Part Two: Water Cops and Microbes

Los Angeles, Phoenix, San Diego and nearly every other city in the American Southwest depend on the dwindling Colorado River. One desert city has become an unlikely model for water conservation: Las Vegas. Over the past two decades, Las Vegas – which hosts over 35 million visitors per year and continues to grow ­– has kept its annual water use around the same, relatively low, level by recycling wastewater and regulating use.