ROCK FALL AS POTENTIAL FOR CAUSE OF THE 2013 RIM WILDFIRE CLAVEY RIVER, CENTRAL CALIFORNIA

On August 17, 2013, at approximately 12:30 PM, the 257,000-acre Rim Fire started on a steep slope near the Clavey River on the Stanislaus National Forest. An Interagency Fire Investigation Team was mobilized to investigate and determine the cause of the fire. A geologist was included on the team because a witness to the fire stated “a rock rolled down the slope and started the fire”. Rockfall activity was assessed at four sites along the Clavey River, where the origin of the fire was identified by fire investigators. This area has a high rockfall hazard within the inner gorge of the Clavey River and rockfalls occur annually. Rock types where the fire started were determined to be argillite and quartzite. Unlike flint and 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. Rockfall deposits were not found at the site where the fire started and were probably moved down the river from high discharge flows during the 1997 flood event. A determination was made that it is unlikely that rockfalls in this area could have resulted in sparks and caused the Rim Wildfire.