BENTHIC GEOLOGIC HABITATS OF GREENWICH BAY, RHODE ISLAND

Greenwich Bay (GBay), Rhode Island, located in northwest Narragansett Bay, is a shallow embayment that is vital to the overall ecosystem health of Narragansett Bay. Side-scan sonar, a method of imaging the seafloor using acoustic energy (sound) was used to map Holocene sediment cover and Late Wisconsinan glacial outcrop in GBay.

An array of benthic habitats were identified and their extent was mapped. There is a general trend of decreasing grain size from east to west in the Bay, ranging from sand (and some gravel) along the shallow shelf areas in the eastern Bay to fine-grained silt and silty sand in the western depositional basin. The eastern bayfloor sand sheet probably represents thin Holocene-age sand over glacial deposits comprised of stratified sand and gravel. The relatively shallow western basin appears to be the depositional site for fine-grained, wave-suspended material of Holocene age in GBay. Wind-generated waves from both the northeast and southeast would tend to transport suspended material to the western part of the Bay. The eastern portion of the relatively deeper mid-bay channel system is occupied by silt and silty sand that may represent Holocene deposition of eroded glacial delta-slope material originally deposited during the retreat of the Laurentide ice sheet. Organic silt and silty clay occupies the shallower, upper reaches of the channel. A sandy depositional platform occupies the margin of the northern and southern shorelines of GBay. It formed by deposition of sand eroded from the shoreline as the shoreline retreated due to wave action during storm events. Sand can be supplied from the depositional platform onshore to the adjacent beaches during depositional cycles.

Human activities in GBay have a considerable impact on the benthic habitats. Significant bottom disturbances from quahog (clam) harvesting were mapped in two areas, and the bayfloor around mooring fields show significant alterations, mostly caused by the ground chain dragged along the bottom. Debris from marina construction/destruction and the remnants of shoreline structures (subtidal and intertidal) litters the bayfloor adjacent to most of the developed shoreline. This work was undertaken to assist the Rhode Island Coastal Resources Management Council (CRMC) with the creation of a Special Area Management Plan for Greenwich Bay and the adjacent watershed.