The analysis of sediment data summarized for nine debris basins in Southern California shows that wildfires substantially increase erosion. However, the transient nature of this effect makes it difficult to assess the potential effect of wildfire from data analysis only. Therefore, a physically based simulation model having fine spatial and temporal resolution is used to predict the hydrologic and sediment fluxes from small burned and unburned catchments in the San Gabriel Mountains. These simulation runs show that fire significantly modifies the hydrologic response, especially sediment production which was increased from 7 to 35 times as a result of fire. The substantial increase in runoff and erosion following fire has also an impact on the hazards associated with the accelerated downstream flooding and debris flows.