GEOMORPHIC HISTORY AND PRESERVATION OF ARCHAEOLOGICALLY SIGNIFICANT AREAS IN THE HANFORD REACH OF THE COLUMBIA RIVER, WASHINGTON STATE

Archaeological sites near rivers may be preserved through burial, altered by exposure, or destroyed through erosion. During the 20^th Century, extensive damming (~14 dams built total) of the Columbia River has left archaeologically significant areas inundated. As the only remaining free-flowing reach of the Columbia River, the Hanford Reach is ideal for research into the geomorphic settings of archaeological sites along the river. In this study, we reconstructed the geomorphic history of known archaeological sites on late-Holocene terraces and flood deposits of the Columbia River to determine whether fluvial or eolian forces were the primary agents for preservation, which sites might be subject to future erosion, and the specific settings most likely to host surface vs. buried archaeological sites. Hydrologic Engineering Center River Analysis System (HEC-RAS) one-dimensional modeling was used to estimate flood discharge and create inundation maps of the two largest historical floods on the Columbia River in 1894 (20,954 m³/sec) and 1948 (19,539 m³/sec). Modeled inputs consisted of river channel geometry from Lidar, elevations of flood debris lines from historical floods, and elevations of slackwater flood deposits. Elevations of historical slackwater flood deposits were modeled to create baseline paleoflood discharge estimates. Particle-size analysis and sedimentological descriptions of sediment samples collected during prior archaeological excavations on Holocene terraces were predominantly medium to fine sand. Identification of archaeologically significant areas within and outside of the zones that were inundated by late Holocene floodwaters, along with the sedimentological analysis, assisted in interpreting whether fluvial or eolian deposits were the agents of site formation and burial. Identifying the geomorphic settings and processes under which sites are most likely to be buried or eroded could play a role in land management decisions to most effectively preserve the cultural resources of this area.