In A Nutshell
Both bays and gulfs are areas where land meets water, but is there a difference other than the name? Yes—a bay has a wider mouth than a gulf, and doesn’t have the same sense of enclosure that a gulf does. Bays are more often formed by the erosion of a coastline or flooding, while gulfs are generally created by the movement of the Earth’s plates to form deeper, more drastically enclosed waters.
The Whole Bushel
The easiest way to describe the difference between a bay and a gulf is to look at how they’re created. Both are a formation where the land meets the water, but a small bay is typically formed by coastal erosion or a rise in the water level and subsequent flooding. Kowloon Bay in Hong Kong was formed when the ocean level rose, flooding the nearby coastline and turning what was once land into a relatively shallow bay. so shallow, in fact, that it’s been largely turned back into land. It kept the name, though, and parts are now protected by shelters and a breakwater. Tidal erosion can also create a bay, such as Guanabara Bay in Brazil.
Bays that are created by the erosion of layers of soft rock alongside more resilient materials are a great example of the wide, sweeping mouth characteristic of this coastal formation. The southern coast of England has what’s known as a discordant coastline, meaning that as you travel along the coast, you’re walking along wide bands of alternating rock. Areas made of soft rock, like sand and clay, will be eroded by the tides, while the more resilient rock won’t erode as fast. The result is a series of bays between jutting points of land called “points” or “headlands.” Small bays can exist within a gulf.
Larger bays can also be created by plate tectonics, and plate tectonics are also often responsible for the creation of gulfs—the difference is what they look like after the earth settles. When a bay is created by movements of the Earth’s surface, it tends to essentially take a large piece out of the coastline and creates a wide, open indent in a land mass. Gulfs, on the other hand, more appropriately resemble massive lakes connected to the ocean by passages that are more narrow than those connecting a bay.
The Gulf of Mexico is the world’s largest gulf, and a great example of the major difference in what a gulf and a bay look like. Bordered by the United States and Mexico, the Gulf of Mexico is hugged by the land and connected to the ocean through various straits and seas.
While gulfs are typically just called “gulfs,” bays can have a number of different names given to them depending on various physical conditions. Fjords are bays that are formed by the movement of a glacier and the reclaiming of the canyons left behind by the sea. A ria is a specific type of bay that is formed by the coastal erosion of an estuary, or the mouth of a river. Lagoons are very shallow bays that are connected to the ocean, but protected by a series of low-lying barrier islands that run along the coastline.
Gulfs tend to open to a marine or ocean environment, while bays can also be found along the coastlines of large lakes and even rivers. This means that the ecosystems supported by both bays and gulfs are very diverse; there are saltwater and marine bays and gulfs, but bays can also be located in freshwater areas.