How would you garden in a flood plain?

If I'm not mistaken the location you mention is fairly close to Los Angeles... If you are looking for a place to visit often, maybe even put a trailer on, it might be interesting.

I would certainly get started with watching the place when it is pouring rain, figure out where the water is going, decide if it would be conducive to some preliminary earthworks, perhaps try some black raspberries and even try heat resistant cranberries on the north side of hills, near the bottom of your slopes. I read there is real interest in trying to cultivate a heat resistant cranberry and it would be really an accomplishment if you succeeded but just a little patch where it looks cooler
As people are saying: look for perennials that grow there.

I would be fascinated to see the abandoned homestead but that's a warning
I imagine a future large earthworks project perhaps like the berm shed at permies, perhaps a trailer downwind. Not something too expensive in case it gets flooded.
Just because it's in a flood plain doesn't necessarily mean you can't have a lot of fun, learning, make improvements, and begin propagating what you want to grow there and elsewhere

I guess the question is how far is it from where you want to live / live when it's flooded?