Under the sand! J/K
Seriously, you probably aren't going to find to much quality river fishing in So. Cal.
Where exactly are you?
When I lived there, all I found near San Bernardino was a few tiny creeks with equally tiny stocker trout, and I was told that these were Rivers and good fish, more than once. If I were you I'd take up lake fishing for Bass, Striper, or Catfish or ocean fishing, they have some awesome saltwater action down there.
Good luck

Tight lines,
Wade