2013 RIM FIRE INVESTIGATION – ORIGIN GEOLOGIC ASSESSMENT OF ROCK FALL POTENTIAL AS CAUSE OF RIM WILDFIRE

On August 17, 2013, at approximately 12:30 PM, a fire started on a steep slope near the Clavey River on the Stanislaus National Forest. The location of the fire is adjacent to the Clavey River, approximately 2.6 miles east of the junction with the Tuolumne River. A U.S. Forest Service crime scene investigation team was mobilized to investigate the cause of the fire. A geologist was included on the team because a witness to the fire stated that a rock rolled down the slope and started the fire. The geologic assessment consisted of accessing the site with the investigation team, collecting representative rock samples at the site, mapping and photographing the area near the origin of the fire, and conducting a literature review for any publications on rock fall initiating wildfires. Research was conducted to determine if earthquakes occurred on the day of the fire, as a possible energy source that could have initiated rock falls at the fire origin. Rock fall potential was determined at a broad scale and at the site scale. Rock outcrops were examined at the fire origin site to determine their rock fall potential. Rock samples were examined to determine their composition, hardness, and potential to cause a spark. Geomorphology was assessed including identifying major landforms, land morphology, slope gradient and roughness. All of this information was then considered to make a determination on the potential for a fire to start as the result of a rock fall in the area. A 560’ section of the Clavey River and the slopes above the river were assessed for rock fall activity and rock fall activity was present. Rock types in this portion of the river and on the slopes where the fire started are quartzite with minor amounts of chert. Examination of the rock samples collected determined that these rocks are all quartzite. Unlike chert, these rocks do not have the characteristics to create a spark when struck against another rock or ferromagnesium material.  Steel and/or other ferromagnesium rocks were not found at the field site. Ferromagnesium is a critical material to create a spark with flint that could ignite combustible material to start a fire. Based on my assessment of the field site and research it is unlikely and highly improbably that rock falls in this area could have resulted in sparks and caused the Rim Wildfire.