River profiles and processes

A river flows from its source, usually in the hills, to its mouth, where it meets the sea.

• The long profile shows how the river's height falls along its length: steep near the source, then flatter towards the mouth.
• The cross profile shows the shape of the valley from side to side. In the upper course it is a narrow, steep-sided V; in the middle course wider, with gentler sides; in the lower course a wide, flat flood plain.

A river erodes its bed and banks in four ways:

• Hydraulic action: the force of the water hits the banks and forces air into cracks, breaking the rock apart.
• Abrasion: rocks carried by the river scrape against the bed and banks, wearing them away.
• Attrition: rocks in the river knock against each other, becoming smaller and rounder.
• Solution: the water dissolves some rocks, such as limestone.

In the upper course, the river mainly erodes downwards (vertical erosion), deepening its valley. Further downstream, it erodes more sideways (lateral erosion), widening the valley.

A river carries its load, the material it has eroded, in four ways:

• Traction: large boulders are rolled along the bed.
• Saltation: pebbles and coarse sand bounce along the bed.
• Suspension: fine particles, such as silt and clay, are carried along in the water, making it look muddy.
• Solution: dissolved minerals are carried in the water, out of sight.

The faster the water and the more there is of it, the more the river can carry, and the bigger the pieces it can move.

A river deposits (drops) its load when it slows down and loses energy, for example:

• on the inside of a bend, where the water is shallow and slow;
• when it floods and spreads out over its flood plain;
• at its mouth, where it meets the sea and slows;
• when its discharge falls after a dry spell.

The biggest, heaviest pieces are dropped first, and the finest, such as silt, last. So material is sorted, with the finest near the mouth.