THE OAK CREEK WATER-SEDIMENT FLOWS OF JULY 12, 2008, INYO COUNTY, CALIFORNIA: A HAZARDOUS RESPONSE TO SIERRAN UPLIFT

Alluvial fans in Owens Valley form at rates on the order of 0.1 to 0.2 mm/year in response to subsidence of the valley. They are the result of debris flows that occur about once in several hundred years in individual drainages. On July 12, 2008 over 32 mm/hr (1.25 in/hr) of precipitation fell on the Oak Creek drainage north of Independence, in Inyo County, California. The drainage had been burned during the Inyo Complex fire of July 6, 2007. Debris and hyperconcentrated flows ran out 6 to 7 km from the mountain front, destroying 17 homes and severely damaging the historic Mt. Whitney Fish Hatchery. Traffic was disrupted on State Highway 395 for nearly a week. Although slopes were extensively rilled, most of the estimated 1-2 million cubic meters of sediment was scoured from channels and deposited over an area of more than 3 km², mostly on younger alluvial fans. Flow surges moved down the North Fork of Oak Creek at estimated speeds of 2 m/sec (~6 mi/hr) to 5.4 m/sec (~12 mi/hr) and were one to three meters high. Sand-rich, hyperconcentrated flows followed the active channel of the north fork of Oak Creek and abandoned channels on the fan filling them, and spread laterally across the interfluves. On the south fork of Oak Creek, boulder-rich debris flows clogged the active channel, leaving a boulder field of at least 1500 m long and 75 m wide, blocking the channel and forcing the south fork to a new course to the west. The largest boulders moved during the storm ranged from 3300 kg (~1300 lbs) to 17,000 kg (11,800 lbs). Older uplifted alluvial fans were unaffected while younger fans were greatly affected. The Oak Creek water-sediment flow is typical of the events that have formed the alluvial fans in the Holocene. Events during late Pleistocene glaciations were probably larger and more frequent.