Rivers

The mouth of a river - where it broadens, or gets wider, as it flows into the sea is called an estuary. Estuaries provide special environments for an enormous variety of wildlife and there are often nature reserves and bird sanctuaries on estuaries.

Estuaries can be classified by their circulation patern eg Salt wedge, Highly stratified, Vertically mixed   or by formation type;Coastal Plain Estuaries,Tectonic Estuaries, Bar-built Estuaries or Fjords.            

However all estuaries independant of fresh/salt water circullation or formation, have areas that are 'washed' either once or twice a day (tidal) with sea water. At high tide the salinity of the estuary will rise as sea water (20-35 parts per thousand of salt dissolved in the water) enters the estuary mixing with freshwater (0-0.5 parts per thousand) coming downstream.

At high tide the advancement of the sea causes the river flow to slow. As the flow is reduced the river loses energy and consequently has to deposit its load of fine-grained sediment. This produces mudflat (An area of mud dropped (deposited) by the sea. You can often see it at low tide as a flat sheet of mud.

The mudflats in an estuary have a very rich fauna. The creatures that live in and on the mud, including micro-fauna, worms and shellfish, provide food for other animals.s. River estuaries are excellent feeding grounds for sea birds. They attract ducks, geese and other birds during migration.

Deltas (not present in the UK)
Another physical feature which occurs where a river meets the sea is a delta. A deltais a fan-shaped channel at a river mouth.

Deltas are features found where rivers that contain much sediment flow into the sea. As the river meets the sea, the flow is is reduced and the river has too little energy to carry the sediment so deposition of the material occurs. Deltas are 3-dimensional features with most deposition occurring below the water surface.

examples of major deltas include

• the Mississippi delta (USA)
• the Nile delta (Egypt)
• the Ganges delta (Bangladesh)
• the Po delta (Italy)

Not all deltas are formed where a river meets the coast. They can form where a river flows into a lake.

The process is the same - a sudden drop, or interruption, in the flow of the river causes it to deposit its load. The photograph below shows some of the major deltas of the world.

Image credit: SeaWiFs, NASA Goddard Space Flight Cente