Long Island Sound

Long Island Sound is a marine sound and a tidal estuary of the Atlantic Ocean.

It is located primarily between the U.S. state of Connecticut to the north and Long Island in New York to the south. The water is a complex mix of freshwater and saltwater. Freshwater enters the sound through many different tributaries and rivers. Saltwater flows in from the Atlantic Ocean. This combination of waters makes the sound a productive environment. It stretches from the East River and the Throgs Neck Bridge in New York City all the way to the Block Island Sound.

The geography of the sound was shaped by massive ice sheets. About 18,000 years ago, the Late Wisconsin Glacier covered this entire region. This glacier was very thick in its interior and along its southern edge. As the ice moved, it scraped surface material from the New England landscape. The glacier then deposited this material, called drift, into the area. This process created several long ridges known as moraines. One major ridge is the Ronkonkoma Moraine, which stretches along southern Long Island. Another is the Harbor Hill Moraine, located along northern Long Island.

Smaller deposits also created recessional moraines along the Connecticut coast. These include the Norwalk area and the Madison-Old Saybrook area. These moraines are smaller and more discontinuous than the southern ones. Some islands, like the Captain Islands, are actually parts of a recessional moraine. Other areas, like the Thimble Islands, consist mostly of exposed bedrock with thin drift. The glacier also left sandy outwash deltas off the coast near Bridgeport and New Haven. These deltas were formed by meltwater streams carrying sand and gravel into the basin.

Before the ocean arrived, the basin was a freshwater body. It was likely formed by ancient stream flows. Glacial meltwater eventually filled the basin to create a lake called Lake Connecticut. This lake stayed freshwater until about 8,000 years ago. At that time, the sea level rose to about 20 meters below its current level. Eventually, seawater overflowed into the basin. This process transformed Lake Connecticut from a nontidal, freshwater lake into a tidal, saline arm of the sea. A ridge called the Mattatuck Sill sits on the west side of the basin. Its lowest point is about 10 meters below sea level.

The sound is home to many different types of plants and seaweeds. In the rocky intertidal zone, you can find brown seaweeds like Fucus and Ascophyllum. Some of these have air bladders to help them float toward sunlight. In the subtidal zone, which is below low tide, large kelp can grow. Some kelp specimens can reach a yard or two in length.

Tidal marshes are also very important. These marshes produce between three and seven tons of vegetation per acre every year. This growth comes mostly from salt marsh grasses. When this plant matter decomposes, it provides nutrients for fish and shellfish.

One very important plant is eelgrass, also called saltwater eelgrass. It is a vascular plant that looks like grass but is a different species. Eelgrass grows in protected bays and coves on muddy or sandy sediments. It grows best in shallow water because it needs sunlight. The roots of the eelgrass help stabilize the mud and trap moving sand. This helps prevent erosion along the shoreline. Eelgrass also provides a calm habitat for many invertebrates and food for waterfowl like brant geese. However, an outbreak of eelgrass wasting disease in the 1930s killed much of the population.

Many people live in the large watershed that drains into the sound. The watershed includes parts of New York, Connecticut, Rhode Island, Vermont, New Hampshire, and Massachusetts. As of the 2010 census, about 8.93 million people live in this area. Numerous rivers flow into the sound, such as the Connecticut River and the Housatonic River. Major cities like Stamford, Bridgeport, and New Haven sit along the Connecticut shore. On the New York side, towns include Rye, Glen Cove, and Smithtown.

This large network of rivers and land makes the sound a vital part of the regional ecosystem.