ROCK-FALL MONITORING IN YOSEMITE NATIONAL PARK

Rock falls, rock slides, and other forms of slope failures are a serious natural hazard in Yosemite National Park. Following the 1980 Mammoth Lakes earthquakes, which triggered a total of nine rock falls and rock slides in Yosemite Valley, the U.S. Geological Survey (USGS) began to collect reports of historic rock falls and to examine and document recent rock falls with the National Park Service (NPS). This effort has been used to prepare rock-fall inventories that provide information on the number, size, frequency, and recognized triggers of rock falls at particular places within Yosemite National Park. As many as 540 reported slope failures have been documented in the park between 1857 and 2004, resulting in 14 deaths and at least 62 injuries. Many more slope failures undoubtedly occurred that were not observed or reported, primarily smaller failures, those that occurred in less frequently occupied areas, those that did not cause any structural damage or injuries, or during early historical periods with fewer park visitors.

Falling rock in Yosemite Valley continues to pose a serious geologic risk because of high potential for continuing failure from steep cliffs, the location of some facilities near the base of the cliffs outside of the Merced River flood plain, and the nearly 4 million annual visitors to the park. Since 1980, there has been an average of about 10 slope failures documented per year. Despite the detailed monitoring of rock-fall events, it is not possible to predict exactly where, when, and what size rock falls will occur. However, the rock-fall inventory provides information for assessing the potential of slope failures within the valley. Scientists and managers know that rock-fall potential can generally be higher during some seasons, such as winter and spring, or during specific events, such as earthquakes or large storms. The NPS developed a Yosemite Valley Plan in November, 2000 restricting future development in rock-fall talus areas depicted on USGS inventory maps. The USGS and NPS are continuing to work closely together to identify and monitor areas that could be hazardous during and following naturally occurring rock falls and to provide timely public information about any potentially dangerous conditions.