The data and interpretations in this report are based on NOAA hydrographic survey H12013 completed off the entrance to the Connecticut River in northeastern Long Island Sound during 2009 and USGS bottom sampling and photography cruises 2009-059-FA and 2010-010-FA completed during 2009 and 2010, respectively. These data yield new geologic perspectives of the dynamic sea floor of the sound and show the composition and terrain of the seabed, providing information on sediment transport and benthic habitat. Together with information available through other similar reports by the USGS, this report provides a fundamental framework for research and resource-management activities in this major east-coast estuary.

Surveyed depths range from about 1 m to 38 m. The shallowest parts of the study area occur off Hatchett Point, on Hatchett Reef, and on the eastern part of Long Sand Shoal near the mouth of the Connecticut River. The deepest parts of the study area occur along its southern, offshore edge and in an elongate, isolated depression extending eastward and westward from the passage between the mainland and Hatchett Reef.

Many sea-floor features visible in the DTM can be geologically interpreted and ongoing sedimentary processes can be identified because the sea-floor features are morphologically distinct. Bedrock outcrops, boulder lag deposits, obstacle marks, shoals, sand waves, and megaripples were observed. Rocky areas, which support a dense sessile fauna and flora that adds dramatically to the overall benthic diversity, reflect high energy conditions characterized by processes associated with erosion or nondeposition.

Sand waves and megaripples, which cover the sea floor in more than 50 percent of the study area, reflect more moderate conditions characterized by processes associated with coarse bedload transport. Transverse bedform morphologies dominate on the shoals where sand supply is abundant; barchanoid bedform morphologies occur along the southern edge of the study area and east of Hatchett Reef where sand supply is more limited. Megaripples, along with current ripples, are typically present on the stoss slopes of the sand waves, showing that transport is ongoing. Sand-wave, megaripple, and obstacle-mark asymmetry shows that net sediment transport is flood-tide dominated and to the west.

Where protected from the strong wave and tidal currents, such as east of Hatchett Point, the sea floor is relatively flat, mud contents of the sediment increase, infaunal communities are better developed, and lower energy sedimentary environments characterized by processes associated with sorting and reworking prevail. Shell beds have accumulated at the western end of the elongate depression between Hatchett Reef and the mainland and around some of the rocky areas, but these beds are generally thin and ephemeral.