On the eastern flank of metropolitan Las Vegas, out past a water park advertising “the world’s largest man-made wave” and a nascent development called Cadence, a red-dirt parking lot teases public access to the system that enabled the past three decades of growth in southern Nevada. Called the Las Vegas Wash, it amounts to a channel that funnels the area’s recycled water back to its aquatic lifeline: Lake Mead.Bushwhacking through the cottonwoods that line the wash and past a massive desert toba