The Los Angeles River is often little more than a trickle as it winds downtown on its way through Long Beach to the Pacific. But this week, it threatened to overflow its concrete banks. Neighborhoods found themselves under feet of water. Mudslides took out hundreds of trees and dozens of buildings, flash flooding swept cars away, and at least three people have died.Many parts of California received more than 10 inches of rain in little more than a day, bringing the entire region to a standstill.