LARGE COLUMBIA RIVER FLOODS AND THEIR IMPACT ON CULTURAL MATERIAL WITHIN THE HANFORD REACH, WASHINGTON STATE

Preserved because of the unusual needs of the Manhattan Project, the Hanford Reach is the only remaining free-flowing reach of the Columbia River and ideal for research into the geomorphic settings of archaeological sites along this river. The 1948 flood, at 19,539 m³/s, was the largest flood in the 105-year gauged record, but little is known about whether archaeological sites were destroyed through erosion or preserved by aggradation, with this event. Consequently, questions remain regarding preservation of cultural materials and the frequency of large floods, such as whether fluvial or eolian forces were the primary agents for preservation; the composition of buried and surficial deposits; and which sites might be subject to future erosion. Hydrologic Engineering Center River Analysis System (HEC-RAS) two-dimensional modeling was employed to create inundation maps for the 1948 flood through the Hanford Reach. Results indicated several known archaeological sites were inundated, with a maximum depth of ~8.1 m near Vernita Bridge and a maximum horizontal inundation of ~672.0 m near Coyote Rapids. Laser particle-size analysis, stratigraphic description and dating of slackwater flood deposits provided sedimentological analogs that distinguished fluvial and eolian deposits collected during this and prior archaeological excavations. Buried archaeological materials were encased predominantly in medium to fine sand. Distinguishing the geomorphic settings and processes under which sites are most likely to be buried or eroded will assist land management decisions to most effectively preserve the cultural resources of this area.