Las Vegas, Nev., the desert-turned-oasis of slot machines, plastic pyramids, and indoor waterfalls, is now undergoing a new kind of metamorphosis: A former 10-mile-long sewage gully is being transformed into a wetlands park that will be one of the largest swaths of locally preserved land in the nation. For 25 years, the Las Vegas “Wash” was an eroded channel draining 150 million gallons of treated sewage and contaminated groundwater per day into nearby Lake Mead. But in 1973, an effort was launched to turn the Wash into a 2,900-acre wetlands preserve. It took 18 years to secure the bond initiative; so far, $14 million has been spent and 150 acres have been redeveloped. When it’s done, the park, which boasts reintroduced cottonwood, willow, and mesquite trees and a growing wildlife population, will have 30 to 50 miles of hiking trails. Elsewhere in Las Vegas, workers are renovating 30 acres of a planned 180-acre, $171 million meadow preserve.