GULLY EROSION OF ARCHAEOLOGICAL SITES IN GRAND CANYON-PHOTOGRAMMETRY AND GIS USED IN GEOMORPHIC STUDIES

Gully erosion along the Colorado River corridor in Grand Canyon National Park has apparently increased in magnitude and frequency in the last few decades, and as a result archaeological sites are being damaged. Remote sensing and GIS allow for low-impact monitoring and high-resolution erosion hazard analysis and mapping, which, in concert with field studies, will give insight to the influence of various soil and terrain characteristics that control the gully problem in Grand Canyon. Ultimate causality of the recent erosion is under debate, with decadal-scale climate change and operation of Glen Canyon Dam influencing baselevel for gullies being potential contributing factors. Erosion control structures have been built in many gullies to stop further damage to archaeological sites, but their performance is uncertain, and an efficient and non-destructive monitoring technique to track knickpoint advancement and assess erosion control efforts is needed.

We are using digital terrain models created from extremely high-resolution photogrammetry, and comparing them to very detailed field topographic surveys and field and lab data. Comparisons with ground survey data in GIS showed that the 20-cm DEMs created from photogrammetry were accurate enough to use for a high-resolution terrain analysis. Area-slope scatter plots differentiate between hillslopes and channels, and indicate that overland flow is the dominant process. An inverse relationship between contributing drainage areas and slopes at gully heads and erosion control structures can be empirically derived from both photogrammetry and field survey data. These slope-area erosion thresholds are represented by power equations, and inserted into a GIS to estimate vulnerability of other archaeological sites to gullying and effectiveness of potential erosion-control structures. Vegetation cover, soil permeability, and soil shear strength, all measured in the field, show inverse relationships with gully erosion activity, and affect the slope-area thresholds between given sites. These trends, along with the fact that gully head locations can be predicted by slope and drainage area relations, imply upcatchment control of gully initiation and knickpoint retreat rather than a baselevel control related to Colorado River stage and Glen Canyon Dam operations.