Investigation of Flood and Debris Flow Recurrence: Andreas Canyon, San Jacinto Range, Southern California

An investigation was conducted to evaluate the existing flood and debris flow hazard at the mouth of Andreas Canyon, a major watershed that drains from the rugged eastern slope of the San Jacinto Mountains, California. Unlike archetypical alluvial fans, which form as a result of streams of water spreading sediment and cutting new channels on the fan surface, the Andreas Canyon fan was constructed principally by debris flow processes. Eleven debris flows of varying ages were mapped on the fan surface, ten of which originated in Andreas Canyon. The debris flows are rather large, with a typical volume of about 10⁵ m³. Nevertheless, hydrologic records available for the watershed suggest that storm events necessary to yield sufficient water to mobilize a debris flow of this size are not extremely uncommon, and should recur every few decades. Archeological records, however, suggest that Andreas Canyon has not experienced a major debris flow in at least 350 years. The absence of debris flows during this period suggests that their occurrence is tied to hydrologic events that are rare or absent in the current climate regime. Details of the type of hydrologic event necessary to produce a debris flow in Andreas Canyon are currently under investigation.