Historical Rock Falls in Yosemite National Park, California

Wieczorek, Gerald; Snyder, James; USGS

This report contains an inventory of historical rock falls reported in Yosemite National Park, with data on the location, date, type of slope movement, trigger, size-volume, types of damage, narrative description and reference(s) for 519 rock falls that occurred between 1857 and early 2004. About 330 of these events occurred within Yosemite Valley and most others were in the near proximity, for example, along El Portal Road in the Merced Gorge. This information was collected from review of published and unpublished historical accounts and field studies of recent rock falls. Many more than 500 rock falls undoubtedly occurred during this historical period, but many went unnoticed or unreported because of the small size of individual rock falls or the lack of impact on trails, roads, structures or utilities. This report is an update of a previous rock-fall inventory of Yosemite National Park (Wieczorek et al., 1992). Following the modern discovery of Yosemite Valley (1851), larger rock falls were mentioned in the writing of many visitors, including Josiah Whitney, State Geologist of California; John Muir, noted naturalist; and Joseph LeConte, Professor of Geology at the University of California. More systematic recording of both small and large rock falls affecting facilities began after 1916 with the monthly National Park Service (NPS) Superintendent’s Reports, because repairs were necessary to maintain damaged trails and roads. Although the bedrock and glacial geology of Yosemite have been studied extensively (Matthes, 1930; Calkins, 1985; Huber, 1987; and Peck, 2002), slope movement processes, such as rock falls and other forms of slope movement, have not been examined systematically. Rock falls, rock slides, and other forms of slope movement are a serious natural hazard in Yosemite National Park and, especially in Yosemite Valley. In this report, the term “rock fall” in the title is used as a generic, collective term for all slope-movement processes in Yosemite, including rock fall, rock slide, debris slide, debris flow, debris slump, and earth slump, individual types of slope movement according to the classification system of Varnes (1978). In addition to damaging roads, trails, and other facilities, rock falls endanger some of the more than 3 million visitors that annually visit Yosemite National Park. Between 1857 and 2003, 12 people have been killed and at least 62 injured by rock falls in Yosemite Valley. The U.S. Geological Survey (USGS) and the National Park Service (NPS) have cooperated to document rock-fall hazards in Yosemite National Park from archival records, aerial photographic interpretation, and field investigations. An initial collection of historical rock-fall information compiled by Snyder (NPS, unpub. data, January 1990) became the basis of the rock-fall inventory (Wieczorek et al., 1992 and this updated report). Additional investigations and mapping of prehistoric, historical and recent rock falls have been conducted in Yosemite (Rempel, 1983; Snyder, 1981, 1986a, 1986b, 1996; Wieczorek et al., 1995; 1998, 1999, 2000; Wieczorek and Jäger, 1996; Wieczorek and Snyder, 1999; Wieczorek, 2002).

Historical Rock Falls in Yosemite National Park, California

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