A LANDSCAPE-SCALE ASSESSMENT OF ARCHAEOLOGICAL-SITE CONDITION AND PRESERVATION IN THE COLORADO RIVER CORRIDOR, GRAND CANYON, ARIZONA

Cultural history in the Colorado River corridor, southwestern U.S., spans more than 8,000 years. Within Grand Canyon National Park, landscape processes that contributed to site formation and change over many centuries are now altered by the operations of Glen Canyon Dam. Most cultural sites in the Colorado River corridor include river-derived sand as an integral component of their geomorphic context, and thus their formation was inextricably linked to water- and/or wind-borne sediment supply from the river. To determine whether archaeological-site erosion risk is elevated today as a result of dam-altered river flow and sediment supply, we employed a holistic, multi-scale field and remote-sensing approach to study landscape conditions at more than 350 sites. We evaluated site geomorphic context, local sand-transport directions, and connectivity to upwind sand sources, and found that few sites have a high degree of modern connectivity to river-derived sand supply. At selected sites studied annually using terrestrial lidar change detection, sand loss by gully erosion and aeolian deflation generally exceeded deposition, such that erosional signals dominated landscape change even at sites with strong connectivity to modern sand supply. Most archaeological sites are at elevated risk of net erosion under present dam operations, due to a combination of reduced sand supply (fluvial and aeolian) through (1) the lower-than-natural flood magnitude, frequency, and sediment supply of modern dam operations, even during controlled floods; (2) reduction of open, dry sand area available for wind redistribution under normal (nonflood) dam operations; and (3) impeded aeolian sand transport due to increased riparian vegetation in the absence of larger, more-frequent floods. We suggest that if dam operations were to increase the sand supply available for windblown transport and also decrease riparian vegetation, the prevalence of active aeolian sand landscapes could increase over time, and the propensity for net erosion of cultural sites could decrease.