Extending over more than 113,000 hectares, the Thomas fire, burning in December 2017, stripped away vegetation and altered the condition of the soil. By removing the above-ground canopy of plants and decreasing water infiltration rates, the fire decreased rainfall thresholds necessary to mobilize large volumes of sediment. Even though the fire was not completely extinguished before the onset of winter rains on 8-9 January, 2018, thousands of people were under evacuation orders as the rainfall fell on vulnerable watersheds. Measured rain intensities were as high as 144 mm/hr. The resulting debris flows and floods (see illustrative videos here) transported ash, mud, boulders, and trees, over 5 m deep at speeds of 20 km/hr through alluvial fans at the base of the mountains, killing at least 20 people and damaging hundreds of homes, vehicles, and infrastructure. The U.S. Geological Survey assessed the burned areas to determine flash flood, mudslide and debris flow hazards. The comparison of images before and after the events published by the BBC are impressive
