Dick
Rustic is on County land, and the land above is designated a County park. This greatly limits what can and cannot be done with the land. In fact, if Rustic had been an invasive, heavy earth moving project as some suggest it should have, it never would have been built.
You are 100 % correct that trees brush and various plants above and through the wash would mitigate the flash flood danger, and you are also correct about the increased fire hazard.
Take a look at Ran's photos above (the top two posted by Brian), especially the first. The entire wash and most of the 12-15 mile watershed above used to be full of fully mature plantlife, including trees and chaparral which had been there for at least 20 years and maybe much longer. Probably 90+ percent of it burned, leaving virtually nothing to stop the water.
This is why I keep emphasizing the connections between the fires 16 months ago and these problems. This stuff takes many years to reestablish itself in this environment, so it will be a while before things will be back to normal. But it will be back normal eventually.
No chance one could go in and plant a bunch of non-natives. They probably wouldnt last anyway, since there is very little water in this valley.