There's a quaint trail in the Arroyo Seco Flood Channel, extending from just north of Avenue 43 in the south for about two miles to York Blvd. It was built by the County of Los Angeles in 1983 as an experiment to test joint use of flood control facilities.
It's a fun ride but the trail is not safe or designed for commuting purposes. The trail is often littered with glass and algae and is especially dangerous during storm conditions because it is located in the bottom of the flood control channel.
Click on the pictures for a full view.