For centuries, the Los Angeles River was unpredictable-- prone to flooding, a raging torrent during rare Southern California rains, and just a trickle and marshlands the rest of the year. To tame it, the U.S. Army Corps of Engineers walled in the river just before World War II. Decades later, roadside signs that proclaimed "Los Angeles River" prodded the question: How can this cement drain, strewn with trash and abandoned refrigerators, be considered...