past rockfalls is amply seen about the valley floor, with debris from some past events spreading more than 1,300 feet beyond the cliff base, according to park geologists. Today, according to a new assessment of rockfall dangers in the scenic valley, "approximately one rockfall occurs each week on average. ... a rockfall of approximately 10,000 cubic meters occurs each year in Yosemite Valley on average, and at least one rockfall greater than 100,000 cubic meters has occured in historic time (the ~600,000 cubic meter 1987 Middle Brother rockfall)."

The assessment, Quantitative Rock-fall Hazard and Risk Assessment for Yosemite Valley, Yosemite National Park, points out that rockfalls long have been seen as a "potent natural force in Yosemite."

"However, due perhaps in part to the smaller number of visitors to Yosemite Valley during the late 1800s and early 1900s, recognition that rockfalls also pose substantial hazard and risk was slower to take hold," the report continues. "This perception changed drastically following the 16 November 1980 rockfall onto the upper Yosemite Falls Trail, which caused three fatalities and at least 19 injuries, representing the greatest mass casualty incident in Yosemite National Park's history."

Though the risk of being killed by a rockfall in the valley is not great, since 1857 15 people have been killed and another 85 injured from rockfalls "and other slope movement," the report notes.

The peer-reviewed 87-page assessment was compiled by staff from National Park Service, U.S. Geological Survey, Research Institute for Geo-Hydrological Protection, and the Georgia Institute of Technology. It notes that, following the Curry Village closures dictated by the 2008 Glacier Point rockfall, "...the remaining risk associated with structures is highest in Curry Village, Camp 4, and the Curry Village Residential Area, respectively, with lesser degrees of risk in the LeConte-Housekeeping Camp, Sunnyside Bench, Castle Cliffs, Wahhga, and El Capitan study regions, respectively."

In determing these risks, the researchers not only took into account how close various structures (including restrooms) were to cliff walls, but also the "typical number" of people in the buildings at any given time. According to that analysis, "the remaining rockfall-related risk associated with structures in Yosemite Valley is highest in the Curry Village (40.7 percent of the total remaining risk), Camp 4 (26.9 percent), and the Curry Village Residential Area (20.6 percent) study regions."

"Lower amounts of risk are associated with structures in the LeConte-Housekeeping Camp (5.8 percent), Sunnyside Bench (3.2 percent), Castle Cliffs (1.7 percent), Wahhoga (1 percent) and El Capitan (0.1 percent) study regions."

Park officials, who did not immediately respond to an email request for details on when the closures would take effect, believe that the closure of buildings and campsites in these areas will "reduce the overall risk associated with structures in Yosemite Valley by 95 percent."

However, as the study notes, "...there are no absolutely safe or zero probability regions for extremely large rock falls or rock avalanches within Yosemite Valley."