THE JUNE 3, 2005, BLACK MOUNTAIN DEBRIS FLOW, IRON COUNTY, UTAH

Reactivation of a landslide on the north side of Black Mountain in southwestern Utah generated a large debris flow on June 3, 2005. The landslide and debris flow were initiated by a combination of rapid snowmelt, weak geologic materials, pre-existing landslide deposits, and steep terrain. The debris flow was triggered when the lower part of the landslide released into a steep, narrow mountain channel where rapid mixing of the landslide mass transformed it into a debris flow. Flow down the steep channel accelerated the debris flow, resulting in a long runout distance. The debris flow traveled down an unnamed valley 1.6 miles before blocking State Route 14 (SR-14) and flowing into Crow Creek. The debris flow demolished a forest of mature trees along the upper valley floor. The debris-flow deposits in the upper part of the unnamed valley are wider, thicker, and rougher than in the lower part. Downstream in Cedar Canyon, water from Crow Creek and its tributaries diluted the debris flow into a sediment-laden flood that caused flood and erosion damage to SR-14 and closed the highway for a week.

A large volume of material remains in the landslide on Black Mountain, and this material could catastrophically release into the narrow mountain channel to form another large debris flow. The main hazards associated with a future large debris flow are direct impact and sediment burial in the unnamed valley and on SR-14. Hazards along Crow Creek and Cedar Canyon (Coal Creek) include flooding, erosion, and creek and culvert blockage. Only partial melting of the 2005 snowpack was needed to trigger landslide movement and the subsequent debris flow. The 2005 snowmelt pattern can be used to anticipate future landslide movement and possible debris flows.