If you want to cross the Rillito River in Tucson, Arizona, anytime between October and July, you probably won’t need a boat, a bridge, waders or even waterproof shoes. During most of the year, the river is an arroyo, a curvy strip of dry sand that holds no more than the memory of water: braided serpentine patterns in the sand, erosion-smoothed stones, debris wrapped around the trunks of the few hardy deciduous trees.