Taking photos from the same vantage point years apart has long been used to study landscape. The technique got its start as a way to document the retreat of European glaciers. For 50 years, the U.S. Geological Survey has been building an archive old photos of desert landscapes and revisiting the sites to take new photos. The result is the largest collection of repeat photography in the world. ...We have a few of the most interesting repeat photographs in this gallery that show changes such as the retreat of glaciers, the birth and death of cactus forests, the excavation of ruins and the shifting of a river channel.