Debris Flow Recurrence On The Upper Snow Creek Alluvial Fan, Forest Falls, San Bernardino County, California

SMSU Event Center BC

Forest Falls, California is subject to destructive debris flows due to the town being built upon a large alluvial fan at the base of a steep mountain slope. During monsoon storms, characterized by extreme, shortduration, rainfall events, the area is subjected to floods and debris flows. Previous work has documented that monsoon events occur on average every 3.5 years, but damaging debris flows occur less frequently, perhaps with returns every decade or longer. The purpose of this study was to characterize debris flow deposition on the upper alluvial fan in terms of change over time. Using modern and historical photographs along with recorded data, one alluvial fan was mapped to determine how the distributary debris-flow channels occur and change over time and, importantly, how these channel patterns and deposition establish a feedback loop that influence the location and patterns of future stream courses. There is evidence of the existence of older channels that no longer flow due to newer debris flows overtopping levies and creating new channels.