The Los Angeles River spans nearly 51 miles — running from Canoga Park to Long Beach. [T]he river changed in the 1930s after a series of catastrophic floods hit the region and the U.S. Army Corps of Engineers channelized the entire length of the river — encasing it in concrete — and turning it into a flood control channel. … [Marissa Christiansen, president of the Friends of the Los Angeles River, hopes] to bring the river back to its roots. And, for Earth Week, FOLAR is planning a huge cleanup. From June to July, the group is asking for help cleaning up 100 tons of trash from the waterway.