The area between Devil’s Gate Dam and the Colorado Street Bridge is called the Central Arroyo, and it is a place where the Arroyo opens up and spreads out over a wide area. Originally, the river meandered its way across this floodplain. However, beginning in 1938, the Los Angeles County Flood Control began to channelize all of the waterways in Los Angeles County – including the Arroyo Seco. The channelization of the Arroyo Seco begins just south of Devil’s Gate Dam and just north of the Brookside Golf Course. From this point on, the Arroyo Seco is paved in concrete.
If you follow Rosemont Avenue south from the 210 (Foothill) Freeway into the Arroyo you will come across a trailhead that will lead to the Arroyo Seco Trail. Head north on the trail for a short distance and you will see where the channelization of the Arroyo Seco begins.