If you'd like to see where a significant amount of that water rushing through Los Angeles flood channels comes from, then take a look at the following video made by Marsha Perloff. Tujunga resident Marsha (Echo Park tries to claim her, but that's another story) lives about a football field away from this basin. This basin did indeed overflow for awhile, but for the most part, the system is working.
Now, for comparison purposes, look at this same basin just days before the rains began. The Army Corp of Engineers had spent weeks dredging it out in preparation for the storms: