The Fire-Flood-Erosion Sequence in California: A Recipe for Disaster

Wildland fires are an inevitable component of many terrestrial ecosystems in California, where alternating episodes of heavy winter rainfall and dry autumnal winds, coupled with extended periods of drought increase the probability of major wildland fires. Factors influencing the intensity and duration of the post-fire hydrologic disturbance include the nature of the local vegetation, burn severity, the geology and topography of the burn area, and the local climate. Many parts of California are underlain by granitic bedrock. Wildfires in granitic terrain result in a common suite of hydrologic aftereffects. The Lowden Ranch Fire took place in an area of granitic bedrock and granite-derived soils in 1999. An investigation of the area in 2002 found only minor evidence of post-fire erosion and sediment transport as a probable result of generally moderate rainfall in the winter of 1999–2000 and the presence of deep, permeable granitic sand in the affected watersheds that reduced runoff. Observations made during our site investigation indicated that by 2002 the area had recovered about 70 percent of its pre-fire erosion resistance. This recovery was due mainly to the reestablishment of ground cover and the breakdown of any hydrophobic layer that had formed in the fire. Comparison of this area with granitic terrains of Southern California burned in 2002 and 2003 and affected by heavy rains in early 2005 indicate that such terrains are relatively resistant to erosion during light to moderate rainfall, but can experience catastrophic erosion during periods of heavy rainfall.