If a river has a wide mouth, this is called an estuary. The river carries a lot of sediment as it travels from its source to its mouth. When the fresh water from the river meets the salty water from the sea, the river drops its sediment. Most is then washed away by the sea, the river and the tides.

An estuary has a mixture of salty and fresh water. The fresh water from the river extends out into the sea. Some of the salty water from the sea travels up the river mouth. This mixture of salt and fresh water is called brackish water.

Many estuaries are suitable as a waterway for ships. Some of the world's greatest ports are found on estuaries.

An estuary by the sea

Facts

• In France, the rise and fall of the tides in the estuary of the River Rance is used to generate electricity. The world's first tidal power station was opened here in 1996.

• There are 5 major types of estuaries: coastal plain, salt marsh, lagoons, fjords and tectonic estuaries.

• Estuaries can change every day. The strength of the tides may change, so the salty water may not go so far into the estuary.

• Estuaries can change over the seasons.

• There are 3 main zones in an estuary: salt water zone, brackish water zone (mixture of salt and fresh water) and a fresh water zone.

• The fresh water zone is usually found further upstream and the salt water zone closer to the sea or ocean.

A salt marsh estuary