The locals will fare just fine. This rain is nothing compared to what we've had in past years--the last El Nino brought us over 15 inches of rain in less than a week and the locals survived those and they've also survived the run offs after the Willow Fire, Old Fire, and numerous other fires over the years.
There will be mudslides--Highway 18 (Arctic Circle) to Big Bear has been closed this storm a couple of times. Nothing new as this occurs every year on that stretch--fire or no fire. It's the nature of the topography of the area. Steep slopes, minimal vegetation, loose decomposed granite and soils leads to slope slippage whenever the ground gets supersaturated.
If you go down into the locals in the early spring during runoff you'll see how much water rips through the channels. Also note how high the trees and brush are pushed during these run offs as the piles are very high up on the banks and there are some large trees in the piles. The fish survive these runoffs just fine.
I think the actual danger to the watersheds is man and what they do to the areas, not nature, but that's just my opinion.
"Should you cast your fly into a branch overhead or into a bush behind you, or miss a fish striking, or lose him,or slip into a hole up to your armpits-keep your temper; above all things don't swear, for he that swears will catch no fish."