The Columbia Slough Trail winds past warehouses, wetlands, freeways and fairways amid 19 miles of the Columbia River's once-wild southern floodplain. Look at the barren levees and lots, and there's no mistaking it's a highly managed, urban waterway. But in some spots, a thin thicket of trees and shrubs separates the working world from the wild one, providing just enough green to enable cyclists, pedestrians and paddlers to forget about the grey.