How are lagoons formed?

A lagoon is a shallow body of water protected from a larger body of water (usually the ocean) by sandbars, barrier islands, or coral reefs. Lagoons are often called estuaries, sounds, bays, or even lakes. They are formed when a shallow water body is separated from a large water body by a small landmass, resulting in the formation of a relief feature that resembles a partially enclosed lake.

Further Reading:

• Erosional landforms
• Geomorphic Processes