How Deep Is the Long Island Sound?

Written by Oak Simmons
Updated: August 14, 2023
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Key Points:

  • Long Island Sound is 230 feet deep at the deepest point.
  • The Long Island Sound consists of brackish water – a mix of freshwater from rivers and saltwater from the Atlantic Ocean.
  • Diamondback terrapins have unique adaptations and behaviors that allow them to live in the brackish water of Long Island Sound.

The Long Island Sound is a marine sound that lies between Connecticut and Long Island, New York. This sound has a long, rich history and its waters are incredibly deep. In fact, the Long Island Sound is 230 feet deep at the deepest point. The sound is brackish, a mix of freshwater from rivers and saltwater from the Atlantic Ocean. This article explores Long Island Sound, including the history of Long Island, as well as the ecology and recreation activities available on the sound.

A flock of seagulls on the beach at Cooper's Beach in Southampton, Long Island, New York.

The Long Island Sound is home to beautiful, sandy beaches and an abundance of wildlife.

©Joe Trentacosti/

The Founding of Long Island

Long Island was first inhabited by the Lenape people, who were later called the Delaware people by Europeans. The ancestral lands of the Lenape people include parts of present-day Delaware, New Jersey, Pennsylvania, and New York, including the Long Island Sound. Lenape people practiced hunting, fishing, farming, and herbal medicine.

The first European to document an encounter with the Lenape people was Giovanni da Verrazzano, an Italian explorer, in 1524. Later, in the 17th century, the island was settled by Dutch, English, and Swedish people. A peace treaty was negotiated between the Lenape people and the Europeans, organized by the English Quaker William Penn. However, the arrival of tens of thousands of new colonists during the next decades reduced the hunting grounds of the Lenape people, disrupting their lifeways. Then, in 1758, the Lenape people signed the Treaty of Easton and moved west to Ohio. The Lenape people moved several times after that, ultimately settling in Oklahoma.

In 1788, New York ratified the United States Constitution, becoming the 11th U.S. state. Long Island’s population grew immensely during the early 20th century, as new trains and subways made it easier to access. Additionally, seven bridges were built across the East River, a narrow estuary that separates Long Island from Manhattan.

Today, Long Island is the most densely populated island in the United States, with a population of over 8 million.

Ecology of the Long Island Sound

The Long Island Sound is home to several important wildlife habitats including the Atlantic Ocean, beaches, and salt marshes. Let’s explore some of the amazing animals that live there.

Diamondback Terrapin (Malaclemys terrapin)

Diamondback Terrapin

Diamondback terrapins are the only species of aquatic turtle that live in brackish water.


The diamondback terrapin is a species of aquatic turtle native to the East Coast, Gulf of Mexico, and Bermuda. Diamondback terrapins have unique adaptations and behaviors that allow them to live in brackish water. First, they have a salt gland that eliminates excess salt. Second, their skin is resistant to salt water. Finally, diamondback terrapins are able to judge the salinity of water and use this ability to search for fresh water to drink. Diamondback terrapins were once hunted nearly to extinction, and today they are listed as Vulnerable by the International Union for Conservation of Nature (IUCN).

Red Fox (Vulpes vulpes)

Close up of a red fox in a forest, UK.

Red fox vocalizations include yips and barks, which can travel great distances.


The red fox is a widespread species of canine native to the Northern Hemisphere. Red foxes live near the salt marshes of the Long Island Sound. They typically visit the salt marshes to hunt, because the marshes are densely populated by prey animals such as shrews, voles, and birds. Red foxes typically live in mated pairs or small family groups.

Saltmarsh Sparrow (Ammospiza caudacuta)

saltmarsh sparrow

Saltmarsh sparrows are threatened by potential sea level rise since they rely on marsh habitats to breed.


The saltmarsh sparrow is a species of New World sparrow (Passerellidae family) native to the East Coast of the United States. Saltmarsh sparrows nest in marshes, using grasses to build cup-shaped nests. They typically raise two broods per year. Saltmarsh sparrows are listed as Endangered by the IUCN due to habitat loss.

What To Do on the Long Island Sound

There are many recreational activities to enjoy on Long Island Sound. Let’s explore some of the best ways to spend time on the sound.


There are numerous beaches to swim at on Long Island Sound. However, water quality can be a concern. Save the Sound, an environmental organization, monitors water quality at beaches on Long Island Sound. Beachgoers can check their reports before heading out to swim. According to their 2023 data, the New York beaches with the best water quality are North Fork at McCabes, Town Beach, and Orient State Park. On the Connecticut side, the cleanest beaches are Hole In The Wall Beach, Esker Point Beach, and Surf Club Beach.


The Long Island Sound is a popular place for all kinds of boating, including sailboats, motorized boats, and yachts.


There is fantastic fishing on the Long Island Sound. Anglers can fish inshore, nearshore, and far offshore. Some common inshore species are flounder, striped bass, and bluefish. Common nearshore species are Spanish mackerel and black seabass. Finally, in the deep offshore waters, anglers can catch yellowfin tuna, swordfish, and mahi-mahi.

Where Is the Long Island Sound Located on a Map?

The photo featured at the top of this post is © Kyle Lee/

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About the Author

Oak Simmons is a writer at A-Z Animals primarily covering North American wildlife and geography. They graduated from the University of California, Santa Cruz in 2018 with a Bachelor of Science in Ecology and Evolutionary Biology. A resident of Washington state, Oak enjoys tracking mammals and watching birds.

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