Most of the Rio Hondo is lined with concrete , but there are two places the river is free-flowing . These are the Peck Road Water Conservation Park and the Whittier Narrows Recreation Area . Most of the Rio Hondo is a concrete-lined channel to serve its primary flood control function , but in two places the river flows over open ground : the Peck Road Water Conservation Park , and the Whittier Narrows Recreation Area .