It's the LA Greenway/Bike Trail
The Los Angeles River Trail (Greenway/Bike Path) : National Recreation Trail Database
They start out calling it a bike path, and then peds and skaters get on it, and then becomes a MUP.
Interesting link... says you can fish in the LA river. Only thing you are going to catch is hepatitis and a diaper or two.
That part of the path is right up next to residences, so people just walk out onto the path to look (smell
) at the mighty LA river....and along comes you cranking away on a silent, well oiled machine and BAM.