Developing a post‐fire flood chronology and recurrence probability from alluvial stratigraphy in the Buffalo Creek watershed, Colorado, USA

Abstract

AbstractStratigraphic and geomorphic evidence indicate floods that occur soon after forest fires have been intermittent but common events in many mountainous areas during the past several thousand years. The magnitude and recurrence of these post‐fire flood events reflects the joint probability between the recurrence of fires and the recurrence of subsequent rainfall events of varying magnitude and intensity. Following the May 1996 Buffalo Creek, Colorado, forest fire, precipitation amounts and intensities that generated very little surface runoff outside of the burned area resulted in severe hillslope erosion, floods, and streambed sediment entrainment in the rugged, severely burned, 48 km2 area. These floods added sediment to many existing alluvial fans, while simultaneously incising other fans and alluvial deposits. Incision of older fans revealed multiple sequences of fluvially transported sandy gravel that grade upward into charcoal‐rich, loamy horizons. We interpret these sequences to represent periods of high sediment transport and aggradation during floods, followed by intervals of quiescence and relative stability in the watershed until a subsequent fire occurred.An alluvial sequence near the mouth of a tributary draining a 0·82 km2 area indicated several previous post‐fire flood cycles in the watershed. Dendrochronologic and radiocarbon ages of material in this deposit span approximately 2900 years, and define three aggradational periods. The three general aggradational periods are separated by intervals of approximately nine to ten centuries and reflect a ‘millennium‐scale’ geomorphic response to a closely timed sequence of events: severe and intense, watershed‐scale, stand‐replacing fires and subsequent rainstorms and flooding. Millennium‐scale aggradational units at the study site may have resulted from a scenario in which the initial runoff from the burned watershed transported and deposited large volumes of sediment on downstream alluvial surfaces and tributary fans. Subsequent storm runoff may have produced localized incision and channelization, preventing additional vertical aggradation on the sampled alluvial deposit for several centuries. Two of the millennium‐scale aggradational periods at the study site consist of multiple gravel and loam sequences with similar radiocarbon ages. These closely dated sequences may reflect a ‘multidecade‐scale’ geomorphic response to more frequent, but aerially limited and less severe fires, followed by rainstorms of relatively common recurrence. Published in 2001 John Wiley & Sons, Ltd.