Taking photos from the same vantage point years apart has been used to study changes in the landscape since the late 19th century. 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. Some of these sets of photos appear in a new book about the technique and the effects of climatic variation and land-use that it can document. We have a few of the most interesting repeat photographs in this gallery that show changes such as the retreat of glaciers seen above, the birth and death of cactus forests, the excavation of ruins and the shifting of a river channel. Captions are modified from USGS information about the photos. Above: Muir and Riggs Glaciers, Muir Inlet, Alaska: 1941, 1950 and 2004 In 1941 (left), Muir Glacier was a large 2,300-foot-thick tidewater glacier that had been retreating since the mid-eighteenth century. Riggs Glacier can be seen in the right of the photo flowing into Muir Glacier, which ends just out of the frame on the right. In the center photo, taken nine years later, Muir Glacier has retreated almost two miles and lost well over 300 feet of ice thickness. It is still connected to Riggs Glacier, and the ridge in the foreground is still bare of vegetation. When the photo on the right was taken after another 54 years, Muir Glacier has retreated out of the photo more than four miles and Riggs has moved back around 2,000 feet and thinned more than 800 feet. Dense vegetation has covered the ridge. Images: Left: USGS/photo by W.O. Field. Center: USGS/photo by W.O. Field. Right: USGS/photo by Bruce Molnia.

Cliff Palace, Mesa Verde National Park, Colorado: 1891 and 1989 This pair of pictures shows the largest cliff dwelling in the park. It includes 150 rooms and 22 kivas (underground ceremonial spaces). The photo on the left from 1891 shows damage to the tall tower that has been repaired by 1989 when the photo on the right was taken. Structures in the foreground that were excavated in 1909 can also be seen in the later image. Images: Left: USGS/photo by Gustaf Nordenskiöld. Right: USGS/photo by Bill Howard.

Huérfano Butte, Santa Rita Experimental Range, Southern Arizona: 1902, 1941 and 2007 In 1902, photographer David Griffiths' horse-drawn buggy sits in front of Huérfano Butte on grassland with a few hackberry plants in the photo on the left. By 1941 in the center photo, the cholla cactus and new mesquite plants have appeared. The cholla is outnumbered by prickly pear cactus and more mesquite has moved in. Images: Left: USGS/photo by David Griffiths. Center: USGS/unknown photographer. Right: USGS/photo by Mitchel P. McClaran.

Saguaro National Park, Southern Arizona: 1935, 2010 The landscape pictured here in front of Agua Caliente Hill near Tucson is dominated by a stand of tall saguaro cactus in the photo on the left, taken in 1935 when the park was a national monument. Brittlebrush, foothill paloverde and mesquite can also be seen. In the photo on the right, taken in March of this year, the saguaro has declined dramatically and very few of the individual plants in the earlier photo remain. But new saguaro have been growing in the area since the 1970s and could eventually fill the landscape again. Houses being built along the park's border are destroying the saguaro there. Images: Left: USGS/unknown photographer. Right: USGS/photo by Raymond M. Turner.

Boundary Monument 75E, Southeastern Arizona: 1893, 1994 and 2009 This monument marks the boundary between Arizona and Mexico in the Chihuahuan Desert. In the photo on the left, taken in 1893, the desert grassland with a few shrubs that are most likely broom snakeweed and velvet mesquite. By 1994 when the center photo was taken, the shrubs have increased. Bush muhly can be seen beneath some of the mesquites, which have remained short, most likely stunted by severe freezes. In the photo on the right from 2009, a vehicle barricade has been built along the border and grass has become thicker. Images: Left: USGS/photo by D. R. Payne. Center: USGS/photo by Raymond Turner. Right: USGS/photo by Raymond Turner.

San Carlos River near Peridot, Southern Arizona: 1940, 1964 and 2000 This gauging station for the San Carlos River on a railroad bridge south of the town of Perido, Arizona. In 1940 when the photo to the left was taken, the river channel has been widened by a flood and a few cottonwood trees are growing along the right side of the river. In the center photo, taken in 1964, vegetation has grown up and blocks the view of the river and the channel has shifted to the right. By 2000 (right) the gauging station is gone, the channel has shifted back to the left and removed some cottonwood trees, while black willow, cottonwood seedlings and tamarisk have grown along the channel. Images: Left: USGS/photo by Roy H. Monroe. Center: USGS/photo by Raymond Turner. Right: USGS/photo by Dominic Oldershaw.

Santa Catalina Highway, near Tucson, Southern Arizona: 1987 and 2006 The image on the left from 1987 was taken just months after a fire came through the area and scorched the saguaros. In the image on the right, taken in 2006, two measuring poles have been placed near where the saguaro used to be in order to estimate how tall they were. From their height, scientists estimate they date back to the 1890s. A few new saguaro are poking up here and there. Images: USGS/photo by Raymond M. Turner.