THE MISSION DEBRIS FLOW: AN EXAMPLE OF A PREHISTORIC LANDSLIDE DAM FAILURE, SANTA BARBARA, CALIFORNIA

The large prehistoric Mission debris flow (10.9x10⁶m³±20%) covers 0.96km² of the city of Santa Barbara with coarse boulder debris to an estimated depth of 10-30m. Pristine geomorphic expression of three morphologic lobes, boulder levees, and plugged and abandoned channels, suggests that the flow maybe a late Holocene event. The Skofield landslide, a large (9.3x10⁶m³±10%) historically inactive landslide located 1.5km above the Santa Barbara piedmont in Rattlesnake Canyon, is identified as the likely source of the Mission debris flow. Close proximity to the flow deposit and similarities in volume, lithology and sedimentology between the landslide source and the Mission debris flow support this interpretation.

Surficial deposits near the landslide and several morphologic elements of the canyon indicate that the Skofield landslide produced a natural dam and temporary lake. We propose that the Mission debris flow was initiated by the failure of this landslide-dam. The former location of the dam is delineated by the coincidence of anomalous topography, high concentrations of unusually large boulders, and a prominent knickpoint in Rattlesnake Creek. Benched topography found at similar elevations on both sides of the failed portion of Rattlesnake Canyon suggest a blockage 30-45m high, potentially impounding 1.2-3.9x10⁶m³ of water as a temporary lake. The existence of a lake is supported by a 0.5m thick accumulation of fine-grained sediment found in a portion of the canyon presumed inundated. The volume of the lake would have been sufficient to mobilize the landslide blockage as a debris flow. The morphology of the Mission debris flow on the Santa Barbara piedmont, debris flow levees in the canyon, and the morphology and deposits of the Skofield landslide are consistent with flow initiation via the formation and failure of a landslide-dam.

The repeat of an event of this magnitude could cause millions of dollars in property damage and substantial loss of life. The results of this study identify a process and hazard that is unevaluated for the region. Due to the similar geology and geomorphic setting the results of this study, specific to Rattlesnake Canyon, may be extrapolated to other small energetic urban watersheds of Southern California.