Stand beside a mountain stream and it is a noisy, tumbling ribbon barely a metre wide. Follow it for a hundred miles and it becomes a broad, silent river sliding past cities. A river is a sculptor with endless patience: given enough time it can slice a valley through solid rock and build new land out at sea. The shape of the whole landscape is written by moving water.
The long profile
From source to mouth a river's journey splits into three courses. The long profile is a side-on view of this slope, steep at the top and gentle at the bottom.
Erosion, transport and deposition
A river does three jobs. It erodes (wears away), transports (carries) and deposits (drops) material. There are four kinds of erosion:
| Process | What happens |
|---|---|
| Hydraulic action | The sheer force of water forcing into cracks and breaking rock apart |
| Abrasion | Pebbles carried by the river grinding against the bed and banks like sandpaper |
| Attrition | Rocks knocking together, becoming smaller and rounder |
| Solution | Slightly acidic water dissolving soluble rocks such as chalk and limestone |
Landforms of the course
- Upper course: V-shaped valleys and waterfalls, where the river cuts down through hard rock over softer rock, forming a plunge pool and gorge.
- Middle course: meanders — sweeping bends. Water flows fastest on the outside, eroding a river cliff, and slowest on the inside, depositing a slip-off slope.
- Lower course: floodplains, ox-bow lakes and deltas where the river drops its load.
Notice how the river's job changes along its course. Near the source it has steep energy for vertical erosion, cutting downwards. In the middle course it swings sideways with lateral erosion, widening its valley. In the lower course it is slow and heavy with sediment, so deposition takes over and it builds new flat land. The same river is a cutter, a carver and a builder at different points along its journey.