Bay

A bay is a coastal indentation that connects to a larger body of water like an ocean, lake, or another bay. Large bays are often called gulfs, seas, sounds, or bights. A small, circular bay with a narrow entrance is a cove, while a long, narrow bay carved by a glacier is a fjord. The term "embayment" can also refer to related features, such as extinct bays or freshwater environments.

Bays can form as river estuaries—for example, the Chesapeake Bay is an estuary of the Susquehanna River. Bays may also be nested within one another; Fig Tree Bay in Cyprus and James Bay, an arm of Hudson Bay in Canada, are examples. Some bays are vast enough to have varied marine geology, such as the Bay of Bengal (2,600,000 km²) and Hudson Bay (1,230,000 km²).

The surrounding land often reduces wind strength and blocks waves. Shorelines of bays vary widely; some have beaches with a steep upper foreshore and a broad, flat fronting terrace. Bays were important for human settlement because they offered easy access to marine resources like fisheries, and later their safe anchorage made them ideal for ports and sea trade.

The United Nations Convention on the Law of the Sea defines a bay as a well-marked indentation in the coastline, where the penetration inland is proportional to the width of its mouth so that it contains land-locked waters, not just a curve. An indentation is not considered a bay unless its area is at least as large as a semicircle drawn across its mouth; otherwise, it is called a bight.

Bays are classified by shape: open bays are widest at the mouth, flanked by headlands; enclosed bays have a mouth narrower than their widest part, flanked by at least one peninsula; semi-enclosed bays are open bays whose exit is narrowed by islands; back-barrier bays are semi-enclosed and separated from open water by barrier islands or spits. A juridical bay is a legal distinction: if a bay meets certain criteria (primarily that its area exceeds that of a semicircle across its mouth), its waters are considered inland waters of a state rather than international or territorial waters. This affects rights to the seabed and minerals, fishing control, innocent passage for seafarers, and whether the coast forms an international border.

A gulf is a large inlet from an ocean or sea into a landmass, typically larger and often with a narrower opening than a bay. Traditionally, the term was used for large, highly indented navigable saltwater bodies enclosed by coastline. Many gulfs, such as the Persian Gulf, Gulf of Mexico, Gulf of Finland, and Gulf of Aden, are major shipping areas.

Bays form through plate tectonics, coastal erosion by rivers, and glaciers. The largest bays resulted from plate tectonics: as the supercontinent Pangaea broke apart along curved fault lines, continents moved apart, leaving large bays like the Gulf of Guinea, the Gulf of Mexico, and the Bay of Bengal—the world’s largest bay. Bays also form from coastal erosion: glaciers create fjords, while rivers create rias with gradual slopes. Softer rocks erode more quickly to form bays, while harder rocks erode more slowly, leaving headlands.