COMPARISON OF ANTHROPOCENE VALLEY-BOTTOM SEDIMENTATION IN GLACIATED NEW ENGLAND AND THE UNGLACIATED MID-ATLANTIC REGION: PRELIMINARY RESULTS

Along the Appalachian Mountains in the northeastern US, widespread human impact on hillslope erosion and valley sedimentation began with the arrival of European settlers and subsequent forest clearing and dam construction during the late 17th century. Evidence from a growing number of studies indicates that dam-related sedimentation is common in the mid-Atlantic Piedmont (e.g., Walter and Merritts, 2008; Pizzuto and O’Neal, 2009). The pervasiveness of Anthropocene valley-bottom deposits is less well known in New England. Both regions have a similar history, timing and intensity of European settlement, but continental ice sheets occupied New England repeatedly during the Pleistocene, while the mid-Atlantic region experienced periglacial processes that diminished southward. We hypothesize that controlling for land use (e.g., comparing basins with similar milldam density), Anthropocene sedimentation in valley bottoms is a larger magnitude event in the mid-Atlantic than New England, because of (1) greater hillslope sediment supply in non-glaciated areas (thicker soils and more loess), and (2) more opportunities for sediment storage in lakes, wetlands and other low-gradient areas in glaciated areas (i.e., more terrestrial accommodation space). The central goal of this new project is to evaluate whether valley-bottom sedimentation during the Anthropocene represents a singular moment in geologic time. In this presentation, we first review the geomorphic history of the study areas, from late Pleistocene to present, focusing on how glacial or periglacial processes influence relief structure and set the stage for Holocene and Anthropocene processes in river valleys. We then present preliminary findings based on fieldwork, mapping of valley-bottom deposits using lidar digital elevation models, and radiocarbon dating. Observations from three watersheds in Maine and Massachusetts, and numerous mid-Atlantic streams, support the hypothesis that preserved millpond deposits are less common in New England, with most occurring in places with large upstream sediment supply from eroding glacial deposits. In the mid-Atlantic, thick (1-5 m) historic deposits are nearly ubiquitous at former dam sites, while in New England current and former dam sites frequently exhibit little evidence for sedimentation.