One of my favorite things about the San Gabriel Mountains. This is a tributary of the Los Angeles River and is a very delightful hiker or nature lover's retreat. It is easily accesible by way of Angeles Crest Highway when you dip down into the Switzer's Picnic Area. From there you hike down into the canyon and continue following the stream, most people continue to Switzer's Fall to play in the pool it creates, but some split off at some of the forks, one of them is to Bear Canyon which is just as nice, but good if you want less people. It is a perfect Sunday getaway if you're not physically unfit or don't like to hike.
Alas, once the Arroyo Seco leaves the mountains it gets dammed by the Devel's Gate Dam and subsequently channelized as it enters the Upper and Lower Arroyo Seco Parks. Then it parallels the Pasadena Freeway that was built on its original riverbed before finally merging with the Los Angeles River.