* Formed when a river meets the ocean along a gently sloping coastline

* Typically have a wide, shallow mouth and a gradual change in salinity from fresh to salt water

* Examples include the Chesapeake Bay (USA) and the Thames Estuary (UK)

Fjord estuaries

* Formed when a glacier retreats and leaves behind a deep, narrow valley that is flooded by the sea

* Typically have a narrow mouth and a steep change in salinity from fresh to salt water

* Examples include the Sognefjord (Norway) and the Milford Sound (New Zealand)

Bar-built estuaries

* Formed when a sandbar or barrier island forms across the mouth of a river

* Typically have a shallow, enclosed basin with a narrow inlet to the ocean

* Examples include the Laguna Madre (USA) and the Ria de Aveiro (Portugal)