An estuary is a dynamic ecosystem having a connection to the open sea through which the sea water enters with the rhythm of the tides. The effects of tides on estuaries can show nonlinear effects on the movement of water which can have important impacts on the ecosystem and waterflow.

An estuary is an area where freshwater from the rivers mixes with saltwater in the ocean/sea. This mixture of freshwater and saltwater is referred to as brackish water. Being a transitional zone from fresh to saltwater, estuaries are a dynamic ecosystem that experiences constant environmental changes.

In the most general terms, an estuary is an ecosystem, comprising both the biological and physical environment, that has developed in a region where rivers meet the sea and fresh-flowing river water mingles with tidal salt water to become brackish, or partly salty.

Estuaries are bodies of water and their surrounding coastal habitats are typically found where rivers meet the sea. Estuaries harbor unique plant and animal communities because their waters are brackish — a mixture of fresh water draining from the land and salty seawater. Estuaries are some of the most productive ecosystems in the world.

An estuary is a body of water that forms when freshwater from the land meets and mixes with saltwater from the ocean. Estuaries come in a variety of sizes and are also known as bays, lagoons, harbours, inlets, sounds, wetlands, and swamps. Estuaries are special environments to which plants and animals have evolved.