OBSERVATIONS DEMONSTRATING THE RUNOFF-INITIATION OF THE JULY 12, 2008 PIUTE WILDFIRE DEBRIS FLOWS, SIERRA NEVADA, CALIFORNIA

On July 12, 2008, the Piute Mountains south of Lake Isabella, CA experienced a period of intense rainfall. At Piute Peak, a remote automated weather station recorded a rainfall intensity rate of 16.2 and 20.4 mm/h during a thirty minute period (De Graff et al., 2011). The rainfall triggered three large debris flows which passed down Erskine Creek, Clear Creek, and Thompson Canyon respectively (De Graff et al., 2011). Much of the headwater source areas for these debris flows were burned by the Piute wildfire eleven days earlier. Within a week of the storm event, field observations were made of extensive rilling throughout the burned area. This indicated debris flow generation was due to erosion and entrainment of material by surface runoff, a process well documented in other parts of the western United States (Parise and Cannon, 2012). Examination at two areas, one in the headwaters of Thompson Creek and the other within a major tributary to the South Fork of Erskine Creek, verified this process in the generation of the July 12th debris flows. The first area was within the first order channels in the headwaters of Thompson Creek near Piute Peak. Numerous rills spaced approximately 5 feet apart and incised up to 6 inches deep were noted in this area which experienced high to moderate soil burn severity. The rills discharged to a defined ephemeral channel. While sediment-laden water clearly flowed in the upper channel reach, its transformation into a debris flow due to progressive sediment bulking was evident by incision into channel sediments bordered by a mud veneer. This same transformation process was visible along the main channel within the headwaters of a tributary to the South Fork of Erskine Creek. Moderate to high soil burn severity was experienced within this area, too. Along this channel, small debris flows issued from minor tributary channels along the length of the main channel and contributed additional material to the main channel debris flow. Matrix-supported levees were present along many of these contributing debris flows. Evidence of more than one surge was seen at the junction between one of these contributing debris flows and the main channel debris flow. One small headwater debris flow failed to reach the main channel and instead ended in a deposit of matrix-supported material.