GEOPHYSICAL MAPPING OF KNOWN ARCHAEOLOGICAL SITES IN RESERVOIR LAKES OF OREGON’S WILLAMETTE VALLEY: TESTING NEW IDENTIFICATION METHODS FOR OFFSHORE INVESTIGATIONS

Following the last ice age, sea-level rise has inundated vast coastlands under up to ~150 meters of seawater. Along the coasts of the Americas, these drowned prehistoric-terrestrial-landscapes represent extensive environments capable of supporting prior human habitation. Existing methods for the identification of submerged archaeological sites, such as dredging, coring, and scientific diving, could damage potential sites, are expensive, and are limited in spatial coverage. A newer method of identification, termed the Human-Altered Lithic Detection (HALD) method, uses acoustic signatures in the water column of Chirp sub-bottom sonar data to detect submerged Stone Age sites containing human-altered lithics. The HALD method has been successfully field-tested in limited geographic settings and conditions, but future research is needed to expand our understanding of the capabilities and limitations of this method.

Inland reservoirs provide an excellent opportunity to study submerged terrestrial landscapes and archaeological sites. These reservoirs, with their annual in-fill and drawdown cycles, serve as natural laboratories, mimicking the natural processes of marine transgression and regression on shorter timescales. Here, we present updated results on geophysical data collected from flood control reservoirs in Oregon's Willamette Valley during summertime high water levels, which were later ground-truthed through terrestrial methods during winter drawdown. Using this approach, we successfully identified several new submerged archaeological sites in drowned river valleys, which additionally contain many previously known archaeological sites of various ages and materials. The characteristic geophysical signatures identified at the submerged sites in these reservoirs can be used to identify submerged archaeological sites elsewhere. Furthermore, our findings facilitate the monitoring and management of landform and archaeological site changes caused by the annual cycles of inundation and exposure. This cost-effective method has the potential to enhance offshore research, contributing to a deeper scientific understanding of submerged archaeological sites and human history.