ANSWER: What the photo shows is Sepulveda Dam and a portion of the huge Sepulveda Flood Control Basin. It is situated just west of Sepulveda blvd. and extends between Magnolia and Victory Blvds. In the general vicinity of Van Nuys.
ANSWER: Yep, it's the spillway of Hansen Dam. (Didn't you notice those foothill peaks peeking through the opening?) And sure, it's racy. They have outboard motorboat races on the lake behind it and sports car races on a track directly below it.