PRESERVATION AND DESTRUCTION OF CULTURAL MATERIAL BY LARGE COLUMBIA RIVER FLOODS: GEOARCHAEOLOGICAL EVIDENCE FROM THE HANFORD REACH

Archaeological sites near rivers may be preserved through burial, altered by exposure, or destroyed through erosion. Preserved because of the unusual needs of the Manhattan Project, the Hanford Reach is the only remaining free-flowing stretch of the Columbia River and ideal for research into the geomorphic settings of archaeological sites along this river. The 1894 (742,000 cfs [20,900 m³/s]) and 1948 (690,000 cfs [19,000 m³/s]) floods were the largest in the historical record through the reach, but their relationship with geomorphic change and site preservation are less understood. To understand how floods have preserved and destroyed archaeological sites, three objectives were applied: 1) stratigraphic reinterpretations and grain-size analysis of earlier archaeological sites; 2) elevation surveys and descriptions of overbank deposits; and 3) hydraulic modeling of historic floods. Findings revealed that archaeological sites were inundated by large magnitude-low frequency floods, with five fluvial strata identified on terrace H4. Hydraulic modeling of a more recent 1997 flood established a maximum specific stream power of 566 W/m². This value, along with its documented geomorphic change, was compared to the findings of the 1894 and 1948 floods, creating a risk potential map for areas of erosion and deposition. Five areas were identified to be at risk for erosion, with the left channel of Locke Island identified as a high risk area. Deposition downstream of Coyote Rapids, Locke Island, and the White Bluffs Boat Launch coincided with velocity decreases, with the last positively identified as slackwater flood deposits. This further solidifies past interactions between the river and archaeological sites and slackwater flood deposits as a geomorphic agent of preservation. Present-day dams upstream have ceased slackwater deposition on these terraces from floods on the scale of the 1948 and 1894 events. The smaller 1997 flood is now a standard for maximum anticipated peak discharges. Future floods of this scale will not assist preservation on archaeological sites, rather they will have the opposite effect of lateral bank erosion and loss of cultural material. In partial compensation, the loss of large episodic floods has allowed the growth of riparian vegetation that will help mitigate future bank erosion.