A COMPARISON OF WATERSHED RESPONSE FOLLOWING THE 1990 AND 2011 WILDFIRES IN MERCED CANYON NEAR EL PORTAL, CALIFORNIA

The 1990 Arch Rock Wildfire and the 2011 Motor Wildfire produced high to moderate soil burn severity on steep slopes within the small tributary watersheds to the Merced River in the vicinity of El Portal, California. More than half the area burned by the Motor Wildfire coincided with slopes burned by the earlier Arch Rock wildfire. Debris flows were generated by the first significant storm during the winter rainy period following each of these wildfires. Three debris flows threatened or impacted residences within the community of El Portal at 11:00 PM on March 3, 1991. Three debris flows; two impacting California Highway 140 and one impacting Mariposa County road 136, occurred about 4:00 AM on January 21, 2012. Within the area common to both wildfire events, the watershed response in 1991 and 2012 consisted of significant water flow in the stream channels which impacted the culverts and road bed of Mariposa County road 136. At several points, deposition of sandy sediment accumulated on the road or near residences in a small area of private land along Mariposa County road 136. The debris flow impacting the County road was unique to the 2012 storm event. The March 3, 1991 storm represented the only significant rainy season storm during a period of drought. The January 21, 2012 storm was the first of a series of storm events. The later storms, which included some larger than the one which triggered the debris flows, only generated high water flows which caused additional erosion and sedimentation. This comparison of watershed response after wildfire occurrence demonstrates the similarity of both the response types (debris flows, high water flows, and depositional features) and triggering storm event for this area of the Merced Canyon despite vegetative recovery during the intervening 20 years. It also illustrates the benefit derived from collecting data about a recurrent natural hazard in a particular geographic area.