The hydrological cycle is also known as the water cycle.

Seas and oceans contain 97 per cent of the world's water, and ice holds 2 per cent. That leaves just 1 per cent of the world's water as fresh water on land or in the air. This water is recycled again and again through the process ofevaporation, condensation and water transfers such as surface run-off.

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  Drainage basin - the area of land drained by a river.
• Catchment area - the area within the drainage basin.
• Watershed - the edge of highland surrounding a drainage basin. It marks the boundary between two drainage basins.
• Source - The beginning or start of a river.
• Confluence - the point at which two rivers or streams join.
• Tributary - a stream or smaller river which joins a larger stream or river.
• Mouth - the point where the river comes to the end, usually when entering a sea.

A river changes shape as it flows from its source (where a river starts) to its mouth (where a river flows into a sea or lake). The shape of both the long profile (a slice through the river from source to mouth) and the cross profile (a slice across the river) changes.

The source of a river is often - but not always - in an upland area. Near the source, a river flows over steep slopes with an uneven surface. It often flows over a series of waterfalls and rapids. Highland areas are usually composed of hard igneous rocks, which are ideal for forming such features.

As a river flows down steep slopes the water performs vertical erosion. This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys.

As the river flows towards the mouth, the slopes become less steep. Eventually the river will flow over flat land as it approaches the sea.

The discharge (amount of water flowing) will increase as the river approaches the sea.

Near the source of a river there is more vertical erosion as the river flows downhill, using its energy to overcome friction (A). As a result the channels are narrow and shallow and may contain large boulders and angular fragments eroded and weathered from the steep valley sides. The sediment in the river creates turbulence and friction.

As the river approaches the mouth, velocity and energy increase due to increased discharge. The river performs more lateral erosion making the channel wider, and smoother (B) and (C). As a result there is less turbulence and friction, making the flow of water more efficient.

River processes shape the land in different ways as the river moves from its source to its mouth.

Erosion

Erosion involves the wearing away of rock and soil found along the river bed and banks. Erosion also involves the breaking down of the rock particles being carried downstream by the river.

The four main forms of river erosion

• Hydraulic action - the force of the river against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and gradually wears it away.
• Abrasion - rocks carried along by the river wear down the river bed and banks.
• Attrition - rocks being carried by the river smash together and break into smaller, smoother and rounder particles.
• Solution - soluble particles are dissolved into the river.

Rivers pick up and carry material as they flow downstream.

The four different river transport processes

• Solution - minerals are dissolved in the water and carried along in solution.
• Suspension - fine light material is carried along in the water.
• Saltation - small pebbles and stones are bounced along the river bed.
• Traction - large boulders and rocks are rolled along the river bed.

Rivers need energy to transport material, and levels of energy change as the river moves from source to mouth.

• When energy levels are very high, large rocks and boulders can be transported. Energy levels are usually higher near a river's source, when its course is steep and its valley narrow. Energy levels rise even higher in times of flood.
• When energy levels are low, only small particles can be transported (if any). Energy levels are lowest when velocity drops as a river enters a lake or sea (at the mouth).

When a river loses energy, it will drop or deposit some of the material it is carrying.

• Deposition may take place when a river enters an area of shallow water or when the volume of water decreases - for example, after a flood or during times of drought.
• Deposition is common towards the end of a river's journey, at the mouth.
• Deposition at the mouth of a river can form deltas - for example, theMississippi Delta.