USING REPEAT PHOTOGRAPHY TO DOCUMENT LANDSCAPE CHANGE IN DEATH VALLEY OVER THE LAST 100 YEARS

Repeat photography is a well-known process used to document landscape changes over the last 150 years. U.S. Geological Survey photographers have been photographing Death Valley landscapes since 1901 with some replicate photographs. In this study, 21 photographs were selected for replicate photography. The photographs were chosen mainly for the ability to find the location. Due to the inaccessibility of some of the photographs, only 19 were used for the final analysis. Photographs with a limited view, low quality, and those with obscure location descriptions were avoided. The year the original photographs were taken ranged from 1901 to 1974. For each replicate photograph, the location was recorded by GPS along with the view azimuth, time of day and other relevant data. Photographs were taken with a digital camera and landscape changes were estimated by side-by-side comparison of photographs. The landscape changes documented by the photographs ranged from no discernible change to minor erosion. For example, Natural Bridge, located 20 km south of Furnace Creek, is Pleistocene Mormon Point conglomerate undercut by an active stream channel that was photographed by Stose in 1940. Natural Bridge shows no evidence of erosion with individual boulders and rills from 1940 still present. At Breakfast Canyon, 2 km south of Furnace Creek, a vertical spire of Furnace Creek Formation shows no evidence of erosion with individual rocks and rills from the 1938 photograph by Thayer still visible. In contrast, Mushroom Rock, 7.6 km south of Furnace Creek, is a precarious basalt boulder with a narrow pedestal and broad head formed by salt-wedge weathering. Compared to the original 1901 photograph by Campbell, Mushroom Rock has lost ~50% of the head material due to rock fall. Along Furnace Creek, ~11 km SSE of Furnace Creek, Thayer photographed a vertical stream cut with a tributary stream in the foreground in 1938. Extending an unconformity visible in the 1938 photograph shows that the top of the stream cut has eroded ~1 m during the intervening 72 years and the tributary stream has incised ~50 cm and transgressed into Furnace Creek. The unequal erosion shown by the replicate photography is illustrative of arid-land erosion where erosion rates are low and frequently dependent on catastrophic events.