A bay is a body of water partially surrounded by land. A bay is usually smaller and less enclosed than a gulf. The mouth of the bay, where meets the ocean or lake, is typically wider than that of a gulf.

In naming bays and gulfs, people have not always made these distinctions. The Persian Gulf, for example, is much smaller than Hudson Bay, Canada.

Bays can also be called lagoons, sounds, and bights. Lagos, the capital of Nigeria, is a bayside city. It sits on Lagos Lagoon, on the Bight of Benin, in the Gulf of Guinea, in the Atlantic Ocean.

Bays form in many ways. Plate tectonics, the process of continents drifting together and rifting apart, causes the formation of many large bays.

The Bay of Bengal, the largest bay in the world, was formed by plate tectonics. Millions of years ago, the Indian subcontinent crashed—and continues to crash—into the the massive Eurasian plate network. The Indian plate is subducting beneath the small Burma plate, forming the Sundra Trench. Because plate tectonics remain an active force in the Bay of Bengal today, the region is home to underwater earthquakes and tsunamis.

Bays are also formed when the ocean overflows a coastline. Kowloon Bay, Hong Kong, was formed as the South China Sea overflowed the coastline of the Kowloon Peninsula. Today, Kowloon Bay has been almost entirely reclaimed from the sea. Kowloon Bay is a major industrial and financial area, and was home to Hong Kong's airport until a new facility was built in 1998.

Another well-known coastal bay is New York Bay. New York Bay is actually two bays (Upper New York Bay and Lower New York Bay) connected by a strait called The Narrows. New York Bay is where the Hudson River meets the Atlantic Ocean.

Many bays are formed as the coastline erodes into the ocean. Guanabara Bay, for example, was formed as the Atlantic Ocean eroded an inlet in South America. Today, Guanabara Bay, also known as the Harbor of Rio de Janerio, Brazil, is one of the Seven Natural Wonders of the World.

A type of bay known as a ria is actually an estuary that has been taken over by the ocean. (An estuary is the mouth of a river.) Rias are often called "drowned rivers." Chesapeake Bay, on the East Coast of the United States, is one of the world’s largest rias. It is the drowned mouth of the Susquehanna River.

Fjords are narrow bays formed by glaciers. A glacier slices through the bedrock of an area, leaving a long, steep canyon when it recedes. The sea seeps into the inlet, forming a fjord.

Bays can also be found along the shores of lakes. Georgian Bay, for example, is a prominent bay in Lake Huron, one of North America’s Great Lakes. Georgian Bay is located in Ontario, Canada.

Freshwater Bay is a bay on the Swan River, near the busy urban area of Perth, Australia. Freshwater Bay has been a center of trade and transportation along the river for centuries.

Bay Ecosystems

Bays have wildly diverse ecosystems. Large bays open to the ocean have marine habitats. Walker Bay, South Africa, is one of the most popular sites to view (and even dive with) great white sharks. Marine mammals such as els and southern right whales are also frequent visitors to Walker Bay.

Bays on lakes and rivers have freshwater ecosystems. The wetlands of Georgian Bay, for example, are home to freshwater reptiles such as rattlesnakes and turtles. These species could never survive in a marine habitat.

Most bays have brackish water. Brackish water has a greater salt content than freshwater, but not nearly as much as the ocean. Many species are uniquely adapted to brackish water. Oyster Bay, Tanzania, adopted the name of one of its most popular species.

The Chesapeake Bay is so large that it features all three types of habitats. The northern part of the bay is almost entirely fresh. It is fed by the outflow of the Susquehanna River, and is home to species such as catfish, which favor freshwater habitats.

The bulk of Chesapeake Bay is brackish. The outflow of rivers mix with the tidal waters of the Atlantic. The eastern oyster, one of the key aquaculture species of the area, thrives in brackish water.

On the southern end of Chesapeake Bay, where it meets the Atlantic Ocean, the ecosystem is almost entirely marine. The blue crab, state crustacean of Maryland, is a mostly-marine species.

Bays and People

Bays are usually much calmer and more protected than seas or oceans. This makes them less likely to face severe damage from waves, tsunamis, and storm surges.

Most bays make excellent harbors and major port cities are often located on them. Mumbai, India, sits on the mouth of the Ulhas River and the Arabian Sea. The bay at Mumbai, originally an archipelago, has been an important trading port between Europe and Asia for thousands of years. Mumbai, also known as Bombay, may have been named after the Portuguese saying for “good bay” (bom bahia), although there is no proof of this. Today, Mumbai is the second-largest city in the world and an important harbor for goods from the Middle East, East Asia, Africa, Europe, Australia, and the Americas.

Some bays have been greatly altered by human activity. The geography of the San Francisco Bay has changed dramatically due to human activity in the 19th and 20th centuries, for instance.

During the California Gold Rush of the 1850s and 1860s, miners in Northern California dumped tons of material into the rivers that empty into the San Francisco Bay. This material material included rocks and soil from mines, as well as chemicals used in the mining process. Eventually, all this material settled in parts of the bay. Wetlands and marshes replaced the freshwater habitats throughout the bay, and pollution increased.

Industry in the 20th century has also changed the shape of the San Francisco Bay. Parts of the bay have been drained to create more land for housing and industry. Industries included salt evaporation ponds, mostly used to create material used in plastics and pharmaceuticals (drugs and medicines). Toxic chemicals used in the transportation industry were also manufactured on land reclaimed from the bay. San Francisco Bay is a strategic point in national defense, and the military has had naval and air stations there for almost a century.

Pollution has altered the ecosystem of the bay, and introduced harmful chemicals into the bay, groundwater, and soil. Today, there are more than a dozen Superfund sites in and around San Francisco Bay. Environmentalists hope the government will restore the natural bay habitat.

Chesapeake Bay

People are trying to restore and protect Chesapeake Bay as well. The Chesapeake's importance as a center of commerce, transportation, and industry predates the Revolutionary War. Native Americans relied on the bay for fishing, trade, and communication long before that.

Millions of people live on the Chesapeake Bay. The bay is anchored by the cities of Baltimore, Maryland, to the north and Norfolk, Virginia, to the south. In between, rural and urban areas dot the bay. Millions more people live in the Chesapeake's watershed, which includes more than a dozen rivers besides the Susquehanna, such as the Potomac, James, and York.

Centuries of civilization have taken their toll. Chesapeake Bay has been polluted by sewage, wastes from industry, and runoff from chemicals used in agriculture. Parts of the Chesapeake Bay are occasionally "dead zones" where there is little life below the surface waters.

Pollution and dead zones are not only bad for the environment. They also threaten the economy of the area. Maryland mussels and crabs are a major industry, harvested by fishermen called "watermen." Before the arrival of Europeans, the animals were a major source of food for Native Americans. However, due to overfishing and pollution in the bay, the number of animals is shrinking.

Thousands of Maryland and Virginia watermen are working with environmental groups and local governments to monitor and restore the habitats of Chesapeake Bay. This ensures a healthy and profitable resource endures for future generations.

Still, many remain worried about the future of Chesapeake Bay. One official, Lisa Jackson of the Environmental Protection Agency, worries. "If we come up short (in plans to clean up the bay), this may be the last generation of watermen on the Chesapeake Bay."