Green cottonwoods flourish by Medano Creek at the edge of the dunefield in Great Sand Dunes National Park & Preserve in Colorado. The tallest dunes in North America spread across 30 square miles, creating a unique high-altitude desert environment and a place of extremes. The sand surface can reach 150 degrees on a summer afternoon, or drop to minus 20 degrees on a winter night. Surfing the dunes is a fun summer activity. Photo by Patrick Myers, National Park Service.