Narragansett Bay is an estuary—a semi-enclosed inlet of the sea in which seawater is diluted by fresh water. Compared to other estuaries, Narragansett Bay is small- to medium-sized. Chesapeake Bay, the largest estuary in the United States, covers more than 30 times the area of Narragansett Bay. Still, Narragansett Bay is big enough to take a good-sized bite out of little Rhode Island. It reaches two-thirds of the way up the state—with the result that no Rhode Islander is more than half an hour's drive from the shoreline—and covers about 10 percent of the state's area.

There are three entrances to Narragansett Bay: the West Passage, the East Passage, and the so-called Sakonnet River, which is not really a river but an arm of the sea. Only the East Passage, with an average depth of 44 feet, is deep enough for large ships.

The Bay's three largest islands are Aquidneck (the Indian name means "longest island"), Conanicut, and Prudence. Some 30 smaller islands, many of them little more than large rocks, also dot the Bay.

Life in the Bay
One-celled floating algae called phytoplankton are the basis of the Bay's food chain—or, more accurately, food web. Like land plants, these microscopic plants use photosynthesis to convert carbon dioxide into organic material that ultimately nourishes all other life in the Bay.

Narragansett Bay, like any estuary, provides a variety of different habitats for living things. Certain plants and animals are concentrated in particular areas where salinity and other conditions are best suited to their needs. For example, the most productive quahog beds are in the less salty, more nutrient-rich waters of the upper Bay. On the other hand, lobster and blue mussel prefer the more oceanlike conditions of the lower Bay.

Geological History of Narragansett Bay
25,000 years ago: With the Pleistocene Ice Age in full force and sea level 300 feet lower than today, Rhode Island lay buried under an ice sheet 400 feet thick. The glacier extended as far as Block Island. Southward from there, some 70 miles of frozen tundra led finally to the Atlantic coast.

10,000 years ago: Earth was warming. The glacier had receded from Rhode Island, and the ocean was rising but had not yet reached its present level. Prehistoric humans lived in the valleys that today are the passages of Narragansett Bay. They could walk across Rhode Island by simply crossing the small streams that ran through these valleys.

9,000 years ago: As sea level continued to rise, the Atlantic Ocean entered the East Passage of the Bay.

5,000 years ago: Narragansett Bay was filled almost to its present level.

Fisheries
The Bay's commercially important species include:

Demersal (bottom-dwelling) fish: winter flounder, summer flounder, tautog, black sea bass
Pelagic fish (fish that feed in the water column): bluefish, striped bass, scup, squeteague (weakfish), menhaden, Atlantic herring, and alewife (for use as lobster bait)
Shellfish: quahog, oyster
Lobster
Squid

The demersal fish, as well as the quahog and oyster, are Bay residents that are able to live in the Bay year round and during all stages of their life cycles. Most of the commercially important pelagic fish, as well as squid, migrate to Narragansett Bay in May or June. Each year, about 100 different species may visit the Bay at one time or another.

Physical Characteristics of Narragansett Bay

Length: 25 miles
Width: 10 miles
Volume: 706 billion gallons at mid-tide
Shoreline: 256 miles, including island shorelines
Drainage basin (watershed): 1,853 square miles

—Excerpted from "An Overview of Narragansett Bay" by Eleanor Ely, published by Rhode Island Sea Grant