USING PHOTOGRAMMETRY TO MONITOR EROSION AT ARCHEOLOGICAL SITES ALONG THE COLORADO RIVER AT GLEN CANYON NATIONAL RECREATION AREA

The Glen Canyon Dam has drastically reduced sediment deposition along the 15-mile stretch of the Colorado River between the Dam and the confluence of the Colorado and Paria Rivers. This impedes river terrace and sand bar formation, with fluvial erosion outpacing deposition on these sediment features. Periodic High Flow Experiments (HFEs) release water through the dam at extremely high rates for a few days to flush sediment downstream from the Paria River drainage, but this sediment is not available upstream of the Paria River in the Glen Canyon reach.

Ongoing erosion damages cultural resources and hinders access to archeological sites situated on these river terraces. Glen Canyon National Recreation Area (GLCA) archaeologists have collected 5-years of high-resolution imagery at four archeological sites to document erosion. During the present project, these images were processed into scaled 3D models using Structure from Motion photogrammetry and compared over time to identify surface changes.

The imagery captures two main erosional features: arroyos and banks. Arroyos experienced no detectable change over the course of a HFE but widened up to 1m over the whole five years. Banks eroded up to 0.2m during a single HFE. In all post-HFE models of banks, a new terrace appeared below the original terrace, perhaps marking the highest water level reached during the HFE. No deposition was detected at the photographed sites. Erosion impacts on archeological features depended on the feature type and its proximity to the river and arroyos. Primary concerns are direct damage or removal of features and future loss of access to sites. The HFE erosion does not appear to contribute to these concerns.

Results from this project will help GLCA archeologists determine where and how frequently imagery should be collected at similar river terrace sites for effective monitoring. The archeological sites along this stretch of the Colorado River provide insight into the prehistoric human occupation in the area and are culturally significant to the tribes whose ancestors inhabited Glen Canyon. Ongoing erosion monitoring using photogrammetry will aid the preservation of valuable cultural resources.