California communities and waterways near scorched hillsides are vulnerable to serious disasters, long after wildfire flames die out. Downpours can wash away millions of tons of soil and rock each year from California slopes, according to a study published this week, led by scientists from the U.S. Geological Survey and California Geological Survey. Researchers found this postfire erosion has dramatically accelerated across Northern California over the past four decades. Burn scars are especially at risk of debris flows, fast-moving slurries of mud and rock that can destroy homes and wipe out roads. But even smaller trickles can unload soil into waterways and clog up lakes. The researchers reported that across the state, 57% of the material flushed from postfire locations was upstream of reservoirs. The cycle could amplify in the future, as the potential for California blazes continues to climb due to climate change.

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