Rain began falling in parched Southern California on Saturday in a boon for firefighters who were mopping up multiple wildfires. But heavy downpours on charred hillsides could bring the threat of new troubles like toxic ash runoff .

Rainfall has provided relief to wildfire-stricken Southern California but posed new threats of toxic ash runoff and debris flows. Authorities are expediting cleanup efforts and implementing flood-control measures to mitigate environmental impacts.