An estuary has a mixture of salty and fresh water. Fresh water from the river and salt water from the sea. This mixture of salt and fresh water is called brackish water.

What is in the Severn Estuary?

The Severn Estuary is at the mouth of three major UK rivers - the Severn, Wye and Avon.

Rivers carry a lot of sediment as they travel from the source to the mouth. Much of this sediment is dropped at the mouth where the rivers flow more slowly. The following photographs show what has been broght down the rivers into the estuary.

Tree Trunk

The tree trunk in the photo below has been transported down the river. At the next hide tide, it could move back upstream or out into the Bristol Channel.

Trunk of a tree with mudflats and M4 crossing behind

Mud

Mud is a mixture of silt and clay. The substrate (bottom) of most estuaries is mud.

Mud

Rocks, stones and sticks

Where are the lighter objects in the photo below?
Why are they found there?
Where are the heavier objects?
How did these objects arrive here?

A variety of objects washed downstream

Seaweed

Where has the seaweed come from?

Seaweed

Notice how smooth the rocks are the in photo above. What do you think has made them like this?

Key Vocabulary

Estuary
The wide mouth of a river.

Silt
A mixture of sand and mud that is carried along than then dropped by the river.

Brackish water
A mixture of salt and fresh water.

Transportation
The movement of pebbles, sand, sticks and mud (the 'load') down a river.

There are several types of transportation:

Flotation. Materials such trees and branches are carried on the surface of the water. If there is a boulder or other large object in the way, then they can get stuck or be deposited behind this object.

Suspension. Very small particles are carried in the water. They are usually deposited in the lower course where the flow of water becomes very slow.

Solution. Very small particles of sediment dissolve in the water. Rivers which carry a lot of sediment in suspension may have brown water.

Saltation. Small pebbles and rocks bounce along the river bed. In turn, they can loosen other small sediments.

Traction. Small pebbles roll along the river bed.