As the rainy season nears in Northern California, areas burned by the summer’s massive wildfires are poised to launch flash floods of mud and ash soup on low-lying lands.
This is because the fires bake the soil surface so that it becomes non-absorbent, or, in scientific terms, hydrophobic. When heavy winter rains hit, the water cannot penetrate the burned soil, and instead rolls downhill — carrying boulders and trees along with it.
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Data from the U.S. Geological Survey can show which fire-scarred hillsides are believed to be at risk for debris flows.
These maps show where the risk is highest from three major fires that burned near Redding in the summer of 2018 — the Carr, Delta and Hirz fires.