An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it. This represents the final feature in the river system in its course from source to mouth. Often it represent an area where the interrelation between human and physical activities are most pronounced.
Estuaries form a transition zone between river environments and ocean environments and are subject to both marine influences, such as tides, waves, and the influx of salt water; and riverine influences, such as flows of fresh water and sediment. The inflow of both seawater and freshwater provide high levels of nutrients in both the water column and sediment, making estuaries among the most productive natural habitats in the world.
This section of the National Geographic website shows a variety of environments, forms and conflicts surrounding estuaries. Follow the link and read through the information and make some brief notes on estuary formation including a variety of examples.