GEOMORPHIC IMPACTS OF FLOODS, HUMANS, AND CLIMATE CHANGE ON THE POST GLACIAL LANDSCAPE OF NEW ENGLAND

Landscape evolution occurs on a variety of timescales ranging from gradual changes driven by prevailing climatic and geologic conditions to rapid transitions caused by naturally occurring extreme events or human perturbations. Rapidly occurring, high magnitude episodes of geomorphic change, such as occurred during the passage of Tropical Storm Irene in 2011 are hazardous and potentially costly events. More gradual changes that result in increased sediment yields may also cause significant impacts associated with infilling of downstream reservoirs or degradation of water quality. This study examines the impacts of extreme events (floods), human land-use, and climate change on sediment yields from a forested watershed in central Vermont in order to help constrain their relative magnitude, determine the frequency of extreme events, and evaluate the interdependency among these forces. Specifically, this study examines a 2,300 year long record of sediment accumulation and event deposition in Amherst Lake, VT, a lake known to be sensitive to flood deposition from the Black River. Over the entire period of record, flood deposits occur with an average frequency of about 1 event per 100 years. The last 100 years stand out as a period of more frequent event deposition and are characterized by average mass accumulation rates significantly higher than at any other time in the record. These results suggest that the past century has been a period of unprecedented landscape change and highlight the lasting geomorphic impact of episodic events. In contrast, a lack of flood deposits and extremely low accumulation rates from ~1600 to 1900 CE suggest a period of minimal landscape evolution. We examine these changes in the mass accumulation rate of sediment along with changes in the magnitude and frequency of flood deposits in the context of climatic change and historical documentation of human land use in the surrounding watershed.