TWENTY-FIVE YEARS OF BOTTOM MAPPING IN SACO BAY, MAINE

Saco Bay hosts the largest sand beach system in northern New England. We began geological studies there in the mid-1980’s to support federal and state efforts to secure long-term sand supplies for eroding beaches. Early work by the USGS connected ten bottom samples with shallow-penetration seismic reflection profiles to define a generally sandy bay with three rock ridges. We added more than 100 bottom samples with connecting high-resolution seismic reflection lines to yield a more complex map with more than 50 polygons ranging from rock to sand to mud. This map retained a bias towards the limited number of bottom samples, but revealed drowned fluvial valleys cut into bedrock and apparent lowstand shorelines. By the early 1990’s we began mapping with side scan sonar and more seismic reflection, as we searched for eroding glacial deposits that the Army promoted as the original source of beach sand. We determined that no such deposits existed, but successfully mapped lowstand shoreline deposits, which we cored to establish a local, relative sea-level curve. Further federal interest in the lowstand sand deposits led to early 21^st Century multibeam mapping in conjunction with side scan sonar mosaics and numerous vibracores. To better understand the context of vibracore sites, we created SWAN models of wave refraction and bottom shear at times of lower-than-present sea level coincident with sample locations. These projected the paleo-direction of longshore currents and allowed identification of drowned lagoons, tidal flats and beaches. Coupling this data with modern dynamic positioning capabilities, we were able to gather many precisely positioned cores with numerous samples suitable for dating to measure sea-level changes. The present-day seafloor map and sea-level curve for this bay reflect the tremendous improvement in geological investigation made possible by improved geophysical methods and navigation. No modern investigation can hope for such success without equipment that permits precise remote sensing of the seabed combined with equivalent navigation.