31 Coastal and Nearshore Processes Affecting Our National Parks

Monday, 15 March 2010
8:00 AM - 12:05 PM
Pratt A/B

PALEO-LANDSCAPES OF THE BOSTON HARBOR ISLANDS NATIONAL PARK AREA - FORMER CHANNELS OF THE NEPONSET RIVER

Paper number: 31-11
Program: Theme Sessions
Time: 11:05 AM – 11:25 AM

Authors

Abstract

Boston Harbor has experienced a dramatic and variable rise in sea level since the retreat of continental ice approximately 17,000 ybp. During the post-glacial lowstand, the entire area of Boston Harbor was sub-aerially exposed and the present-day Mystic/Chelsea/Charles River, Neponset River, Fore River, Back River and Weir River all traversed the plain. The subsequent transgression has submerged the plain, allowing for backfilling and drowning of the river channels. Since colonization, the submerged plain has seen major anthropogenic disturbances including filling of shallow flats for development, dredging of navigation channels to facilitate marine commerce and emplacement of cables and tunnels. Presently, Boston Harbor consists of numerous islands and several river estuaries. The changes in sea level coupled with anthropogenic activities have created a patchwork of preservation potential for unraveling the Holocene submergence and backfilling of the former river systems in Boston Harbor.

We are undertaking a project to identify the landscape changes to Boston Harbor since deglaciation. Our initial research focuses on the Neponset River and Dorchester Bay. The Dorchester Bay area has seen minimal dredging east of Castle Island and landmaking in the Neponset River Estuary and along Old Colony Beach and Columbia Point. This, coupled with the close proximity to the University of Massachusetts-Boston campus provided the ideal location to begin a paleo-drainage study of Boston Harbor.

The team deployed boomer seismic reflection and high-resolution sub-bottom profiling equipment to image the subsurface architecture of Dorchester Bay and sidescan sonar equipment to map the surficial texture and features. All data were imported into a GIS framework for spatial analysis and integration with other data sources.

The high-resolution sub-bottom profiling data from Dorchester Bay has revealed a preserved and filled former river channel. In places, the channel exceeds 5 m in depth. The banks of the paleo-channel do not show signs of erosion and suggest filling and flooding in a quiet water environment. The paleo-channel correlates with the present-day Neponset River. Boomer seismic data and sidescan sonar imagery are still under review and interpretations are pending.