the river system - source to estuary
Ref: B03

Every river has a ‘source’, a place where the river begins. The sources of many rivers in Cornwall, for example the Fowey and the Lynher, are to be found on Bodmin moor. Rain falling on high ground drains off the moor to form small channels, and the beginnings of these rivers.

Withybrook on Bodmin moor

As a river flows across the land it grows in size, smaller channels and streams, known as tributaries, join it. This increases the size of the river as rainwater from a larger area is able to enter the river through these tributaries.

As the amount and speed of water in a river increases so does the river’s ability to transport materials. The more water it has the more energy it has to erode and carry sediment, carry leaves and wood – branches and even whole trees. Rivers can be very powerful, they move a lot of material on their journey from source to sea.

River Lynher aftera rainstorm, evidence of cattle erosion on the far bank

The type of terrain (land) over which a river travels and its gradient (G1) will make a difference to the appearance of the river. Rivers on steep gradients will try and cut down into the terrain rather than widen out over an area when the amount of water entering a river increases. This is because it creates a faster pathway by cutting down. When, however, there is a sudden increase in water through heavy rain or a thunderstorm, the river will not have time to cut down into the land and alter its shape to accommodate the extra water. Instead the river will flood over nearby land, especially in the lower part of the river where the land is usually much flatter. Land either side of a river is called the floodplain because of this.

De Lank in flood at Delphi Bridge

At different stages on its journey a river will offer a home to different plants and animals, creating a variety of habitats from source to sea. As the energy of a river increases the things it can carry and its ability to create different features e.g. overhanging banks (see Fisheries F01 – point 6), gravel islands (F01 – point 7) is increased. The amount of places for plants and animals to live is then increased, so the habitat has more variety and different places suitable for different creatures.

Salt Marshes on the Lynher estuary

When a river nears the sea it usually becomes much flatter, loses energy and the heavier materials settle on the river bed. Where it meets the sea it is called an estuary or the ‘mouth‘ of the river. At this point the river will drop its sediment for a last time as it loses the rest of its energy when joining a bigger mass of water. The freshwater of the river will then mix with the saltwater of the sea.