An estuary is a partially enclosed body of water (such as a bay, lagoon, sound, or slough) where two other bodies of water, usually saltwater and freshwater, meet and mix. Freshwater estuaries occur where freshwater from a river or stream mixes with water from a lake that is chemically distinct. Scentists classify these environments using two characteristics: geology and water circulation.

Estuaries are often called the nurseries of the sea because so many marine animals depend on them as a food source or spend some part of their lives there. Many plants and animals, including many species of concern, are especially adapted to live in these unique environments. Estuaries are also a major stopover point for migratory animals, and coastal communities rely on estuaries for tourism, shipping and transportation, and fishing.

Each estuary displays unique beauty. One estuary may be enclosed by marshes and barrier islands, while others have a coastline or reef border. Examples include Chesapeake Bay, San Francisco Bay, Boston Harbor, Tampa Bay, and Puget Sound.