Much-needed rain has begun to fall over Southern California, bringing relief to the drought-stricken region but also the threat of toxic runoff.

So far, many clues to the origins of the deadly Eaton fire, which started in the area just after 6 p.m. that evening and went on to kill 17 people, have pointed to the brushy hillside where a tangle of electrical lines stretch up Eaton Canyon.

With parts of Los Angeles County still smoldering from wildfires, the expected rain this weekend would seem like a welcome relief. But how the rain falls could make the difference between a disaster respite or a disaster repeat.