The beach is made up of pebbles, which are continually being moved by the waves along the beach from west to east because that is the direction the wind is normally blowing.
Because it moves this way it is gradually blocking up the mouth of the river and so in about 6 years time the river mouth will have moved.
The river will also be allowed to flood. This will take place in stages: the 1st stage is for it to flood a field to the west .
Later the meanders will join into the river and eventually the whole valley will be flooded or turn to salt marshes. A salt mash is a piece of land which the sea water has gone on the land making it wet and salty. If you put your foot in the grass it would sink.