Geological work of running water and its landforms

Running water (rainwater, streams and rivers) is the most important agent of landscape change. Its work has three parts.

1. Erosion

Done by hydraulic action (force of water), abrasion (carried load scrapes the bed), attrition (load wears itself) and solution. Erosion is vertical in the youth stage and lateral in old age.

2. Transportation

• Solution: dissolved salts.
• Suspension: clay and silt.
• Saltation: sand bouncing along the bed.
• Traction: pebbles and boulders rolled along the bed.

3. Deposition

Occurs when velocity falls (gentle gradient, river mouth, floods).

Landforms

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: vertical cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: hard rock lying over soft rock; the plunge pool forms at the base.
• Pot holes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: side cutting in the old stage.
• Interlocking spurs: ridges between bends of a young river.

Depositional work occurs when the velocity and load capacity fall (at gentle slope, at a mouth, in floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad flat valley floor built by flood deposits.
• Natural levees: low ridges of coarse silt built along the banks during floods.
• Braided bars and channel (point) bars: sand and gravel bars inside the channel.
• Delta: fan-shaped deposit at the river mouth (Ganges-Brahmaputra delta).
• Ox-bow lake and meander scars: cut-off bends.

Stages of a river